Molecules 2010, 15, 442-459; doi:10.3390/molecules15010442
Vitamin B6: A Molecule for Human Health?
Hanjo Hellmann * and Sutton Mooney
Washington State University, Abelson 435, P.O. Box 66224, Pullman, WA, USA
* Author to whom correspondence should be addressed; E-Mail: firstname.lastname@example.org.
Received: 5 November 2009; in revised form: 16 January 2010 / Accepted: 20 January 2010 /
Published: 20 January 2010
Abstract: Vitamin B6 is an intriguing molecule that is involved in a wide range of
metabolic, physiological and developmental processes. Based on its water solubility and
high reactivity when phosphorylated, it is a suitable co-factor for many biochemical
processes. Furthermore the vitamin is a potent antioxidant, rivaling carotenoids or
tocopherols in its ability to quench reactive oxygen species. It is therefore not surprising
that the vitamin is essential and unquestionably important for the cellular metabolism and
well-being of all living organisms. The review briefly summarizes the biosynthetic
pathways of vitamin B6 in pro- and eukaryotes and its diverse roles in enzymatic reactions.
Finally, because in recent years the vitamin has often been considered beneficial for human
health, the review will also sum up and critically reflect on current knowledge how human
health can profit from vitamin B6.
Keywords: vitamin B6; PDX; de novo; salvage; health
The B vitamins are a group of water soluble, chemically quite distinct compounds to which other
than vitamin B6, vitamin B1 (thiamine), B2 (riboflavin), B3 (niacin or niacin amide), B5 (pantothenic
acid), B7 (biotin), B9 (folic acid), and B12 (various cobalamins) also belong . Historically, it was
believed that only one vitamin B existed with a critical function for maintenance of growth and health
and prevention of characteristic skin lesions in animals and human . However, with ongoing
research it became obvious that vitamin B actually comprised a group of compounds that was
collectively called the ‘vitamin B complex’.
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Vitamin B6 (vitB6 from here on) itself is an enzymatic co-factor required for more than 140
biochemical reactions including transaminations, aldol cleavages, α-decarboxylations, racemizations,
β- and γ- eliminations, and replacement reactions. Most of these reactions are related to amino acid
biosynthesis and degradation, but vitB6 is also involved in other processes including sugar and fatty
acid metabolism . It comprises a set of three different pyridine derivatives called pyridoxine (PN;
1), pyridoxal (PL; 2), and pyridoxamine (PM; 3). They differ in a variable group present at their 4-
position with PN carrying a hydroxymethyl group, and PL (2) and PM (3) having an aldehyde and an
aminomethyl group, respectively. Furthermore, all three B6 vitamers are phosphorylated by a kinase,
which is a requirement for their role as cofactors in enzymatic reactions (Scheme 1). While
pyridoxamine-5’-phosphate (PMP; 4) has been reported to function as a co-factor, it is pyridoxal 5’-
phosphate (PLP; 5) that is the biologically most active form [4,5].
A growing number of interesting and helpful new resources have been established in the last years
that focus primarily on vitB6 related issues. For example, an online database has been launched that
allows searching whole genomes for PLP-dependent enzymes, and which also provides information on
critical aspects such as the biochemical pathways requiring PLP (5) and the classification of PLP-
dependent enzymes (http://bioinformatics.unipr.it/cgi-bin/bioinformatics/B6db/home.pl) . In
addition, a database has been established that allows searching for mutated PLP-dependent enzymes in
various organisms (http://www.studiofmp.com/plpmdb/home.htm) .
2. Suggested Reaction Mechanisms of VitB6 for Amino Acid Metabolism
In most cases PLP (5) is covalently bound to the ε-amino group of a conserved lysine residue in the
active center of a PLP-dependent enzyme, with its 5’-phosphate group being buried in a conserved
phosphate-binding cup . It is suggested that reactions are initiated by the formation of a geminal
diamine intermediate between the aldehydic carbon atom of PLP (5) and an amino group of the
substrate. This is followed by its rapid breakdown and the formation of an external aldimine (Schiff
base) between PLP (5) and the substrate causing the release of the lysine residue of the enzyme from
PLP (5). From this point on subsequent reactions mainly depend on the specific, participating enzymes
that guide and modulate the next steps leading to e.g. racemisations, β- and γ- eliminations.
