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Hydroxy Acids: Production and Applications
Chapter 4
T.C. Bhalla,* Vijay Kumar and S.K. Bhatia
Department of Biotechnology, Himachal Pradesh University, Shimla 171 005,
Himachal Pradesh, India;
*Email: bhallatc@redi mail.com
1. INTRODUCTION
Hydroxy acids are organic acids having one or more hydroxyl group attached directly to the
carbon chain of an aliphatic or alicyclic carbon atom. Hydroxy acids occur in nature and can
also be synthesized through chemical or enzymatic methods. Many of the hydroxy acids occur
in plants i.e. sugar cane, tomatoes, oranges, lemons, grapes, apples, etc. and in animal tissues.
Since time immemorial, human civilization use many hydroxy acids in form of crude extracts
of plants and their various parts for curing many ailments and diseases. Ancient Sumarian and
Egyptian knew the analgesic property of willow leaf and use its extract for curing joint pain
and infl ammation, now it is well known that it contain salicylic acid as an active ingredient
(Jack, 1997). There are many traditional preparations in ‘Ayurveda’ an ancient Indian science,
containing one or more hydroxy acids, used as antiaging, anti-infl ammatory and for other skin
disorders (Datta et al., 2011). Formulations containing hydroxy acids have been used in clinical
practice for decades to treat a variety of skin infection. These acids have a broad range of
application in various fi elds e.g. cosmetics (Saint-Leger et al., 2007), pharmaceutical and food
processing (Kornhauser et al., 2010). In cosmetics hydroxy acids are used for the treatment of
various skin diseases such as in treating photoaging, acne, pigmentation disorders and psoriasis
(Wang, 1999; Kornhauser et al., 2010). Hydroxy acids have transformed skin care since their
introduction to dermatology (Van Scott & Yu, 1974; Green, 2006; Green et al., 2009). A range
of pharmaceutically important chiral synthons are being synthesized using hydroxy acids as
precursor, e.g. α-hydroxyphenylacetic acid (mandelic acid) used in the synthesis of antitumor
agents (Surivet & Vatele, 1999), antiobesity agents, semi synthetic penicillin (Furlenmeier et
al., 1976) and cephalosporin (Terreni et al., 2001). Hydroxy acids and their derivatives are
useful starting materials in synthetic organic chemistry and can be synthesized by biochemical
resolution of racemates and by asymmetric synthesis. Biochemical processes are preferred over
chemical processes as these allow ecofriendly synthesis with high chemo- and regio-selectivity
Advances in Industrial Biotechnology
Ram Sarup Singh, Ashok Pandey & Christian Larroche (Eds.)
IK International Publishing House Pvt. Ltd., India pp 56-76
Hydroxy Acids: Production and Applications 57
and yield. However, the information on synthesis of hydroxy acids through biological route is
scanty and only a few reports are available. In this chapter synthesis of hydroxy acids with a
focus on enzymatic routes and their applications are discussed.
2. CLASSIFICATION OF HYDROXY ACIDS
Hydroxy acids can be classifi ed on the basis of position of hydroxyl group on the carbon skeleton
i.e., α, β, γ (Fig. 1), number of hydroxyl group i.e. mono or poly hydroxy acids (Fig. 2) and
nature of carbon skeleton i.e. aliphatic or aromatic (Fig. 3). However, hydroxy acids can be
broadly categorized in to four major groups: (a) α-hydroxy acids (AHAs), (b) β-hydroxy acids
(BHAs), (c) polyhydroxy acids (PHAs)/polycarboxy hydroxy acids (PCHAs), and (d) aromatic
hydroxy acids acid (ArHAs).
Fig. 1: Hydroxy acids: -Hydroxy acids, -Hydroxy acids, - Hydroxy acids.
Fig. 2: Monohydroxy acids and polyhydroxy acids.
Fig. 3: Aliphatic, aromatic and aryalkyl hydroxy acids.
58 Advances in Industrial Biotechnology
2.1 α-Hydroxy Acids
α-Hydroxy acids are naturally occurring organic carboxylic acids, where hydroxyl group is
present on α carbon atom of the skeleton (Kornhauser et al., 2010). Glycolic acid is the smallest
molecule of all the hydroxy acids and a natural constituent of sugar cane juice and lactic acid
is another example, found in sour milk and tomato juice (Maddin, 1998). All other AHAs
may be considered derivatives of glycolic acid. α-Hydroxy acids are found in plants (chain
from 12 up to 24 carbon atoms) and in animal wool waxes, skin lipids and specialized tissues,
mainly in brain. Salvia nilotica contains 2-hydroxylinoleic and 2-hydroxyoleic acids along with
hydroxylinolenic acid (Bohannon & Kleiman, 1975). 2-Hydroxytetracosanoic acid (cerebronic
acid) and 2-hydroxy-15-tetracosenoic acid (hydroxynervonic acid) are constituents of the
ceramide part of cerebrosides (glycosphingolipides) found mainly in nervous tissue and in some
plants. The testis and spermatozoa of bear and rat contain sphingomyelin with 2-hydroxylated
n-6 tetra- and pentaenoic acids with very long carbon chain (up to 34 carbon atoms) (Robinson
et al., 1992). The AHAs may be divided into three sub groups: (a) alkyl AHAs, (b) arylalkyl
AHAs, and (c) polycarboxyl AHAs.
