Line narrowing and site selectivity in tryptophan fluorescence from proteins and glasses: Cryogenic studies of conformational disorder and dynamics

Fluorescence line narrowing and site selectivity are observed for red edge excitations (≈300 nm) of tryptophan in alcohol glasses and proteins at cryogenic temperature (2-200 K). Structured and broad fluorescence bands are observed. Both components are excitation wavelength dependent. These two spectral features are assigned to emission from ¹L_b and an exciplex version of ¹L_a (designated ¹L_a^exc) respectively. The latter can be populated either through barrier crossing from ¹L_b or directly via highly excited vibrational levels of ¹L_a^exc. The barrier separating ¹L_b and ¹L_a^exc appears to be on the order of a few hundred cal/mole suggesting that in solution at ambient temperatures the ¹L_a^exc state should dominate the emission process. Influence of protein relaxation upon the ¹L_a^exc emission is not apparent until T>100 K. A model incorporating the conformational substate description of proteins and the rotamer model for tryptophan is developed and discussed in terms of the presented results.