Buffer effects on electric signals of light-excited bacteriorhodopsin mutants.

The effects of glycyl-glycine and bis-trispropane buffers on the light-excited electric signals due to proton motion in the molecule were studied for the bacteriorhodopsin (bR) mutants D38R, D96N, E204Q, R227Q, D85N, D85T, R82Q/D85N, and D85N/D96N in purple membranes and for delipidated purple membrane containing the wild-type bR. The results show additional charge motion caused by the buffers in all cases. Arrhenius parameters calculated from the temperature dependence of the difference signals (with buffer minus without buffer) are similar to the parameters found for the wild-type bR in the case of these buffers: the values of the activation enthalpies are mostly in the range 25-50 kJ/mol; all the activation entropies are negative. The results are evaluated with the cluster hypothesis outlined previously.