3. Three Different Biosynthetic Pathways for VitB6 Are Known
Three different pathways for vitB6 biosynthesis have been described which will be just briefly
summarized, as they were topics of other recent reviews [8,9]. In eubacteria like Escherichia coli, the
vitamin can be de novo synthesized by the concerted activities of the pyridoxine biosynthesis proteins
A and J (PdxA (EC 126.96.36.1992) and PdxJ (EC 188.8.131.52), respectively) which use 4-phospohydroxy-L-
threonine (4HPT; 6) and deoxyxylose 5’-phosphate (DXP; 7) to synthesize pyridoxine 5’-phosphate
(PNP; 8) (Scheme 1) [10–12]. In bacteria, archaea, and eukarya a second de novo pathway is known
that synthesizes PLP (5) from ribose 5’-phosphate (9) or ribulose 5’-phosphate (10), in combination
with either glyceraldehyde 3’-phosphate (11) or dihydroxyacetone phosphate (12) and glutamine (13)
(Scheme 1) [13–17].
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Scheme 1. The three known pathways for PLP biosynthesis: one salvage pathway, and
two de novo pathways, a DXP-dependent one and a DXP-independent one. Chemical
structures: (5) PLP; (7) deoxyxylulose 5’-phosphate, (6) 4-(phosphohydroxy)-L-
threonine; (11) glyceraldehyde 3’-phosphate; (12) dihydroxyacetone phosphate; (9)
ribose 5’-phosphate; (10) ribulose 5’-phosphate, (13) glutamine, (3) PM, (4) PMP, (1)
PN, (8) PNP, (2) PL.
Here two pyridoxine biosynthetic enzymes (PDX) are active: while PDX2 functions as a
glutaminase that deaminates glutamine to glutamate in order to supply nitrogen for the PLP
heterocycle, PDX1 arranges the final ring closure [18–24]. Because of a different sugar precursor used
for the biosynthesis of the vitamin, the de novo pathway from eubacteria is known as the DXP-
dependent pathway, while the other is the DXP-independent pathway . In addition to the two de
novo pathways, most organisms also have a salvage pathway that converts the different B6 vitamers to
PLP (5). This is achieved by the concerted activities of an oxidase, PDXH (EC 184.108.40.206), and a kinase,
PDXK (EC 220.127.116.11) (Scheme 1) [8,25]. Most animal organisms, including humans, have a salvage
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pathway, however, they lack the enzymatic machinery for de novo synthesis and rely on external
uptake of the vitamin from food .
4. VitB6 and Its Healthy Face
Since its discovery in 1932 by the Japanese scientist S. Ohdake, vitB6 has been discussed in
relationship to health issues . In these early works from, for example, Ohdake or the Hungarian
scientist P. Györgi, vitB6 was associated with pellagra, a skin disease that is based on multi-vitamin
deficiencies that mostly occurs in context with niacin undersupply [27–29]. A search through the
public literature data basis (http://www.ncbi.nlm.nih.gov/) for health aspects associated with vitB6
yields a surprisingly high number of articles (>900). Furthermore, the current Recommended Dietary
Allowance per day by the National Institute of Health (NIH) of the USA is around 2 mg with an
upward tolerance of 100 mg per day for adults. A recent U.S. study, which tested the blood PLP levels
in around 8,000 patients, demonstrated a widespread deficiency of the vitamin among all tested
subgroups, and the authors suggested an increase of the daily allowance from around 2 mg to 3 to
4.9 mg per day . It has been reported for animal models, that continuous uptake of very high doses
(e.g. 400 mg/kg) can lead to peripheral sensory neuropathy and nerve degeneration [31,32]. These
problems are generally reversible when supplementation is stopped. Additionally some studies have
suggested that increased levels of the B6 vitamers and some derivatives can generate toxic
photoproducts as a result of UV irradiation [33–35]. However, the applied daily dosages were far
beyond any physiological concentrations an organism is normally exposed to, making it unlikely that
such vitB6 induced impacts will be observed. Because of the great interest in vitB6 as a therapeutic and
pharmaceutical compound, its reactive capability, and its potent antioxidative characteristics, we
summarize in the following paragraphs some of the relevant topics related to these issues.