2.1.1 Alkyl AHAs
Hydroxyl group is attached at α carbon of an alkyl carbon chain of hydrocarbon; it is called
alkyl AHAs. Some alkyl AHAs are listed in Table 1.
Table 1: Alkyl -hydroxy acids.
Common name Structure formula Systematic name
Glycolic acid CH2OHCOOH α-Hydroxyethanoic acids
Lactic acid CH3 CHOHCOOH α-Hydroxypropanoic acid
Methyl lactic acid (CH3)2COHCOOH α-Methyl,α-hydroxypropanoic acid
α -Hydroxybutyric acid CH3CH2 CHOHCOOH α-Hydroxybutanoic acid
DL-α-Hydroxy valeric acid CH
3(CH2)2CHOHCOOH α-Hydroxypentanoic acid
DL-α-Hydroxy caproic acid CH3(CH2)3CHOHCOOH α-Hydroxyhexanoic acid
α-Hydroxy enanthoic acid CH3(CH2)4 CHOHCOOH α-Hydroxyheptanoic acid
α-Hydroxy caprylic acid CH3(CH2)5 CHOHCOOH α-Hydroxyoctanoic acid
α-Hydroxy pelargonic acid CH3(CH2)6 CHOHCOOH α-Hydroxynonanoic acid
α-Hydroxy capric acid CH3(CH2)7 CHOHCOOH α-Hydroxydecanoic acid
α-Hydroxy hendecanoic acid CH3(CH2)8 CHOHCOOH α-Hydroxyundecanoic acid
α-Hydroxy lauric acid CH3(CH2)9 CHOHCOOH α-Hydroxydodecanoic acid
α-Hydroxy myristic acid CH3(CH2)11 CHOHCOOH α-Hydroxytetradecanoic
α-Hydroxy palmitic acid CH3(CH2)13 CHOHCOOH α-Hydroxyhexadecanoic acid
DL-α-Hydroxy stearic acid CH3(CH2)15 CHOHCOOH α-Hydroxyoctadecanoic acid
Cerebronic acid CH3(CH2)21CHOH-COOH α-Hydroxytetraeicosanoic acid
Hydroxy Acids: Production and Applications 59
2.1.2 Arylalkyl AHAs
Arylalkyl AHAs is formed when a phenyl group is attached to α carbon of the alkyl AHA,
and the example of these group are mandelic acid, benzilic acid and 3-phenyllactic acid Some
important Aryl AHAs are listed below in Table 2.
Table 2: Arylalkyl -hydroxy acids.
Common name Chemical structure Systematic name
Mandelic acid C8H8O32-Phenyl α-hydroxyethanoic acid
Benzilic acid C14H12O32,2-Diphenyl α-hydroxyethanoic acid
Phenyllactic acid C9H10O33-Phenyl α-hydroxypropanoic acid
Atrolactic acid C10H12O22-Phenyl 2-methyl α-hydroxyethanoic acid
2.1.3 Polycarboxy hydroxy acids
AHA may contain of more than one carboxyl group e.g. malic acid, occurring in apples is also
called apple acid, and the tartaric acid present in grapes (Hale, 1962), has been called fruit
acid in the past. Citric acid occurring in oranges and lemons has one hydroxy group and three
carboxyl group. The α or β refers to the position of the hydroxy group in the hydroxy acids.
When a hydroxy acid has more than one carboxyl group it can be an AHA and a BHA at the
same time. For example, malic acid, tartaric acid and citric acid can be both AHA and BHA
(Table 3).
Table 3: Polycarboxy -hydroxy acids.
Common name Structure Systematic name
Tartonic acid HOOCCHOHCOOH 2-Hydroxypropane-1,3-dioic acid
Malic acid HOOCCH2CHOHCOOH 2-Hydroxybutane-1,4-dioic acid
Citramalic acid HOOCCH2C(CH3)OHCOOH 2-Hydroxy-2-methylbutane-1,4-dioic acid
Tartaric acid HOOCCHOHCHOHCOOH 2,3-Dihydroxybutane-1,4-dioic acid
Citric acid C(OH)(COOH)(CH2COOH)23-Carboxy-3-hydroxypentane 1,5-dioic acid
Isocitric acid HOOCCH2OHCH(COOH)CH2COOH 3-Carboxy-2-hydroxypentane-1,5-dioic acid
Homocitric acid HOOCCH2C(OH)(COOH)(CH2)2COOH 3-Carboxy-3-hydroxyhexane 1,6-dioic acid
Homoisocitric acid HOOCCHOHCH(COOH)(CH2)2COOH 3-Carboxy-2-hydroxyhexane-1,6-dioic acid
2.2 β-Hydroxy Acids
β-Hydroxy acids are organic acids containing hydroxyl functional group separated by two carbon
atoms (Kornhauser et al., 2010). Optically active β-hydroxycarboxylic acids and their derivatives
have been used as starting materials for the synthesis of optically active bioactive compound such
as vitamins, antibiotics, pheromones, and fl avor compounds (Ren et al., 2010). Most common
β-hydroxy acids are 3-hydroxypropanoic acid (β-hydroxypropanoic acid), 3-hydroxybutanoic
60 Advances in Industrial Biotechnology
acid (β-hydroxybutyric acid), 4, 2-phenyl-3-hydroxypropanoic acid (tropic acid), 3-hydroxy-
3,7,11-trimethyldodecanoic acid and 9,10,16-trihydrohexadecanoicacid (aleuratic acid). Some
BHAs are also considered AHAs as they contain a hydroxyl group in the α-position to one
carboxyl group and in the β-position to the other carboxyl group. Malic acid and citric acid
are prominent representatives in this category (Kornhauser et al., 2010).
