Table 3.

Protonation constants of some α-amino acids in water and water-ethanol mixtures [Temp. = 25°C and I = 0.1 mol·dm⁻³ NaCl].

AA	Water			30% E—70% W			50% E—50% W			70% E—30% W
	log⁡K 1	log⁡K 2	log⁡K 3	log⁡K 1	log⁡K 2	log⁡K 3	log⁡K 1	log⁡K 2	log⁡K 3	log⁡K 1	log⁡K 2	log⁡K 3
Cys	10.29	8.36	1.71	10.25	8.23	2.60	10.81	7.06	2.93	12.07	6.33	2.82
Tyr	10.14	9.03	2.17	10.46	9.03	2.48	10.90	9.06	3.25	10.70	8.80	2.69
Trp	9.33	2.35	—	9.17	2.83	—	9.05	3.21	—	8.78	2.77	—
Lys	10.69	9.08	2.04	10.20	8.77	1.71	10.07	8.76	2.68	9.64	8.48	2.19
His	9.08	6.02	1.70	8.86	6.21	3.00	8.53	5.85	2.93	8.32	5.73	2.99

Note. E: ethanol, W: water, Cys = L-cysteine, Tyr = L-tyrosine, Trp = L-tryptophane, Lys = L-lysine, and His = L-histidine. ^aData taken from reference [50]; log⁡K ₁: corresponds to 11 species (i.e., L⁻ + H⁺⇌ LH); log⁡K ₂ corresponds to 12 species (i.e., LH + H⁺⇌LH₂ ⁺). The % is expressed as (volume/Volume).