4.1. Therapeutic applications by using drugs against PLP-dependent enzymes
PLP-dependent enzymes are highly diverse and the reactions they facilitate are estimated to
represent 4% of all known catalytic activities; hence, many of them are being explored as targets for
therapeutic agents (for an excellent overview see ). We chose three major examples for this review
to illustrate the potentials of this approach in disease control: malaria, sleeping sickness, and cancer
One of the most threatening human diseases is malaria, with more than 300-500 million infected
people worldwide and an annual death toll of up to one million people (http://www.unicef.org/health/
index_malaria.html). Several approaches are currently underway in an effort to affect the life cycle or
metabolism of the pathogen Plasmodium falciparum, the cause of malaria. One such approach is to
impair biosynthesis of xanthurenic acid (14), which is essential for gametogenesis and fertility of the
pathogen [36–38]. The acid is synthesized as part of the L-tryptophan (15) degradation pathway from
L-kynurenine (16) via 3-hydroxykynurenine (17) by the activity of the PLP-dependent kynurenine
aminotransferase (EC 18.104.22.168)  (Scheme 2A).
A possible strategy involves developing specific drugs that reduce activity of the aminotransferase.
This might lower the levels of 3-hydroxykynurenine (17) in P. falciparum infected mosquitoes
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potentially reducing or even preventing malaria transmission to humans. A similar direction was
recently proposed by channeling synthetic pyridoxyl-amino acid adducts into the pathogen, which can
phosphorylate these compounds mediated by PDXK kinase . After binding by a PLP-dependent
enzyme, such phosphorylated compounds should inhibit these enzymes and affect further metabolism.
Müller and co-workers successfully tried pyridoxyl-tryptophan methyl ester to inhibit proliferation of
P. falciparum opening up the possibility for a novel malaria treatment in the future . Because P.
falciparum expresses PDX1/PDX2 proteins, which humans lack, a potential approach can also be to
target these de novo pathway proteins by specific drugs . However, no specific approach has been
reported so far.
African sleeping sickness is another severe epidemic disease with an estimated 300–500 thousand
people affected in various African countries (http://www.sbri.org/diseases/african.asp). It is caused by
the protist Trypanosoma brucei and transmitted by flies of the Genus Glossina. A target to treat
sleeping sickness in affected patients is the PLP-dependent enzyme ornithine decarboxylase (ODC;
E.C. 22.214.171.124). It catalyzes the step from L-ornithine (18) to the diamine putrescine (19), an initial step
in the production of polyamines (Scheme 2B). α-Difluoromethylornithine (DFMO) is a proven
irreversible inhibitor of ODC activity and works by forming a covalent bond with a cysteine residue of
ODC after decarboxylation [42–44]. Although DFMO is an approved drug in treating sleeping
sickness caused by T. brucei, the precise reason for its effectiveness is not fully resolved because
human and T. brucei ODCs are comparably affected by the agent . It is suggested that this effect is
based on metabolic differences: a more rapid turnover of the host’s ODC on the one side, and on the
other side T. brucei’s high demand for the synthesis of the polyamine trypanothione, a specific dithiol
essential for the detoxification system of Trypanosomes and Leishmania parasites [46,47].
Targeting PLP-dependent enzymes is also discussed in context with cancer. Here an interesting
candidate is, for example, serine hydroxymethyltransferase (SHMT; EC 126.96.36.199), which catalyzes the
reversible transfer of the Cβ of serine (21) to tetrahydrofolate (22) to form glycine (23) and 5,10-
methylenetetrahydrofolate (24) (Scheme 2C). Because of 5,10-methylenetetrahydrofolate (24), which
serves as a methyl donor in many reactions, SHMT activity is critical for one-carbon metabolism, the
biosynthesis of methionine, lipids, formyl-tRNA and pyrimidine. The latter is of special interest as
apparently SHMT activity is coupled to some extent with increased demand for DNA biosynthesis. For
example there is evidence in tumors with highly proliferating, mitotically active cells, that serine is
preferentially channeled for DNA biosynthesis [48,49]. Consequently SHMT is a proposed target in
developing drugs for chemotherapy [50,51].
