TRIFLUOROETHANOL DRIVES PLATELET FACTOR-IV SUBUNIT ASSOCIATION RATE TOWARD THE SMOLUCHOWSKI-STOKES-EINSTEIN DYNAMIC DIFFUSION LIMIT

Under certain solution conditions, platelet factor 4 (PF4) monomers (7800 Da), dimers, and tetramers exist in solution in slow exchange on the H-1 NMR (500 MHz) time scale (Mayo, K. H.; Chen, M. J. Biochemistry 1989, 28, 9469). In the presence of increasingly low concentrations of trifluoroethanol (less than 1-5%), the normally observed slow monomer-dimer-tetramer exchange is driven into an intermediate and then fast chemical exchange regime. This indicates that PF4 subunit exchange rates are dramatically increased to in excess of 1000 s-1. While other chemical agents are known to dissociate aggregate states, trifluoroethanol is unusual in that it enhances protein molecular association rates. In the slow and slow-intermediate exchange regimes, association/dissociation kinetic parameters have been derived from exchange-induced line width increases. In the absence of trifluoroethanol, the PF4 monomer association jump rate is 18 s-1 (1.6 x 10(5) M-1 s-1), which is about 3-4 orders of magnitude below the traditionally derived Smoluchowski-Stokes-Einstein theoretical rate of diffusional encounter. When PF4 subunit exchange is brought into the NMR fast exchange regime, i.e., 1000 s-1 and above, trifluoroethanol effectively has driven PF4 subunit association toward this theoretical dynamic diffusion limit.