Prototropism of [2,2'-bipyridyl]-3,3'-diol in albumin-SDS aggregates

In this present investigation, attempt is made to use [2,2'-bipyridyl]-3,3'-diol (BP(OH)(2)) as a marker to study albumin-SDS interactions and to obtain structural information about these aggregates. It is also intended to contemplate the effect of these aggregates on the excited-state proton-transfer dynamics of BP(OH)(2). Steady-state and time-resolved fluorescence spectroscopic techniques are employed to elucidate the nature of interaction of two homologous carrier proteins, human serum albumin (HSA) and bovine serum albumin (BSA), with negatively charged surfactant sodium dodecyl sulfate (SDS). Both spectral and temporal behavior of BP(OH)(2) in these albumin-SDS aggregates strongly affirm an initial competitive binding of SDS in high-energy binding sites of albumin. Unlike normal SDS micelles, the absence of formation of the monocation of BP(OH)(2) at the negatively charged interface of SDS is rationalized by screening of the micellar interface in the presence of denatured protein which wraps around these surfactant aggregates. An enhanced extent of excited-state proton transfer is manifested by a corresponding increase in fluorescence quantum yield of BP(OH)(2) in these aggregates. Temporal evolution of BP(OH)(2) at different emission wavelengths fortifies the formation of normal micelles post saturation. All our observations are found to corroborate with the necklace and bead model proposed for protein-surfactant aggregates.