2.3 Polyhydroxy Acids
Polyhydroxy acids are organic carboxylic acid having multiple hydroxyl groups (Kornhauser et
al., 2010). Many PHAs are also AHAs as these are derived from carbohydrates and are important
metabolites in the intermediary metabolism. PHAs may be further divided into different groups
aldonic acid (a monocarboxylic acid having a chain of three or more carbon atoms and formally
derived from an aldose by oxidation of the aldehydic group), aldaric acid (both ends of an
aldose chain are oxidized to carboxylic acids). Some important PHAs are listed in Table 4.
Table 4: Polyhydroxy -hydroxy acid.
Common name Structure formula Systematic name
Glyceric acid HOCH2CHOHCOOH 2,3-Dihydroxypropanoic acid
Erythronic acid, threonic acid HOCH2(CHOH3)COOH 2,3,4-Trihydroxybutanoic acid
Ribonic acid, arabionic acid, xylonic acid HOCH2(CHOH)4COOH 2,3,4,5-Tetrahydroxypentanoic acid
Allonic acid, altonic acid, gluconic acid HOCH2(CHOH)5COOH 2,3,4,5,6-Pentahydroxyhexanoic acid
Allheptnoic acid, altroheptnoic acid,
glucoheptonic acid
HOCH2(CHOH)6COOH 2,3,4,5,6,7-Hexahydroxyheptanoic acid
2.4 Aromatic Hydroxy Acids
Aromatic hydroxy acids (phenolcarboxylic acids) are a type of organic compounds containing
a phenolic ring and carboxylic acid functional group (C6-C1 skeleton). Hydroxyl group
whether one or many are directly attached to benzene ring. They can be categorized into
monohydroxybenzoic acids (ortho, meta and para-hydroxybenzoic acid), dihydroxybenzoic acids
(gentisic acid, protocatechuic acid) and trihydroxybenzoic acids (gallic acid, phloroglucinol
carboxylic acid) (Table 5). Hydroxybenzoic acids are a major group of phenolic acids; other
is hydroxycinnamic acids.
Table 5: Aromatic hydroxy acids.
Common name Structure formula Systemic name
o-Coumaric acid C9H8O3(E)-3-(2-Hydroxyphenyl)prop-2-enoic acid
p-Coumaric acid C9H8O3(E)-3-(4-Hydroxyphenyl)-2-propenoic acid
m-Coumaric acid C9H8O3(E)-3-(3-Hydroxyphenyl)-2-propenoic acid
Ferulic acid C10H10O4(E)-3-(4-Hydroxy-3-methoxy-phenyl) prop-2-enoic acid
Sinapic acid C11H12O53-(4-Hydroxy-3,5-dimethoxyphenyl) prop-2-enoic acid
Contd...
Hydroxy Acids: Production and Applications 61
Ca eic acid C9H8O43-(3,4-Dihydroxyphenyl 2-propenoic acid
Salicylic acid C7H6O32-Hydroxybenzoic acid
m-Hydroxybenzoic acid C7H6O33-Hydroxybenzoic acid
p-Hydroxybenzoic acid C7H6O34-Hydroxybenzoic acid
Vanillic acid C8H8O44-Hydroxy-3-methoxybenzoic acid
Syringic acid C9H10O54-hydroxy-3,5-dimethoxybenzoic acid
Protocatechuic acid C7H6O43,4-Dihydroxybenzoic acid
Gentisic acid C7H6O42,5-Dihydroxybenzoic acid
Gallic acid C7H6O53,4,5-Trihydroxybenzoic acid
Phloroglucinol carboxylic acid C7H6O52,4,6-Trihydroxybenzoic acid
2.5 Other Important Hydroxy Acids
2.5.1 γ-Hydroxy acids
γ-Hydroxybutyric acid (GHB) is a naturally occurring short-chain fatty acid, a metabolite of
γ-amino butyric acid (GABA). GHB also known as 4-hydroxybutanoic acid, is a naturally
occurring substance found in the human central nervous system, as well as in wine, beef, citrus
fruits and almost all animals in small amounts. It is also categorized as an illegal drug in many
countries. It is currently regulated in Australia and New Zealand, Canada, most of Europe and in
the US. GHB as the sodium salt, known as sodium oxybate, used to treat cataplexy and excessive
daytime sleepiness in patients with narcolepsy (O’Connell et al., 2000; Drasbek et al., 2006).
