STUDY ON INTERACTIONS OF DIVALENT METAL IONS WITH ASPARTIC ACID IN BINARY COMPLEXES

Article (PDF Available)with 52 Reads
Cite this publication
Abstract
The acidity constants of aspartic acid (ASP) were determined by potentiometric pH titration. The stability constants of the 1 : 1 complexes formed between M 2+ : Mn 2+ , Co 2+ , Cu 2+ or Zn 2+ and Asp 2-, were determined by potentiometric pH titration in aqueous solution (I = 0.1 M, NaNO 3 , 25°C). The order of the stability constants was reported. It is shown that the stability of the binary M (Asp) complexes is determined solely by the basicity of the carboxylate or amin groups. The observed stability order for aspartate follows the Irving-Williams sequence. It is shown that Asp can exert a direct influence on reaction rate through both the kind of metal ions and the kind of donor groups of Asp.
________________________________________
Available online at www.sadgurupublications.com
*Author for correspondence; E-mail: sajadi@sharif.ac.ir
J. Curr. Chem. Pharm. Sc.: 2(1), 2012, 32-36
ISSN 2277-2871
STUDY ON INTERACTIONS OF DIVALENT METAL IONS WITH
ASPARTIC ACID IN BINARY COMPLEXES
S. A. A. SAJADI*
Sharif University of Technology, Institute of Water & Energy, P.O. Box 11155-8639 TEHRAN, IRAN
(Received : 27.11.2011; Revised : 09.12.2011; Accepted : 11.12.2011)
ABSTRACT
The acidity constants of aspartic acid (ASP) were determined by potentiometric pH titration. The stability constants
of the 1 : 1 complexes formed between M2+: Mn2+, Co2+, Cu2+ or Zn2+ and Asp2-, were determined by potentiometric pH
titration in aqueous solution (I = 0.1 M, NaNO3, 25°C). The order of the stability constants was reported. It is shown that the
stability of the binary M (Asp) complexes is determined solely by the basicity of the carboxylate or amin groups. The
observed stability order for aspartate follows the Irving-Williams sequence. It is shown that Asp can exert a direct influence
on reaction rate through both the kind of metal ions and the kind of donor groups of Asp.
Key words: Aspartic acid, Divalent metal ions, Potentiometric titration, Acidity, Stability constants.
INTRODUCTION
Aspartic acid (L-Aspartic acid) (Fig. 1) and glutamic acid play important roles as general acids in
enzyme active centers, as well as in maintaining the solubility and ionic character of proteins. Aspartic acid
can help protect the liver from some drug toxicity and the body from radiation. Aspartic acid also can help
form the ribonucleotides that assist production of DNA and RNA and aids energy production from
carbohydrate metabolism. Aspartic acid may also help improve the function of the immune system, and may
play a role in protecting against toxins and neural and brain disorders. Aspartic acid reportedly helps treat
chronic fatigue. Aspartic acid can be easily converted to glucose when demand for glucose exceeds
supply1,2.
Aspartic acid is one of the 20 amino acids commonly found in animal proteins. Aspartic acid is the
carboxylic acid analog of asparagine. Aspartic acid is alanine with one of the β-hydrogens replaced by a
carboxylic acid group. Aspartic acid is a part of organic molecules containing an amino group, which can
combine in linear arrays to form proteins in living organisms. Its acidic side chain adds a negative charge
and hence a greater degree of water-solubility to proteins in neutral solution and has been shown to be near
the active sites of some enzymes. Aspartic acid is a non-essential amino acid having an acidic carboxyl
group on its side chain which can serve as both an acceptor and a donor of ammonia. It is converted to l-
asparagine by binding with ammonia. It forms carbamyl-l-aspartic acid which roles purine as well as
pyrimidine biosynthesis. The peptide RDDANG binds calcium with a significantly greater affinity than does
peptide DRNADG. The stronger bonding of calcium to a peptide with Asp residues next to each other has
J. Curr. Chem. Pharm. Sci.: 2(1), 2012 33
analogy with organic diacids that have nearby carboxylic groups and have lower pKa1’s than those with
remote functional groups3,4. The close proximity of one carboxyl group to another enhances the acidity of
the latter. It is of great interest the knowledge of the affinity of aspartic acid for other metal ions and the
structure which shows the position of functional groups in M-Asp complexes. This would help us to
understand such reation mechanism much better.
