Protonation Equilibria of Biologically Active Ligands in Mixed Aqueous Organic Solvents

Table 5

Protonation constants of dipeptides in water and water-ethanol mixture Temp. = 25°C and  mol·dm⁻³ NaCl.

Dipeptides	Water		20% E—80% W^c		40% E—60% W^c		60% E—40% W^c
	Gly-Gly	8.15^a	3.11^b	7.66	3.33	6.92	3.54	6.98	3.81
GlyVal	8.23^d	3.15^d	7.67	3.52	7.06	3.91	6.92	4.18
GlyLeu	7.91^e	3.15^e	7.60	3.38	7.17	3.94	7.02	4.22
GlyPhe	7.98^d	3.03^d	7.62	3.33	6.96	3.65	6.98	3.91
GlyThr	8.50^c	3.20^c	7.74	3.19	7.07	3.37	7.05	3.67
GlyMet	8.22^f	3.11^f	7.64	3.32	7.02	3.62	6.96	3.91

Note. E: ethanol; W: water; ^adata taken from reference [51]; ^bdata taken from reference [52]; ^cdata taken from reference [42]; ^ddata taken from reference [53]; ^edata taken from reference [54]; ^fdata taken from reference [55]. corresponds to 11 species (i.e., L⁻ + H⁺ LH); corresponds to 12 species (i.e., LH + H⁺). The % is expressed as (volume/Volume).