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. 1968 Dec;8(12):1458–1474. doi: 10.1016/S0006-3495(68)86566-X

Mixed Chelates of Ca(II)-Pyridine-2,6-Dicarboxylate with Some Amino Acids Related to Bacterial Spores

T Tang, K S Rajan, N Grecz
PMCID: PMC1367448  PMID: 5713454

Abstract

The high resistance of bacterial spores to heat has been repeatedly postulated to be due to stabilization of spore biopolymers by metal chelate compounds. Binding of calcium dipicolinic acid (Ca(II)-DPA) with spore proteins and amino acids has been discussed in the literature, but equilibrium data are generally lacking. By means of potentiometric pH titrations at 25°C and an ionic strength of 1.0 (KNO3), the formation of Ca(II)-DPA (1:1 and 1:2) chelates and the interactions of Ca(II)-DPA chelate with a mole of each of three typical amino acids viz., cysteine, alanine, and glycine has been investigated. Analysis of the potentiometric data indicates that calcium and DPA forms 1:1 and 1:2 chelates with log KML1 = 4.39 ± 0.01 and log KML2 = 2.25 ± 0.01. In the presence of an equimolar amount of each of the amino acids under consideration, the Ca(II)-DPA chelate forms mixed ligand (ternary) chelate yielding the following stepwise stability constants: log K1 = 4.17 ± 0.01, log K2 = 0.78 ± 0.01 for cysteine, log K1 = 4.06 ± 0.01, log K2 = 0.65 ± 0.01 for alanine, and log K1 = 4.30 ± 0.02, log K2 = 0.11 ± 0.01 for glycine. Methods for calculating the stability constants of the mixed ligand system have been developed. On the basis of the potentiometric equilibrium data, possible structures for the various calcium chelate species are discussed. The data suggest that the differences in heat resistance of various strains of bacterial spores may conceivably be related to the differences in composition and stability of coordination complexes in the spore.

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Selected References

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