Conformational Dynamics of Tryptophan: a Proposal for the Origin of the Non-exponential Fluorescence Decay

Tryptophan has been widely used as a probe of protein environment and dynamics. Proper interpretation of the experimental results requires a detailed understanding of tryptophyl photoprocesses, so much effort has been made to understand the fluorescence decay of zwitterionic tryptophan. For a review of these efforts, see ref. [l] . It is now well established that the fluorescence decay of tryptophan (3 < pH < 9) is non-exponential [2-61. The decay at neutral pH has generally been fit to a sum of two exponentials; a third exponential appears at higher pH due to the presence of anionic tryptophan [7,8]. It has generally been assumed that the non-exponentiality of the zwitterion is due to the presence of two or more conformers which do not interconvert during the fluorescence lifetime [9-141 and that the lifetimes correspond to the conformers in a simple manner. However, as pointed out by James and Ware [ 15,161, it is by no means self evident how many stable conformations (i.e. confined by barriers greater than a few k&“) tryptophan has, or on what time scale these conformers interconvert. Given this uncertainty it is quite possible that the “double exponential decay” obtained by numerous workers is, in reality, a distribution of lifetimes that, at the precision levels commonly em-