Fig. 1: Chemical structure of L-Aspartic acid
EXPERIMENTAL
Materials
The L-aspartic acid (extra pure) was purchased from Merck, Darmstadt. The nitrate salt of Na+, Mn2+,
Co2+, Cu2+ and Zn2+ (all pro analysi) were from Merck. All the starting materials were of reagent grade and
used without further purification. Potassium hydrogen phthalate and standard solutions of sodium hydroxide
(titrasol), nitric acid, EDTA and of the buffer solutions of pH 4.0, 7.0 and 9.0, were from Merck. All
solutions were prepared with deionized water. Water was purified by Milil-Q water purification system,
deionized and distillated.
pH titrations
Reagents
Carbonate-free sodium hydroxide 0.03 M was prepared and standardized against sodium hydrogen
phthalate and a standard solution of nitric acid 0.5 mM. M (II) nitrate solution (0.03 M) was prepared by
dissolving the above substance in water and was standardized with standard solution of EDTA 0.1 M
(triplex).
Apparatus
All pH titrations was performed using a Metrohm 794 basic automatic titrator (Titrino), coupled with
a thermo stating bath Hero at 25°C (± 0.1°C) and a Metrohm combined glass electrode (Ag/AgCl). The pH
meter was calibrated with Merck standard buffer solutions (4.0, 7.0 and 9.0).
Procedure
For the determination of acid dissociation constants of the ligand Asp an aqueous solution (0.03 mM)
of the protonated ligand was titrated with 0.03 M NaOH at 25°C under nitrogen atmosphere and ionic
strength of 0.1 M, NaNO3. For the determination of binary (one ligand and Cu2+) system, the ratios used
were 1 : 1, Cu (II) : Ligand and 1 : 1, Cu (II) : Asp, 0.3 mM. This solution was titrated with 0.03 M NaOH
under the same conditions mentioned above. Each titration was repeated seven times in order to check the
reproducibility of the data.
Calculation
The acid dissociation constants, HAspH
K)(
2and HAspH
K)( for H2(Asp) were calculated by an
algebraicmethod. The equilibrium evolved in the formation of 1 : 1 complex of Asp and a divalent metal ion
may be expressed as equations 7 and 9.
S. A. A. Sajadi: Study on Interaction of Divalent Metal Ions with….
34
RESULTS AND DISCUSSION
The potentiometric pH-titrations (25°C, 0.1 M, NaNO3) were carried out to obtain the acidity and
stability constants which are summarized in Table 1.
Table 1: Logarithm of the stability constants of binary complexes of M2+ at 25°C, 0.1 M, NaNO3*
S. No. Species logK Site
1 H2(Asp) 3.72 ± 0.03 -CO2H
2 H(Asp) 9.90 ± 0.03 -NH3
3 Mn2+ 3.91 ± 0.03 -
4 Co2+ 6.69 ± 0.06 -
5 Cu2+ 8.78 ± 0.02 -
6 Zn2+ 5.35 ± 0.06 -
*The given errors are three times the standard error of the mean value or the sum of the probable
systematic errors
Acidity constants
Aspartate ion (Asp2-), -O2CCH2CH (NH2) CO2-, is a two-basic species and thus it can accept two
protons, given H2 (Asp) for which the following deprotonation equilibriums hold:
H2 (Asp) H+ + H (Asp) …(1)
HAspH
K)(
2 = [H (Asp)-] [H+]/[H2 (Asp)] …(2)
H (Asp) H+ + Asp2 …(3)
HAspH
K)( = [Asp2-] [H+]/[H (Asp)-] …(4)
The two proton in H2 (Asp) are certainly bound at the terminal acetate and amino groups, i.e., it is
released from HO2CCH2CH(NH3+) CO2- according to equilibrium (1) & (2).