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Scheme 2. Enzymatic reaction that are targets for pharmaceutical approaches. Shown are
only the PLP-dependent enzymes. (A) Synthesis of xanthurenic acid (14) from L-
tryptophan (15), via the intermediates L-kynurenine (16) and 3-hydroxykynurenine (17).
(B) Synthesis of putrescine (19) from L-ornithine (18), leading subsequently to the
synthesis of spermidine (20) and other polyamines. (C) Synthesis of tetrahydrofolate (22)
and serine (21) to glycine (23) and 5,10-methylenetetrahydrofolate (24). Note: in this and
the next Schemes only the PLP-dependent enzymes are shown.
4.2. VitB6 in context with cardiovascular disease and blood pressure
Other aspects in which vitamin B6 is directly discussed to play an important role are cardiovascular
disease and high blood pressure. Coronary heart disease (CHD) is one the major reasons for death
worldwide. It is caused by atheromata, which are swollen artery walls due to the accumulation of cell
debris containing e.g., fatty acids and cholesterols that negatively affect blood flow. Though the impact
of vitB6 is controversially discussed (compare for example  and ), a variety of works indicate
positive effects of vitB6 on CHD. For instance, a large study in Japan, comprising 40,803 subjects,
recently showed that vitB6 has the potential to reduce the risk of CHD, and especially nonfatal
myocardial infarction (MI), among middle-aged (40–59 years) non-multivitamin supplement users
. Here, an increase of daily supplementary vitB6 intake from 1.3 to 1.6 mg already significantly
reduced the number of affected patients with reported CHDs and MIs . Similarly, the Coronary
Health Project and other studies indicate a correlation between increased vitB6 intake and reduced risk
of CHD [25,54–56]. It is noteworthy that often other vitamins like folates or cobalamins are tested in
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these studies as well with similar positive effects in reducing the risk of CHD. The precise reason(s)
for the beneficial impact of vitB6 are unclear. One suggested reason is that vitB6, like folates and
cobalamins, can lower homocysteine (26) levels in the blood by converting the amino acid to cysteine
(25) or methionine, respectively. VitB6 is required as a cofactor for cystathionine-β-synthase (EC
188.8.131.52), a PLP-dependent enzyme that converts homocysteine (26) to cysteine (25) via a
cystathionine (27) intermediate (Scheme 3) . Because high levels of homocysteine are often
associated with an increased chance for atherosclerotic diseases, it is considered a risk factor like e.g.
high blood pressure, active smoking, or adverse blood lipid profiles . But, as stressed above, it is
not generally accepted whether vitB6, folates, or cobalamins do indeed reduce the blood homocysteine
levels, as a recent review indicates, thus still awaiting additional proof .
Scheme 3. Synthesis of L-cysteine (25) from L-homo-cysteine (26) via the intermediate L-
VitB6 appears also to have a beneficial role in reducing hypertension or high blood pressure.
Several articles showed that supplementary treatments with the vitamin could lower blood pressure
[59–64]. Like for CHD the biochemical or physiological reasons are unresolved. However, it is
suggested that the blood pressure lowering role of vitB6 might be connected with the level of blood
aldehydes. These are highly reactive compounds that, potentially by binding to sulfhydryl groups of
membrane proteins, activate Ca2+-channels and increase cytosolic free calcium in the blood, ultimately
leading to an increase in peripheral vascular resistance and blood pressure . Consequently, it is not
uncommon for people with excessive alcohol consumption to have increased levels of acetaldehyde,
which is often accompanied by high blood pressure . Treatment with N-acetylcysteine can
normalize blood pressure in spontaneous hypertensive rats, most likely because the amino acid
competes with membrane proteins for the reaction with the aldehydes, causing a reduced Ca2+ flux
. In addition, it is well established that acetaldehyde is detrimental to PLP (5) stability .
