Modification of the free sulphydryl groups of bovine serum albumin to probe conformational transitions in the neutral region

Abstract

The free SH group in bovine serum albumin has been modified by covalent coupling with 2-chloromercuri-4-nitrophenol and 2-chloromercuri-2,4-dinitrophenol. The ionization of the phenolic OH group of the former label when bound to albumin can be followed spectrophotometrically. The pK of this group was influenced by the presence of Ca2+. This is not a direct effect but proceeds via an effect of Ca2+ on the protein conformation. Similar results were obtained by following the c.d. signal of this label. This conformational change seems to be different from the one which can be detected by measuring the induced c.d. of a non-covalently bound ligand like diazepam.