Table 6.

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

Esters	Water		30% E—70% W		50% E—50% W		70% E—30% W
	log⁡K 1	log⁡K 2	log⁡K 1	log⁡K 2	log⁡K 1	log⁡K 2	log⁡K 1	log⁡K 2
CysOMe	9.17	6.38	9.23	6.09	9.34	5.88	10.82	6.24
CysOEt	9.36	6.54	9.37	6.17	9.48	5.97	11.13	6.28
TyrOMe	9.73	7.04	10.10	6.73	10.49	6.42	10.62	6.17
TyrOEt	9.71	7.05	10.13	6.75	10.52	6.45	10.63	6.19
TrpOMe	10.66	7.10	11.27	6.78	11.56	6.51	11.39	6.29
TrpOEt	10.79	7.10	11.28	6.98	11.70	6.80	11.77	6.45
LysOMe	9.99	6.98	9.80	6.90	9.52	6.63	9.12	6.46
LysOEt	10.32	7.18	9.70	6.81	9.45	6.60	9.00	6.35
HisOMe	7.10	4.96	6.72	4.78	6.54	4.61	6.34	4.39

Note. E = ethanol; W = water; ^adata taken from reference [56]. 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).