2.5.2 Hydroxy fatty acids
The prostaglandins and other eicosanoids are hydroxy fatty acids. 2-(D)-hydroxy fatty acids
are more conventional lipid components and are important constituents of sphingolipids of
microorganism, plants and animals (Nichols & Maraj, 1998; Kishimoto & Hauin, 1963; Yang
et al., 2012). The chain-lengths vary from about C16 to C26, and they are normally saturated,
although monoenoic components are also known. Sphingomyelin containing 2-hydroxylated
polyenoic very-long-chain fatty acids has been found in mammalian testes and spermatozoa
(Robinson et al., 1992). Such fatty acids together with isomers containing iso/anteiso-methyl
branches have been found in the glycerophospholipids especially phosphatidylethanolamine of
sponges (Schmitz & McDonald, 1974).
2.5.3 ω-Hydroxy acids
ω-Hydroxy acids (OHAs) are a class of naturally occurring straight chain aliphatic organic
acids with a carboxylic group at position 1 and α hydroxyl group at position n. The C16 and
C18 omega hydroxy acids e.g. 16-hydroxy palmitic acid and 18-hydroxy stearic acid are key
monomers of cutin in plant cuticle (Kolattukudy & Walton, 1972; Kolattukudy, 1996).
3. HYDROXY ACIDS IN PLANTMICROBE SYMBIOSIS
Phenolic acids are the main hydroxy acids made by plants. These compounds have diverse
functions and are immensely important in plant-microbe interactions. Phenolic compounds act
Contd...
62 Advances in Industrial Biotechnology
as signaling molecules in initiation of legume rhizobia symbioses, establishment of arbuscular
mycorrhizal symbiosis and can act as agents in plant defense. Flavonoids are a diverse class
of polyphenolic compounds that have received considerable attention as signaling molecules
involved in plant-microbe interactions compared to more widely distributed simple phenolic
acids e.g. hydroxybenzoic and hydroxycinnamic acids (Mandal et al., 2010).
In response to microbial attack, plants activate defense responses that lead to induction
of a broad spectrum of antimicrobial compounds i.e. aromatic hydroxy acids. Plant phenolics
compounds produced during host-pathogen interactions act by several mechanisms in plant
defense (Robbins, 2003). There is a complex interrelationship between phenolic acids or their
derivatives such as fl avonoids and the ecology of the plant microbe symbiosis system. They
undergo transformation in the soil because some microorganisms have the capacity to utilize them
as carbon sources. Over the last few decades, various functions for these phenolic compounds
in root nodules have been investigated (Mandal et al., 2010).
4. SYNTHESIS OF HYDROXY ACID
Hydroxy acids are widely distributed in nature i.e. present in all life forms. Most of them
are synthesized by plants and microorganism in various metabolic pathways. Advancement in
scientifi c knowledge has led to the development of many chemical as well enzymatic methods
for their production.
4.1 Chemical Synthesis
There are several chemical methods for the synthesis of individual hydroxy acids which depend
on their commercial importance and the availability of starting materials. Mandelic acid,
α-hydroxyisobutyric acid, glycolic acid and hydroxybenzoic acids are some of the hydroxy acids
having immense potential in cosmetic and pharmaceutical and other industries. Their chemical
synthesis has been described below:
4.1.1 Chemical synthesis of mandelic acid
Mandelic acid (aryl aliphatic hydroxy acid) is an important hydroxy acid having wide applications.
Chemically it is synthesized from benzaldehyde and acetophenone as shown below:
4.1.1.1 From benzaldehyde: Mandelic acid is prepared from benzaldehyde through cyanohydrins
(Fig. 4). It is prepared by reacting benzaldehyde with NaCN in the presence of bisulphate,
followed by treatment with hydrochloric acid (Corson et al., 1941).
Fig. 4: Mandelic acid synthesis from benzaldehyde.
Hydroxy Acids: Production and Applications 63
4.1.1.2 From acetophenone: Dichloroacetophenone produced by the chlorination of
acetophenone in the presence of glacial acetic acid which is subsequently reacted with NaOH
and HCl to produce mandelic acid as shown in Fig. 5 (Aston et al., 1955).
Fig. 5: Mandelic acid synthesis from acetophenone.
4.1.2 Chemical synthesis of α-hydroxyisobutyric acid
α-Hydroxyisobutyric acid (α-HIB) is another important α-hydroxy acid, synthesized chemically
from isobutyric acid by its catalytic oxidation. An aqueous environment, with molecular oxygen
in the presence of thallic bromide catalyst and a catalytic amount of copper, iron or the halide
as a co-oxidant leads to synthesis of α-HIB (Fig. 6).
Fig. 6: Synthesis of -hydroxyisobutyric acid by the oxidation of isobutyric acid.
