Protein binding studies of luminescent rhenium(I) diimine complexes

Interaction of four luminescent rhenium(I) diimine complexes, [Re(CO)₃(N-N)L]PF₆ ((N-N = 2,2-bipyridine, L = py-3-COOH) 1a, (N-N = 2,2-bipyridine, L = py-3-CONH₂) 1b, (N-N = 1,10-phenanthroline, L = py-3-COOH) 2a, (N-N = 1,10-phenanthroline, L = py-3-CONH₂) 2b with bovine serum albumin (BSA) at physiological pH has been examined using UV-Vis absorption and luminescence spectroscopy, excited state lifetime measurement and circular dichroism (CD). In the presence of BSA, the luminescence of Re(I) complexes is quenched due to the locking-in of the probe into the protein environment. Interestingly the probe is released from the protein environment in the presence of sodium dodecyl sulfate (SDS) resulting in the restoration of the original luminescence along with a red shift in the emission maximum. These observations are explained in terms of binding constants (K_a) of probe with protein and surfactant and the nature of the binding has been investigated from Scatchard plot and Hill’s coefficient (n) value. These studies point out that the interaction between Re(I) complexes and BSA is cooperative in nature.