Protonation Equilibria of Biologically Active Ligands in Mixed Aqueous Organic Solvents

Table 7

Protonation constants of α-amino acid-t-butyl and benzyl esters at 25°C for different water-EtOH mixtures  mol·dm⁻³ NaCl^a.

Esters	20% E—80% W	30% E—70% W	40% E—60% W	50% E—50% W	60% E—40% W	70% E—30% W	80% E—0% W
GlyO-t-Bu	7.66	7.51	7.40	7.30	7.20	7.05	6.64
AlaO-t-Bu	9.01	8.86	8.75	8.55	8.39	8.16	8.14
ValO-t-Bu	7.74	7.56	7.15	6.95	6.85	6.51	6.50
LeuO-t-Bu	7.79	7.63	7.38	7.21	7.02	6.92	6.75
PheO-t-Bu	7.15	7.00	6.67	6.56	6.50	6.40	6.33
IleO-t-Bu	7.75	7.65	7.35	7.18	7.12	6.89	6.64
GlyO-Bz	7.27	—	7.07	—	6.94	—	6.85
AlaO-Bz	7.35	—	7.17	—	7.01	—	7.00
ValO-Bz	7.16	—	6.88	—	6.77	—	6.68
SerO-Bz	6.75	—	6.60	—	6.51	—	6.54

Note. E = ethanol; W = water; ^adata taken from ref. [57]. corresponds to 11 species (i.e., L⁻ + H⁺ LH); corresponds to 12 species (i.e., LH + H⁺). The % is expressed as (volume/Volume).