Equilibrium Constants of Interaction between Pyridoxal-5'-Phosphate Coenzyme and Glycine and Its Oligopeptides in Aqueou
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Equilibrium Constants of Interaction between Pyridoxal-5'-Phosphate Coenzyme and Glycine and Its Oligopeptides in Aqueous Buffered Saline V. P. Barannikova,* and E. A. Venediktova a Krestov
Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, 153045 Russia *e-mail: [email protected] Received February 7, 2020; revised March 6, 2020; accepted March 17, 2020
Abstract—Electronic absorption spectroscopy is used to study the patterns of bonding of oligopeptides of glycyl-glycine and glycyl-glycyl-glycine using the coenzyme pyridoxal-5'-phosphate at 293 K in a phosphate buffer solution at pH 7.35. The results are compared to the similar process for monomeric glycine. The interaction constants and the spectral parameters of the reaction products are determined. The observed increase in the peptide–coenzyme bonding equilibrium constants in the Gly–(Gly–Gly)–(Gly–Gly–Gly) series is due to a drop in the basicity of the reaction’s amino group and an increase in the proportion of peptides with non-ionized amino groups in the solution. An increase in the chain length of the peptide is accompanied by a reduction in the coefficient of extinction of the resulting Schiff base. Keywords: interaction between oligopeptides and pyridoxal-5'-phosphate, electronic absorption spectroscopy DOI: 10.1134/S0036024420110035
INTRODUCTION Pyridoxal-5'-phosphate (PP) acts as a coenzyme in many processes of the metabolizing of amino acids (AAs) and peptides [1]. It is known that Schiff bases (SchBs) are the products of the interaction between PP and amino acids and peptides under model conditions [1, 2]. Earlier studies of the thermodynamics of the reactions between PP and various amino acids [3] showed that the efficiency of pyridoxal phosphate coenzyme bonding depends on the structure of the amino acids and the charges of their ionic groups. In this work, we performed a comparative study of the efficiency of the bonding of glycine and its oligopeptides with coenzyme PP in an aqueous buffer solution. The condensation reaction of an amino acid or peptide with PP at pH 7.35 is described by the equation H HO
O C
−O
POH2C O N −
OOC
CH2
COO− OH + H2N Rn CH2 CH3 Rn N
−O
HO
CH POH2C
OH + H2O
O N
CH3
where the R denotes the fragment O CH2CNH
Index n = 0 for glycine; n = 1 for glycyl-glycine, and n = 2 for glycyl-glycyl-glycine.
EXPERIMENTAL In this work, we used pyridoxal-5'-phosphate (Aldrich) with a 98% content of the main substance, along with ≥99% pure glycine, diglycine, and triglycine (Merck). Prior to use, the reagents were dried to a constant weight in a vacuum at 50°C. The reaction was conducted in aqueous phosphate-buffered saline (0.04165 M NaH2PO4/0.20492 M Na2HPO4) at pH 7.35 and a temperature of 293 ± 0.3 K. The pH value of the phosphate buffer solution was checked using a pH meter and adjusted to the exact value (pH 7.35) by adding the components of the mixture. The use of a buffer solution ensured the stability of the ionic forms of the reagents in solution during the reaction. Peptide conce
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