Photoionization used as a tool for the study of biomembranes: fate of tetramethylbenzidine photocation in purple membrane.

Photoionization of hydrophobic probes has been developed in micelles or synthetic vesicles. Studies of the yields, compartmentation, and lifetimes of the photo-produced charged species have gathered reliable information on the interfacial and structural properties of these assemblies. Such an approach has never been applied to biological membranes. The present system is tetramethylbenzidine as the probe in native or modified (deionized and/or bleached) purple membrane from halobacteria. The data on photocation formation yields (phi ion) and lifetimes (tau 1/2) allow two main conclusions to be made: (1) tetramethylbenzidine, as the cation, is buried in the membrane core, and (2) its incorporation does not alter the biological activity of the protein. In this biological membrane the photocation lifetime and yield present the same trend of variation with the surface potential but to less of an extent than in model membranes. Bleaching of purple membrane completely modifies the photoionization process and the photocation decay. In addition, these experiments reveal a tight correlation between membrane structure and probe photoionization. Further evidence for structural modification of purple membrane, either by deionization or by bleaching is pointed out.