Tryptophan imaging of membrane proteins.

A theoretical analysis of resonance energy transfer between protein tryptophan and the n-(9-anthroyloxy) (AO) fatty acid probes has been carried out to evaluate its potential use in determining the tryptophan distribution in membrane proteins. The Förster theory for two-dimensional energy transfer was formulated to calculate multiple donor (tryptophan) transfer efficiencies to ensembles of AO probes at different depths in the bilayer. The variation of transfer efficiency with AO probe depth is found to be a sensitive function of tryptophan position and the protein radius but not the dipole-dipole orientation factor or the decay heterogeneity of the donor. For single tryptophan-containing proteins the model predicts that the tryptophan position can be determined with a precision of about 2 A. Although for multiple tryptophans there is appreciable deterioration in resolution, it is still possible to determine the essential features of the distribution such as its first two moments. The positions determined by this method are the projections of the tryptophan positions on a plane perpendicular to the membrane surface, since the probes distribute uniformly around the protein. To analyze the data, a Monte Carlo approach has been developed to search for tryptophan distributions compatible with the observed efficiencies and to display the results in terms of a tryptophan density map. It is shown that even for cases in which little is known about the quantum yield distribution, significant information can be determined about the tryptophan spatial distribution.