Constrained tryptophan has a monoexponential decay

摘要

The fluorescence decay of tryptophan is complex. Various models have been proposed to account for the multiexponential decay, including ground-state rotamers, excited-state proton transfer, and lifetime distributions. In order to test the rotamer model, we are studying a series of tryptophan derivatives in which rotation about the C{sub}α-C{sub}β and C{sub}β-C{sub}γ bonds is constrained. The ground-state conformations are determined by X-ray diffraction, molecular mechanics, and 1{sup left}H NMR. Or preliminary results for a tryptophan derivative with the a-amino group incorporated in a six-member ring are described below. 3-Carboxy-1,2,3,4-tetrahydro-2-carboline was synthesized by refluxing tryptophan with formaldehyde in alkaline solution. The derivative was purified by recrystallization from hot water after treatment with decolorizing charcoal. TLC showed a single spot on silica gel plates developed with butanol/acetic acid/water (4:1:1) and visualized by UV, iodine, and ninhydrin. matrix showed ions at m/z = 217 ([MH]{sup}+), [MH + glycerol]{sup}+, FAB mass spectrometry using a glycerol and [MH + 2,3, and 4 glycerols]{sup}+. The fluorescence decay of the tryptophan derivative was measured by time-correlated single photon counting. Decay curves were acquired at 295 nm excitation wavelength and different emission wavelengths between 315 and 375 nm. The data were analyzed by reference deconvolution in a global program. The decays were fit by a monoexponential function and the fluorescence lifetime was independent of emission wavelength. The fluorescence decay was a single exponential from pH 5.5 to 11.3 (Table 1). The lifetime decreased with increasing pH.