Thermodynamic Constants of Glycyl Peptides: Solvation and Buffers for the Physiological pH Range |
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Authors: | Venthan B Naidoo Munessar Sankar |
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Institution: | (1) Department of Chemistry, University of Durban-Westville, Private Bag X54001, Durban, 4000, South Africa |
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Abstract: | The pK
2 values for the dissociation of the NH
3
+
charge center of the glycyl peptides, e.g., glycyl-D-asparagine, glycyl-DL-serine, glycyl-L-leucine, and glycyl-DL-methionine have been determined at 10 temperatures in the range 5–50°C by measurements of the emf of cells without liquid junction, utilizing hydrogen electrodes and silver–silver bromide electrodes. The thermodynamic quantities, H
o, C
p
o
and S
o were calculated from the temperature coefficients of the dissociation constants. The pK
2 values at 25°C are 8.268 (glycyl-D-asparagine), 8.277 (glycyl-DL-serine), 8.323 (glycyl-L-leucine), and 8.408 (gly-cyl-DL-methionine). These values show that changes in the substituents on the -carbon atom have very little effect on the dissociation of the NH
3
+
, with the possible exception of glycyl-DL-methionine. The suitability of these compounds as buffers in the physiologically important pH range 7–9 is of interest. The thermodynamics of the solute–solvent interaction is interpreted in terms of a mixture model. The peptides chosen for study include both polar and nonpolar substituents. |
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Keywords: | Thermodynamic constants ampholytes glycyl-D-asparagine glycyl-DL-serine glycyl-L-leucine glycyl-DL-methionine solvation buffers physiological pH range calorimetry |
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