It is also closed to the deprotonation of acetate groups which occurs at the terminal acetate groups of
tartaric acid5,6. Asp2- can release one more proton from the neutral (NH2) site of their amin residue, hence,
here in addition equilibrium (Eq. 5) should be considered. This takes place above pH 2.
Asp2 H+ + (Asp-H)3 …(5)
H
Asp
K = [(Asp-H)3-][H+]/[ASP2-] …(6)
This reaction is not here further considered.
Stability of binary and ternary complexes
If we abbreviate for simplicity Mn2+, Co2+, Cu2+ and Zn2+ with M2+, one may write the following two
equilibriums Eq. 7 and 9:
M2+ + H (Asp) M (H; Asp)+ …(7)
J. Curr. Chem. Pharm. Sci.: 2(1), 2012 35
MAspHM
K);( = [M (H; Asp)+]/[M2+] [H(Asp)-] …(8)
M2+ + (Asp)2 M(Asp) …(9)
MAspM
K)( = [M (Asp)]/[M2+] [Asp2-] …(10)
The experimental data of the potentiometric pH– titrations may be completely by considering the
above mentioned equilibriums (Eq. 1-10) if the evaluation is not carried into the pH range where hyrdoxo
complex formation occurs.
Potentiometric analyses
The results of all potentiometric pH-titration i.e. acidity and stability constants are summarized in
Table 1. The deprotonated amino acid Asp2- can accept in total two protons to give the acid H2Asp. First one
of this two protons of carboxylate residue is released; its pKa is low (2). The next proton is the second
proton from the carboxylate group. However, Asp2- can release one more proton from neutral - NH2 site. It is
about pH > 12 (eq. 3). The measured acidity constants in this work show good agreement with the same
value received by other authors5,10. However, the carboxyl group is a far stronger acid than the amino group.
N
O
C
C
C
Cu
C
O
O
O
Fig. 2: Schematic structures of the species with interactions according to equilibrium
(Eq. 9) for Cu (Asp)
0
2
4
6
8
10
Mn
2+
Co
2+
Cu
2+
Zn
2+
log K
Fig. 3: Irving-Williams sequence-type plot for the 1 : 1 complexes of Mn2+ to Zn2+
with aspartate (Table 1)
The stability constants of the binary complexes (Fig. 2) were refined separately using the titration
data of this system in a 1 : 1, ligand : Cu2+ ratio in the same conditions of temperature and ionic strength
(according Eq. 7-10). As they were in good agreement with reported value5,11. All the stability constants of
S. A. A. Sajadi: Study on Interaction of Divalent Metal Ions with….
36
Table 1 show the usual trend. The obtained order is Mn2+ < Co2+ < Cu2+ > Zn2+. The observed stability order
for aspartate follows the Irving-Williams sequence12 (Fig. 3).
REFERENCES
1. IUPAC-IUBMB Joint Commission on Biochemical Nomenclature. Nomenclature and Symbolism for
Amino Acids and Peptides. Recommendations on Organic & Biochemical Nomenclature, Symbols &
Terminology etc. Retrieved on 05-17 (2007).
2. D. L. Nelson, M. M. Cox, Lehninger, Principles of Biochemistry, 3rd Ed., Worth Publishing: New
York (2000).
3. Philip E. Chen, Matthew T. Geballe, Phillip J. Stansfeld and Alexander R. Johnston, Hongjie Yuan,
Amanda L. Jacob and James P. Snyder, Stephen F. Traynelis, and David J. A. Wyllie, Structural
Features of the Glutamate Binding Site in Recombinant NR1/NR2A N-Methyl-D-Aspartate Receptors
Determined by Site-Directed Mutagenesis and Molecular Modeling, Molecular Pharmacology, 67,
(2005) pp. 1470-1484.
