Trifluoroethanol Drives Platelet Factor 4 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 ¹H 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 × 10⁵ 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.