Protonation Equilibria of Biologically Active Ligands in Mixed Aqueous Organic Solvents

Table 11

Protonation constants of some α-amino acids in water and DMSO-water media at 25°C and  mol·dm⁻³ .

System			30% DMSO	50% DMSO	70% DMSO
Gly	1	1	9.51 ± 0.01	9.79 ± 0.01	10.29 ± 0.01
	1	2	12.92 ± 0.01	13.85 ± 0.02	15.20 ± 0.01
L-Ala	1	1	9.65 ± 0.009	9.85 ± 0.02	10.48 ± 0.02
	1	2	13.07 ± 0.01	13.96 ± 0.03	15.36 ± 0.03
L-Val	1	1	9.34 ± 0.01	9.57 ± 0.01	9.99 ± 0.01
	1	2	12.61 ± 0.02	13.61 ± 0.02	14.88 ± 0.03
L-Ser	1	1	8.95 ± 0.01	9.12 ± 0.02	9.89 ± 0.01
	1	2	12.16 ± 0.02	12.95 ± 0.03	14.71 ± 0.03
L-Isl	1	1	9.31 ± 0.01	9.51 ± 0.02	9.91 ± 0.02
	1	2	12.75 ± 0.01	13.56 ± 0.03	15.02 ± 0.03
L-Leu	1	1	9.46 ± 0.01	9.77 ± 0.05	10.18 ± 0.01
	1	2	12.80 ± 0.02	13.81 ± 0.05	14.98 ± 0.02
L-Phe	1	1	9.06^d	9.35^c	—
	1	2	12.44	13.43
L-Met	1	1	9.07^d	9.48^c	—
	1	2	12.69	13.48
L-Thr	1	1	8.95^d	9.32^c	—
	1	2	12.30	13.38

^aData taken from reference [87]; and are stoichiometric coefficients corresponding to ligand and H⁺, respectively; ^cdata taken from reference [88]. Species 11 refers to L⁻ + H⁺ LH; species 12 refers to L + 2H⁺; the % is expressed as (volume/Volume).