Pressure effects on tryptophan and its derivatives

The high pressure effects on fluorescence of free tryptophan (Trp) and its derivatives, N-acetyl-tryptophan (AT), N-acetyl-tryptophanamide (NATA), tryptophanamide (TA), and tryptophan, containing 6-polypeptides in aqueous solution, were investigated in a pressure range from 0.1 to 650 MPa. It was found by analyzing the center of spectral mass in the wavelength range from 300 to 450 nm that high pressure shifted the fluorescence spectra of all these species to red direction: 421 cm(-1) for Trp, 305 cm(-1) for AT, 310 cm(-1) for NATA, 265 cm(-1) for TA, and 220 cm(-1) for single tryptophan containing 6-polypeptides. All the fluorescence efficiencies (i.e., quantum yield) of the compounds were reduced with pressure except free tryptophan where its fluorescence efficiency was enhanced with pressure. Glycerol, ethanol, and pH obviously influenced the pressure effects on their fluorescence characteristics. Since the tryptophan fluorescence is usually used as a probe for protein structural investigation, these findings suggested that the intrinsic pressure effect on tryptophan (or its derivatives) must be taken in consideration to explain the phenomenon observed in high pressure study on biomolecules when using the usual fluorospectroscopic approaches. In the present investigation, the mechanisms involved for pressure effects on tryptophan and its derivatives were explored and discussed.