4.1.3 Chemical synthesis of glycolic acid
This acid is prepared by the reaction of chloroacetic acid with sodium hydroxide followed by
re-acidifi cation to get glycolic acid and sodium chloride as shown in Fig. 7 (Ebmeyer et al.,
1998). It can also be synthesized by the oxidation of ethylene glycol in alkaline environment
(Kuzetsov et al., 1962). Other methods, not apparently in use, include hydrogenation of oxalic
acid with nascent hydrogen and the hydrolysis of the cyanohydrin derived from formaldehyde.
64 Advances in Industrial Biotechnology
Fig. 7: Chemical synthesis of glycolic acid from chloroacetic acid.
4.1.4 Chemical synthesis of hydroxybenzoic acid
Hydroxybenzoic acids are produced commercially from potassium phenoxide and carbon dioxide
in the Kolbe-Schmitt reaction (Lindsey and Jeskey, 1957). The modifi ed Kolbe–Schmitt reaction
is well known as the carboxylation reaction of alkali metal phenoxides producing hydroxybenzoic
acid under conditions of high temperature and pressure (Iijima & Yamaguchi, 2007) (Fig. 8).
Several phenolic compounds such as salicylic acid, p-hydroxybenzoic acid, γ-resorcylic acid,
and p-aminosalicylic acid are produced through this reaction. p-Hydroxybenzoic acid can also
be synthesized by aerobic oxidation of p-hydroxyacetophenone (Minisci et al., 2004) (Fig. 9).
Fig. 8: Modifi ed Kolbe–Schmitt reaction for synthesis of hydroxybenzoic acid.
Fig. 9: Synthesis of p-hydroxybenzoic acid by aerobic oxidation of p-hydroxyacetophenone.
4.1.5 Limitation of chemical methods
Chemical synthesis of hydroxy acid remains a laborious and tedious task due to requirements
of heavy metals catalyst and toxic reagents. They require harsh conditions (high temperature
and pressure) and involve numerous synthetic steps employing intermediate leaving and
protecting groups. Most of the times chemical processes are both energy intensive, requiring
Hydroxy Acids: Production and Applications 65
high temperature, pressure and produce a signifi cant amount of by-products due to side reaction
caused by extreme reactivity of the resident hydroxyl oxygen (Miller and Peretti, 2001; Kirimura
et al., 2010).
4.2 Biological Synthesis of Hydroxy Acids
There are several pathways for the biosynthesis of individual hydroxy acids, depending on
the plant and microorganisms. However, most of the aromatic hydroxy acids are produced in
plants via shikimic acid through the phenylpropanoid pathway, as by-products of the monolignol
pathway and as breakdown products of lignin and cell wall polymers in vascular plant (Martens,
2002; Croteau et al., 2000). Additionally, some phenolic acids are of microbial origin (Moorman
et al., 1992). They can be derived directly from the shikimate pathway (Mandal et al., 2010) or
can also be produced by the degradation of hydroxycinnamic acids in a similar manner to the
β-oxidation of fatty acids and the major intermediates are cinnamoyl-CoA esters. Knowledge
of the mechanisms and particularly the enzymes involved in the biosynthesis of hydroxy acids
and their derivatives is rather limited.
4.2.1 Enzymatic methods for synthesis of hydroxy acids
Enzymatic production of hydroxy acids offers signifi cant advantages over traditional methods,
due to specifi city of enzymes and mild reaction conditions. Enzymatic processes for the
synthesis of fi ne chemicals are gaining importance because of mild reaction conditions,
eco-friendly biotransformation, ease of biocatalyst production, stereo and regio-selective
transformation (Kirimura et al., 2010). Various hydroxy acids i.e., glycolic acid, mandelic acid,
α-hydroxyisobutyric acid and p-hydroxybenzoic acid have been synthesized using nitrilase from
their corresponding nitriles at ambient conditions. Different types of microbial and enzymatic
processes are used for the synthesis of various hydroxy acids. A few of them are discussed below:
4.2.1.1 Mandelic acid biosynthesis
4.2.1.1.1 Hydrolysis of mandelonitrile: The nitrilase of Alcaligenes faecalis 8750 (Yamamoto
et al., 1992), Alcaligenes sp. ECU0401 (Zhang et al., 2011), A. faecalis ZJUTB10 (Xue et al.,
2010) have been used for conversion of R-(-) mandelonitrile into R-(-) mandelic acid. S-(+)
mandelonitrile in reaction was recycled via formation of benzaldehyde and hydrogen cynide (Fig.
10). Enzymatic synthesis of mandelic acid is advantageous over its chemical counterpart due to
high enatio-selectivity of nitrilase enzyme (Pandey et al., 2011). Pseudomonas putida MTCC
5110 also contains highly enatioselective nitrilase for the hydrolysis of racemic mandelonitrile
to R-(-) mandelic acid (Kaul et al., 2004). Genetically engineered E. coli cells were used for
the production of R-(-) mandelic acid in stirred tank reactor with high yield and enantiomeric
excess (Banerjee et al., 2009).
