Pressure dependence of fluorescence quenching reactions in proteins

The effect of hydrostatic pressure (0-2.6 kbar) on the acrylamide quenching of the fluorescence of indole derivatives and several single-tryptophan-containing proteins has been studied using phase fluorometry at 25 degrees C. For the model system, N-acetyl-L-tryptophanamide in water, there is essentially no pressure dependence of the quenching rate constant, kappa q. For the internal Trp residue of ribonuclease T1 and cod parvalbumin, there also is essentially no pressure dependence of the apparent kappa q at low pressure. Thus, the activation volume, delta V not equal to, for these quenching processes is approximately zero. Such small delta V not equal to values are expected for diffusion-limited reactions in water at this temperature. The low, apparent delta V not equal to values for the globular proteins characterize these quenching processes as involving very small amplitude fluctuations in the protein structures. Only for the poised tetramer in equilibrium monomer equilibrium of melittin were we able to observe a significant effect of pressure on kappa q and this is due to the pressure-induced shift in the equilibrium position.