Conformational changes of bovine serum albumin as a consequence of adsorption mimicked by freezing molecular motion

Abstract

Bovine serum albumin (BSA) was modified at its free sulfhydryl group (residue 34 near the N-terminus) with the maleimido-tempo (MT) spin probe to produce spin-labelled BSA (BSA-MT). The immobilization of the spin label of BSA-MT at low temperature in an aqueous environment was studied. The aim was to mimic and evaluate the immobilization of the spin label of BSA-MT as a consequence of adsorption at the solid-liquid interface. The difference between electron spin resonance (ESR) spectral parameters of BSA-MT recorded in buffer (hyperfine splitting constant 2 T II =64.1 G; ratio of the low magnetic field maxima s w =0.45 ) and adsorbed on the surface of controlled pore glass beads was matched to the difference between the ESR characteristics of BSA-MT recorded in buffer at room temperature (2 T II =64.1 G; rotational correlational time τ c=8.7 ns; s w =0.45 ) and at −57°C (2 T II =70.0 G; s w =0.82 ). The ESR spectrum of BSA-MT recorded at −57°C in buffer had a comparable shape and spectral parameters to those of BSA-MT adsorbed on the surface of the glass beads and recorded at 25°C. The data suggest that the spin label mobility of BSA-MT adsorbed on the solid surface can be mimicked by decreasing the temperature of the solution to −57°C. Changes in the mobility of BSA-MT in buffer over the temperature range from +20 to +70°C are discussed with reference to unfolding of the protein. The molecular motion of BSA-MT increased non-linearly with linear temperature increases of the BSA-MT solution. An abrupt change in the mobility of BSA-MT was recorded at +40°C. The adsorption of BSA-MT onto the surface of the controlled pore glass beads increased with increasing temperature.