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Because PLP (5) is needed for the biosynthesis of cysteine (25), it is suggested that the mechanism of
PLP (5) on the blood pressure is either a direct one, by buffering the detrimental activity of aldehydes,
or occurs indirectly, by influencing the rate of cysteine (25) biosynthesis [61,63]. Still, it is worth
mentioning that the connection of PLP (5) with lowered blood pressure is curious as it is required for
dopamine biosynthesis (see Section 4.3 below), a known vasopressor that actually stimulates
contraction of the muscular tissue of the capillaries and arteries . A correlation between high blood
pressure and PDXH/Pnpo gene expression was found in hypertensive Dahl-S rats . These rats are
sensitive to a high salt diet and develop high blood pressure in response to such foods . Okuda and
co-workers could show for Dahl-S rats supplied with a high salt diet that, in comparison to a control
group, gene expression of the oxidase is down-regulated. These findings indicate that high blood PLP
(5) levels are needed for coping with salt uptake and argue that the vitamin is beneficial for preventing
high blood pressure . However, it would be interesting to know to what extent neurotransmitter
production in these rats is affected as well and whether this correlates with the hypertensive status.
Scheme 4. Example for Advanced Glycation Endproduct Reaction of glucose with
proteins or PM. On the right half: glucose (28) can react with lysine residues of proteins
29 to form an imine (Schiff base) 30 and an Amadori product 31. This latter can be
further oxidized by metals to form the final AGE. pb, peptide bond. On the left half: PM
(3) has been proven to be the most potent B6 vitamer to compete for the formation of
AGEs . It is suggested to form first an imine 32 with the glucose, followed by a
double cyclization to afford 33 rather than formation of an Amadori product .
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4.3. VitB6 in context with diabetes, AGE and ALE
A variety of articles about diabetes mellitus focus on the impact of vitB6 on blood sugar levels and
arteriosclerosis [60,71,72]. For example, a recent study could show that endothelial dysfunction is
normalized by treatment with folates and vitB6 in children with type 1 diabetes . Endothelial
dysfunction is an indicator for the progression of arteriosclerosis that is often developed early in
diabetes mellitus patients. Endothelial function can be assessed as flow-mediated dilation of the
brachial artery with high-resolution ultrasound. MacKenzie and co-workers treated patients over eight
weeks with vitB6 or folate, which resulted in improved flow-mediated dilation from an average 3.5%
to 8.3% and 2.6% to 9.7%, respectively, and with a combination of both to over 10% .
Other work also supports the notion of a positive impact of vitB6 on endothelial cells, indicating
that the vitamin is indeed affecting the status of this tissue [74–76]. Additionally vitB6 seems to have a
positive role against progressive kidney disease, which is frequently associated with diabetic
nephropathy [72,77,78]. A possible reason for the advantageous results of vitB6 on mammalian tissues
is discussed to be the vitamin’s ability to react with reducing sugar and lipids in the blood to prevent
formation of advanced glycation or lipoxygenation endproducts (AGE and ALE, respectively)
(Scheme 4) [39,72,73,79].
Such products can accumulate when reducing sugars like glucose (28) or fructose or
polyunsaturated fatty acids are highly abundant in the blood or in cells. This can be the case under
stress conditions (e.g. oxidative stress) or for patients that suffer from diabetes or arteriosclerosis,
respectively. The accumulation of AGEs and ALEs are on the long run detrimental and can lead to
severe tissue damage in the body. Here, vitB6 might effectively prevent AGE and ALE formation
making it a good candidate as a therapeutic agent in treating side effects in diabetes and
arteriosclerosis patients [80,81].
4.4. VitB6 in context with neurological activity
VitB6 is required for the biosynthesis of several neurotransmitters like serotonin (34), dopamine
(35), and γ-aminobutyric acid (GABA) (36). Serotonin (34), or 5-hydroxytryptamine, is synthesized
from L-tryptophan (15) and requires the activities of tryptophan hydroxylase (EC 184.108.40.206) and the
PLP-dependent enzyme DOPA (L-dihydroxyphenylalanine) decarboxylase [(common synonyms are:
L-aromatic amino acid decarboxylase, tryptophan decarboxylase, 5-hydroxytryptophan decarboxylase;
(EC 220.127.116.11)], which catalyzes the step from 5-hydroxy-L-tryptophan (37) to serotonin (34). The
enzyme also catalyzes the biosynthesis of dopamine (35) from L-DOPA (38). Here the initial precursor
is L-tyrosine (37), which is converted to L-DOPA (38) by the activity of L-tyrosine hydroxylase (EC
18.104.22.168) (Scheme 5A, B). GABA (36) in turn is synthesized by a decarboxylation reaction from
L-glutamate (40) based on the activity of L-glutamate decarboxylase (Scheme 5C) (EC 22.214.171.124).
