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In bulk water at neutral pH, a bimodal excited state lifetime distribution characterizes the fluorescence decay of tryptophan (Trp) in the blue edge of the fluorescence emission spectrum (centers 'ri =0.58 ns, t2 = 3.15 ns, proportions: cq= 0.33, a = 0.67 at 320 nm). A monomodal distribution is found in the red edge (center: t= 3.18 ns at 390 nm) as described in the litterature. The decay of N-acetyl-tryptophanamide (NATA) is also biexponential at short emission wavelength with similar decay values as Trp but with different proportions (centers: ti =0.61±0.13 ns, t2 = 3.01±0.05 ns, proportions: cq= 0.05, a = 0.95 at 320 nm) and monomodal at long emission wavelength (center: t = 3.00±0.05 ns at 390 nm). No evidences for a fast build-up at long emission wavelength can be observed. The fluorescence decays of indole, N-methyl indole and NATA in ethanol are described in the blue edge of the emission spectrum by the existence of a bimothl distribution of time constants (centers: t—2O30 ps and t2 3.7-4.3 ns) and in the red edge of the fluorescence emission spectrum, by a bimodal distribution with the same centers as in the blue edge and identical preexponential values but opposite in sign. In isobutanol, the rising time value increased to —90 ps. The Trp fluorescence decay in ethanol shows three separated time constant populations in the blue edge of the emission spectrum (centers ti =20 p5; 'c = 0.43 ns and t3 = 2.47 ns). In the red edge, a fast rising of the fluorescence (center: —70 ps) is observed, together with a long decaying component (2.67 ns) and a shorter one (0.46 ns). For NATA in reverse micelles, similar observations are obtained in the red edge with a rising time of —30 ps, a value independent of the degree of immobilization of the water molecules. These observations support the existence of excited state reactions involving interactions with the solvant molecules (exciplex formation or solvent-cage relaxation ). The example of annexin V is proposed to illustrate the case of a protein: the data support the existence of a fast relaxation process (—30 ps) observed at low temperature for the lone buried tryptophan residue. According to the tridimensionnal structure of the protein, this can be interpreted as resulting from the existence of a H-bonding involving the E-OH group of a threonine residue (Thr224) and the indole nitrogen.
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