4.2.1.2 α-Hydroxyisobutyric acid
4.2.1.2.1 Hydrolysis of acetone cyanohydrins: α-Hydroxyisobutyric acid has been produced
by hydrolysis of acetone cyanohydrin by two step reaction using nitrile hydratase and amidase
of Alcaligenes sp. 10674 (Bhatia et al., 2012). Various microbes e.g., Acidovorax, Comamonas,
66 Advances in Industrial Biotechnology
Pseudomonas and Penicillium can be used for the production of α-HIB from acetone cyanohydrins
using nitrilase or nitrile hydrates/amidases (Fig. 11).
4.2.1.2.2 Biooxidation of tert-butanol: A biooxidation route for the synthesis of α-HIB by
bacterial degradation pathway of methyl tert-butyl ether (MTBE) via tert-butanol has been
reported (Lopes et al., 2008). In Mycobacterium austroafricanum IFP 2012, a none-heme alkane
monooxygenase is hydroxylating tert-butanol and resulting diol is further oxidized by two novel
dehydrogenases, designated as MpdB and MpdC, to the carboxylic acid. The enzyme system
for the oxidation of tert-butanol has also been described for the MTBE-degrading bacterial
strains Methylibium petroleiphilum PM1 (Hristova et al., 2007) and Aquincola tertiaricarbonis
L108 (Schafer et al., 2007).
4.2.1.3 Glycolic acid: Glycolic acid (GLA) is synthesized chemoenzymatically from glycolonitrile
(GLN) to ammonium glycolate, using a nitrilase derived from Acidovorax facilis 72W (Wu
et al., 2007) (Fig. 12). Ammonium glycolate is than subsequently converted to GLA by ion
exchange. Glycolic acid (GLA) and polyglycolic acid (PGA) are used in various industrial and
medical products.
Fig. 12: Chemoenzymatic synthesis of glycolic acid from formic acid and HCN.
4.2.1.4 p-Hydroxybenzoic acid: p-Hydroxybenzoic acid has been synthesized from
p-hydroxybenzonitrile using nitrilase activity of resting cells of Gordonia terrae (Kumar &
Bhalla, 2013). This reaction is advantageous over its chemical counterpart as it occurs at 35oC
and pH 8 and product obtained is free from its isomeric form i.e. ortho or meta-hydroxybenzoic
acid (Fig. 13).
Fig. 10: Mechanism of nitrilase catalyzed hydrolysis of mandelonitrile.
Hydroxy Acids: Production and Applications 67
5. APPLICATIONS OF HYDROXY ACIDS
Hydroxy acids have wide application in cosmetic and pharmaceutical industry (Green et al.,
2009; Kornhouser et al., 2010). Beside these, they have application in many other areas such
as chemical, textile, transportation, petroleum, food processing and water treatment. Some
important applications of hydroxy acids in chemical, polymer, food, cosmetics and medical
industries are summarized below:
5.1 Chemical Industries
Mandelic acid is used as a precursor for the synthesis of various important pharmaceutical
compounds e.g. β-lactam antibiotics, anti-infl ammatory drug i.e., 2-(1, 3-dioxo-2,3-dihydro-1H-2-
isoindolyl) ethyl 2-hydroxy-2-(substituted phenyl) acetates) synthesized from the combination of
N-(2-hydroxy ethyl) phthalimide and substituted mandelic acids (Varala et al., 2008), antiobesity
drug i.e., β-phenethanolamines derived from mandelic acid is used as antiobesity agent, antitumor
agent i.e., cyanogenic glycosides of mandelic acid (amygdalin and prunasin) are used as antitumor
agent (Fukuda et al., 2003). The derivatives of α-HIB are used as pharmaceutical intermediate
and complex forming agent for lanthanide and actinide heavy metals. Dehydration of α -HIB
leads to the formation of methacrylic acid. Esters of methacrylic acid can be polymerized to
polymethyl methacrylate (PMMA) which is used for the production of acrylic glass (Chisholm,
2000; Bauer, 2002). Other branched C4 carboxylic acids i.e., chloro and amino derivatives of
α-HIB as well as isobutylene glycol and its oxide are also used in polymer synthesis (Fig. 14).
5.2 Medicine
Hydroxy acids have numerous applications in medical fi eld. α-Hydroxy acid can be used to
improve the anti-infl ammatory effi cacy of cortisteroids. Polymer of α-hydroxyisobutyric acid
i.e., polymethyl methacrylate has a good degree of compatibility with human tissue, and can be
Fig. 11: Production of -hydroxyisobutyric acid from acetoncyanohydrin via nitrilase or nitrile
hydratase/amidase mediated hydrolysis.
68 Advances in Industrial Biotechnology
used for replacement intraocular lenses in the eye when the original lens has been removed in
the treatment of cataracts (Atchison, 1990; Khanna & Cernovsky, 2012). In orthopedic surgery,
PMMA bone cement is used to affi x implants and to remodel lost bone (Khanna & Cernovsky
2012). α-Hydroxybutyrate is an early biomarker of insulin resistance and glucose intolerance
in a non-diabetic population (Gall et al., 2010).
