Investigation of slow dynamics of the sulfhydryl in the solution and gel states of bovine serum albumin: A vector electron paramagnetic resonance study

Serum albumin has one reactive sulfhydryl (Cys-34) that is one of the impor tant binding sites. Cys-34 is located in the crevice on the surface of the albumin molecule and is therefore restricted in its motion. Bovine serum al bumin (BSA) Fraction V forms a transparent gel at pD 4.0 (F-form) in D2O at protein concentrations above 7% (BSA*-gel). We studied the molecular motio n of Cys-34 on BSA in the solution and gel states by the vector electron pa ramagnetic resonance (EPR) method using a maleimide spin label. The rotatio nal correlation times of the spin label bound to Cys-34 in the BSA solution and BSA*-gel were in the order of 10(-6) and 10(-5)s, respectively. A long er rotational correlation time of the Cys-34 spin label in the BSA*-gel sug gested that the gel network formed in BSA may drastically slow the motion o f Cys-34. The integrated value obtained from the vector EPR spectra also sh owed an extremely dramatic slowing of the Cys-34 spin label during the gel formation. On the other hand, the values for order parameter and the inclin ation of the principal axis (z) of the Cys-34 spin label to the rotational axis (ILL) were the same in the BSA solution and BSA*-gel.