Characterization of human serum albumin forms with pH. Fluorescence lifetime studies.

Fluorescence lifetimes of human serum albumin (HSA) tryptophan 214 residue were measured in solution at different pH (from 2 to 12). The results indicate that tryptophan emission occurs with three lifetimes at all pH. However, lifetimes and pre-exponential values are dependent on the pH and thus on the protein form. Three different protein populations have been differentiated: one population for pH 2 and 3 (extended form), the second one from pH 4 to 9 containing HSA migrating (F), normal (N) and basic (B) forms. Another type of population is obvious for pH higher than 9, characterizing the aged (A) form of HSA.