(R)-(-)-Mandelic acid (R-MA) is an important chiral intermediate for the production
of pharmaceuticals such as semisynthetic penicillins, cephalosporins, antitumor agents, and
antiobesity agents (Tang et al., 2009). Mandelic acid condensation polymer act as a microbicide
against human immunodefi ciency virus (HIV) and herps simplex virus (HSV) (Lourens et al.,
Fig. 13: Bioconversion of p-hydroxybenzonitrile to p-hydroxybenzoic acid by nitrilase.
Fig. 14: -HIB a precursor for the synthesis of various important compounds.
Hydroxy Acids: Production and Applications 69
2002). This polymer blocks the binding of HIV and HSV to cells by targeting the envelope
glycoprotein gp120 and gb2, respectively (Herold et al., 2002). Mandelic acid is also used as
urinary antiseptics. Mandelic acid with acid sodium phosphate as the acidifying agent, with
sodium bicarbonate is used for the treatment of urinary tract infection (Van Putten, 1979).
Benzilic acid derivatives have antihistamine and anesthetic properties (Forbes and Marshall,
1951). Phenyllactic acid (PLA) has been found in cultures of Lactobacillus plantarum that show
antifungal activity (Lavermicocca, 2003).
β-Hydroxypropanoic acid and their derivative hydrazides and thiosemicarbozides show anti-
infl ammatory, analgesic and antimicrobial activity. β-Hydroxybutyric acid is used as an energy
source by the brain when blood glucose level is low.
γ-Hydroxybutyric acid (GBA) used as a neuroprotective therapeutic nutrient to treat cataplexy
and narcolepsy. Poly (hydroxyl) polymer can be used as scaffolds for tissue engineering or a
drug carrier for controlled drug delivery. Mechanism is based on the cleavage of the polymer
chain into smaller fragments by biodegradation and subsequent release of drug into the medium
(Wee et al., 2004). Biodegradable polymers from glycolide and lactide fi nds applications in
the controlled delivery of pharmaceuticals, such as hormones LHRH (Luteinizing hormone
releasing hormone), TRH (thyroid releasing hormone), calcitonin and various steroids (Asano
et al., 1989; Heya et al., 1991), antibiotics, including ampicillin, gentamicin, polymyxin B and
chloramphenicol (Benoit et al., 1997; Calhoun and Mader, 1997).
Salicylic acid is a well-known aromatic carboxylic acid used as a precursor for the production
of acetylsalicylic acid and the later is widely used as a non-steroidal anti-infl ammatory drug i.e.
aspirin (Jack et al., 1997; Jeffreys, 2004). Salicylic acid itself is an antipyretic, anti-infammatory
and analgesic (Levy, 1981). Methyl salicylate is used as a liniment to soothe joint and muscle
pain, and choline salicylate is used topically to relieve the pain of aphthous ulcers, a type of
mouth ulcer.
5.3 Food Industries
Lactic acid is also known as milk acid and play a very important role in many biochemical
processes. Lactic acid may also be found in various processed foods, usually either as a pH
adjusting ingredient, or as a preservative (either as antioxidant or for control of pathogenic
micro-organisms) (Narayanan et al., 2004). Potassium lactate, sodium lactate and calcium lactate
are the neutralized salts of lactic acid. Potassium lactate is used in many fresh and cooked meat
products for shelf-life control, color preservation and reduction of sodium content. Sodium lactate
has a mild saline taste and is therefore suitable for fl avor enhancement in meat products as well.
Esters of hydroxybenzoic acids known as parabens, fi nds important applications as dietary
antioxidant (Tomas-Barberan & Clifford, 2000), natural fl avour (Lemini et al., 2002) and
preservatives. Ferulic acid and vanillic acid are useful as a precursor in the manufacturing of
vanillin, a synthetic fl avoring agent often used in place of natural vanilla extract (Walter et al.,
1997).
5.4 Cosmetics
α-Hydroxy acids are one of the most powerful and most popular tools available in the skin
care industry. Mandelic acid has also long been known to have antibacterial properties, which
70 Advances in Industrial Biotechnology
are especially benefi cial in the treatment of acne and oily skin. The side effects of mandelic
acid are minimal, especially when compared to similar skin care ingredients such as glycolic
acid (an alpha hydroxy acid) or hydroquinone (a skin lightening ingredient). Mandelic acid is
recommended in various skin diseases condition i.e., acne, melasma, wrinkles (Taylor, 1999).
The moisturizing property of lactic acid makes it ideal for treating dry skin (Narayanan et
al., 2004). Polymers of α-hydroxyisobutyric acid i.e. methacrylic acid has various application in
cosmetics. It is used to permanently raise indented scars and fi ll in folds like the nasolabial folds
(Clark, 1996).
