"Interaction between drugs and water-soluble polymers". Part III. Binding position of ibuprofen with bovine serum albumin determined by measuring nuclear magnetic resonance relaxation time.

The binding of ibuprofen (IB) to bovine serum albumin (BSA) was predominantly attributed to a hydrophobic interaction based on the thermodynamic parameters obtained by equilibrium dialysis. Little variation in the proton chemical shift of IB was observed when concentrations of the IB were changed (1-10mM), or with the addition of BSA (7.25×10-5M). The spin-lattice relaxation time (T1) of IB was almost concentration-independent, but decreased in the presence of BSA to 36-45% for the phenyl group and its neighborhood, and to 70-97% for the other positions. The spin-spin relaxation rate (1/T2) of IB was also almost concentration-independent, but significantly increased ca. 37 times for the phenyl group and 12-24 times for the alkyl group in the presence of BSA. The ratio of the spin-spin relaxation rate of the free IB to the bound IB ((1/T2)b/(1/T2)f) of the phenyl group was 2-3 times larger than that of the alkyl group as shown by a contour plot (Chart 1). The binding of IB to BSA was considered to involve mainly the phenyl group of IB.