Serotonin (34) acts on the central nervous system where it affects a diverse range of conditions
including appetite, sleep, or cognitive functions, and it is also well known for its ability to improve the
overall mood . In comparison, dopamine (35) affects the sympathetic nervous system where it is
involved in the regulation of blood pressure and heart rate, while GABA (36) is a major inhibitory
neurotransmitter in mammals that widely controls the excitability of neurons [83,84]. Consequently,
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low levels of vitB6 have been associated with depression and also dysfunction of the brain (e.g.,
epilepsy), and it is even considered by some authors as an ‘anti-stress’ agent [85–88]. In this context it
is interesting to note that some plants, like Ginkgo biloba, synthesize derivatives of vitB6 that are
suggested to inhibit the salvage pathway enzyme PDXK and thereby to impair neurotransmitter
biosynthesis in the brain [8,10,89,90].
Scheme 5. Synthesis of neurotransmitters in the brain. (A) Serotonin (34) is synthesized
from L-tryptophan (15) via the intermediate 5-hydroxytryptophan (37). (B) Dopamine
(35) is formed from L-tyrosine (39) via DOPA (38), and (C) GABA (36) is formed in a
decarboxylation reaction from L-glutamate (40).
4.5. The antioxidative capacity of vitB6
Only recently was the potent antioxidant ability of vitB6 recognized. Here groundbreaking work
from the group of Margaret Daub showed that the vitamin is highly efficient in quenching reactive
oxygen species with a similar potential like described for carotenes and tocopherols [91–93].
Consequently, results from different organisms showed that reduced levels of the vitamin are
connected with severe susceptibility to abiotic stress (oxidative, salt, drought, UV-B) [25,93–95].
Given the great consideration for other antioxidants like vitamins C and E or phenolics as ‘anti-aging’
compounds by the food industry and consumers, it will be interesting to see whether this relatively
novel antioxidant will be embraced in similar ways in the future.
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4.6. Other aspects related to vitB6 and health
VitB6 has also been brought into context with various other health aspects. Because these
connections between dietary levels of vitB6 with disease control are not well established, and could be
related to the central or pleiotropic role of vitB6 as a cofactor, we will only briefly list some examples
that might be of broader interest. For instance, several groups have made connections between high
doses of vitB6 with reduction of tumor growth potentially by suppressing cell proliferation and
While much of this work has been done in cell cultures, experiments in mice demonstrated
significant tumor reduction at the minimum recommended levels of vitB6 with optimum reduction at
two to five-fold higher levels (up to 35 mg/kg) with no major side effects reported . The immune
system depends on vitB6 as deficiencies cause ‘atrophy of lymphoid organs, marked reduction in
lymphocyte numbers, impaired antibody responses, and decreased IL-2 production’ . Likewise,
normal vitB6 levels appear to be critical for patients with asthma or carpal tunnel syndrome [101,102];
and finally it appears to be critical for women suffering from premenstrual syndrome (PMS; fatigue,
depression, fluid retention etc.) with an apparent correlation between these symptoms and low vitB6
The biosynthesis of vitB6 and its function as a co-factor have been well resolved in the last years,
leaving currently open what drives the activities of the different participating enzymes in the cell.
Overall the latest studies indicate that vitB6 can be beneficial as a nutritional supplement, but can also
be used as a pharmacological agent for disease treatment. Similarly, the diversity of PLP-dependent
enzymes and the reactions they catalyze yield a wide range of targets for therapeutic approaches.
However, the precise mechanisms of how vitB6 is beneficial are often still elusive, and to solidly
define them is probably one of the most challenging tasks in the near future.
We would like to thank our Russian collaborator Cleatus for critical reading. We also would like to
thank WSU for supporting this work.
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