4. M. S. Dunn, B. W. Smart, DL-Aspartic Acid, Organic Syntheses, Collected, 4, (1963) p.55.
5. A. E. Martel, Critical Stability Constants of Metal Complexes, 26 (2006).
6. S. A. A. Sajadi, A. A. Alamolhoda and A. Nazari Alavi, Inorg. Chem. (AIJ), 6, 1 (2011).
7. Handbook of Chem. & Physics, Chemical Rubber Publishing Company,55, D-129 (1975).
8. J. L. Miranda, J. Felcman, Polyhedron, 22, 225-233 (2003).
9. J. Felcman, J. L. Miranda, J. Braz. Chem. Soc., 8, 575 (1997).
10. IUPAC Stability Conatants Database, Release 3, Version 3.02, Coplied by L. D. Pettit, H. K. J. Powel,
Academic Software Timble, UK (1998).
11. H. Sigel, A. D. Zuberbuehler and O. Yamauchi, Anal. Chim. Acta., 255, 63 (1991).
12. H. Irving, R. J. P. Williams, J. Chem. Soc., 3192-3210 (1953).
This research hasn't been cited in any other publications.
    • J L Miranda
    • J Felcman
    J. L. Miranda, J. Felcman, Polyhedron, 22, 225-233 (2003).
    • H Irving
    • R J P Williams
    H. Irving, R. J. P. Williams, J. Chem. Soc., 3192-3210 (1953).
    • J Felcman
    • J L Miranda
    J. Felcman, J. L. Miranda, J. Braz. Chem. Soc., 8, 575 (1997).
    • M S Dunn
    • B W Smart
    • Dl-Aspartic Acid
    M. S. Dunn, B. W. Smart, DL-Aspartic Acid, Organic Syntheses, Collected, 4, (1963) p.55.
    • D L Nelson
    • M M Cox
    D. L. Nelson, M. M. Cox, Lehninger, Principles of Biochemistry, 3 rd Ed., Worth Publishing: New York (2000).
    • S A A Sajadi
    • A A Alamolhoda
    • A Nazari Alavi
    S. A. A. Sajadi, A. A. Alamolhoda and A. Nazari Alavi, Inorg. Chem. (AIJ), 6, 1 (2011).
    • H Sigel
    • A D Zuberbuehler
    • O Yamauchi
    H. Sigel, A. D. Zuberbuehler and O. Yamauchi, Anal. Chim. Acta., 255, 63 (1991).
  • Article
    Full-text available
    We have used site-directed mutagenesis of amino acids located within the S1 and S2 ligand binding domains of the NR2A N-methyl-D-aspartate (NMDA) receptor subunit to explore the nature of ligand binding. Wild-type or mutated NR1/NR2A NMDA receptors were expressed in Xenopus laevis oocytes and studied using two electrode voltage clamp. We investigated the effects of mutations in the S1 and S2 regions on the potencies of the agonists L-glutamate, L-aspartate, (R,S)-tetrazol-5yl-glycine, and NMDA. Mutation of each of the corresponding residues found in the NR2A receptor subunit, suggested to be contact residues in the GluR2 alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit, caused a rightward shift in the concentration-response curve for each agonist examined. None of the mutations examined altered the efficacy of glutamate as assessed by methanethiosulfonate ethylammonium potentiation of agonist-evoked currents. In addition, none of the mutations altered the potency of glycine. Homology modeling and molecular dynamics were used to evaluate molecular details of ligand binding of both wild-type and mutant receptors, as well as to explore potential explanations for agonist selectivity between glutamate receptor subtypes. The modeling studies support our interpretation of the mutagenesis data and indicate a similar binding strategy for L-glutamate and NMDA when they occupy the binding site in NMDA receptors, as has been proposed for glutamate binding to the GluR2 AMPA receptor subunit. Furthermore, we offer an explanation as to why "charge conserving" mutations of two residues in the binding pocket result in nonfunctional receptor channels and suggest a contributing molecular determinant for why NMDA is not an agonist at AMPA receptors.