Protonation Equilibria of Biologically Active Ligands in Mixed Aqueous Organic Solvents

Table 4

Protonation constants of some α-amino acids in water and water-ethanol mixtures Temp. = 25°C and  mol·dm⁻³ NaClO₄^a.

AA	30% E—70% W		40% E—60% W		50% E—50% W		60% E—40% W		70% E—30% W
	Gly	9.54	2.81	9.39	2.89	9.35	2.90	9.20	3.05	9.05	3.06
Ala	9.70	2.88	9.55	2.97	9.45	3.05	9.18	3.06	9.12	3.08
Val	9.50	2.85	9.41	2.95	9.30	3.15	9.07	3.16	9.02	3.17
Leu	9.60	2.87	9.40	3.01	9.30	3.06	9.07	3.17	9.05	3.25
Ile	9.55	2.92	9.47	2.98	9.31	3.15	9.20	3.17	9.10	3.24
Phe	9.12	2.75	8.95	2.95	8.79	2.98	8.70	3.05	8.60	3.00
Ser	9.18	2.88	9.00	2.87	8.77	2.89	8.68	2.91	8.66	2.90
Met	9.05	2.67	8.99	2.85	8.85	2.88	8.67	3.00	8.62	3.02

Note. E: ethanol, W: water, Val = L-valine, Ala = L-alanine, Leu = L-leucine, Ile = L-isoleucine, Phe = L-phenylalanine, Ser = L-serine, and Thr = L-threonine; ^adata taken from reference [49]. corresponds to 11 species (i.e., L⁻ + H⁺ LH); corresponds to 12 species (i.e., LH + H⁺). The % is expressed as (volume/Volume).