The subsequent effect of interaction between Co2+ and human serum albumin or bovine serum albumin

A notable hysteretic effect has been observed in the interaction of Co(II) with human serum albumin (HSA) or bovine serum albumin (BSA) using UV-Visib le spectrometry at physiological pH (7.43), which shows that the binding be tween Co(II) and HSA or BSA may induce a slow transition of HSA or BSA from the conformation of weaker affinity for Co(II) to one of stronger affinity (A-B transition). The rate constants and activation parameters of this tra nsition were measured and are discussed. It is inferred that such a conform ation transition may occur due to the binding of the first Co(II) ion with the peptide segment of N-terminal residues 1-3, which results in a 'hinged movement' of the relatively hydrophobic 'valley' in the IA subdomain. This process leads to a slow conformational transition in the albumins, makes th e other binding sites of Co(II) er,posed, and shows a positive cooperativit y effect. The LMCT (ligand-to-metal charge transition) bands of the Co(II)- HSA and Co(II)-BSA systems also show a kind of hypochromic effect featuring a dipole-dipole interaction mechanism. This phenomenon is rarely reported. (C) 2001 Elsevier Science B.V. All rights reserved.