Salicylic acid is known for its ability to ease aches and pains and reduce fevers. As with other
β-hydroxy acids, salicylic acid is a key ingredient in many skin-care products for the treatment
of Seborrhoeic dermatitis, acne, psoriasis, calluses, corns, keratosis pilaris and warts (Steele et
al., 1988). Because of its effect on skin cells, salicylic acid is used in several shampoos used
to treat dandruff. Parabens are a group homologous series of hydroxybenzoic acid, esterifi ed at
the C-4 position (including methyl-, ethyl-, propyl-, butyl-, heptyl- and benzyl-paraben), used
singly or in combination to exert an antimicrobial effect and are present in approximately 80%
of cosmetics (Soni, 2005).
5.5 Polymer Synthesis
Mandelic acid can be used for the synthesis of polymer and copolymer with other hydroxy acids
(Moon et al., 2003). Lactic acid is used as a monomer for producing polylactic acid (PLA)
which has application as biodegradable plastic (Narayanan et al., 2004). This kind of plastic
is a good option for substituting conventional plastic produced from petroleum oil because of
low emission of carbon dioxide that contribute to global warming. Glycolic acid is used as a
monomer in the preparation of polyglycolic acid and other biocompatible copolymer (PLGA).
With increasing environmental concerns and advances in medical technologies, biodegradable
polymers have been heavily researched. These polymers have already seen extensive use in the
medical sector as surgical sutures, drug delivery systems, internal fi xation devices and tissue
engineering scaffolds (Dunn and Vert, 1999; Mayer et al., 1994). Polymer of glycolic acid can
be synthesized by different methods: (a) polycondensation of glycolic acid, and (b) ring-opening
polymerization of glycolide. Ring-opening polymerization of “glycolide” (cyclic diester of
glycolic acid) is the most common process used for the synthesis of polyglycolic acid (Fig. 15).
Various copolymers can also be synthesized by using different monomer units of hydroxy
acid (Fig. 16). PLGA or poly (lactic-co-glycolic acid) is a copolymer which is used in
therapeutic devices, owing to its biodegradability and biocompatibility. PLGA is synthesized by
means of random ring-opening co-polymerization of two different monomers, the cyclic dimers
(1, 4-dioxane-2,5-diones) of glycolic acid and lactic acid (Astete & Sabliov, 2006). The esters
of p-hydroxybenzoic acid known as parabens are used as preservatives in cosmetics, food and
pharmaceutical products (Rastogi et al., 1995; Soni et al., 2001, 2002; Soni, 2005). Salicylic
acid, another aromatic hydroxy acid, is widely used in organic and polymer synthesis. Poly
(anhydride-esters) was successfully synthesized by incorporating salicylic acid into the polymer
backbone (Erdmann & Uhrich, 2000).
Hydroxy Acids: Production and Applications 71
Fig. 15: Glycolic acid polymer.
Fig. 16: Copolymer of glycolic acid and lactic acid.
5.6 Other Applications
Polymer of hydroxyisobutyric acid i.e. Polymethyl methacrylate (PMMA) is a transparent
thermoplastic, often used as a light or shatter-resistant alternative to glass. It is sometimes
called acrylic glass. Chemically, it is the synthetic polymer of methyl methacrylate (Khanna
& Cernovsky 2012). PMMA acrylic glass is commonly used for constructing residential and
commercial aquarium. PMMA was used in laserdisc optical media. (CDs and DVDs use both
acrylic and polycarbonate for higher impact resistance.) It is used as a light guide for the
backlights in thin fi lm transistor liquid crystal display (TFT-LCDs). p-Hydroxybenzoic acid serve
as monomers for synthesis of liquid crystal polymers (LCP) (McQualter et al., 2005). LCPs are
thermo-tropic polyesters and the major monomer used in the manufacture of these copolymers is
the aromatic hydroxyacid, p-hydroxybenzoic acid (McQualter et al., 2005) and fi nds applications
in electrical and optical connectors, integrated circuit boards, vehicle ignition components, and
mobile phone components. Salicylic acid (SA) is an important signalling molecule in plant
defense against biotrophic pathogens. It is also involved in several other processes such as heat
production, fl owering, and germination (Raskin et al., 1992; DeFraia et al., 2008).
6. CONCLUSIONS
Hydroxy acids are important organic acids having one or more hydroxyl group. They are available
naturally as well as synthesized by different chemical and enzymatic reactions. These acids have
immense market potential and are used as cosmetics, drug or drug precursors, building blocks
in chemical synthesis, monomer of various biopolymers and bioplastics, drug delivery, tissue
engineering and production of various commodity chemicals. There are number of chemical
processes available for the synthesis of hydroxy acids, but have some serious disadvantages in
term of requirement of energy, harsh conditions, costly catalyst and formation of by-products,
often results in a threat to environment. Some of the hydroxy acids e.g., glycolic acid, mandelic
acid, p-hydroxybenzoic acid, α-hydroxyisobutyric acid, have been produced through enzymatic
72 Advances in Industrial Biotechnology
process using microbial nitrilases or nitrile hydratase-amidase bi-enzymatic system. In order
to produce large number of industrially important hydroxy acid through biological/enzymatic
routes, exploration of microbial diversity for enzymes having wider substrate specifi cities with
little substrate/product inhibition is needed in future.
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