Studies on the temperature effect on bacteriorhodopsin of purple and blue membrane by fluorescence and absorption spectroscopy.

Fluorescence and absorption spectra were used to study the temperature effect on the conformation of bacteriorhodopsin (bR) in the blue and purple membranes (termed as bRb and bRp respectively). The maximum emission wavelengths of tryptophan fluorescence in both proteins at room temperature are 340 nm, and the fluorescence quantum yield of bRb is about 1.4 fold higher than that of bRp. As temperature increases, the tryptophan fluorescence of bRb decreases, while the tryptophan fluorescence of bRp increases. The binding study of extrinsic fluorescent probe bis-ANS indicated that the probe can bind only to bRb, but not to bRp. These results suggest that significant structural difference existed between bRb and bRp. It was also found that both kinds of bR are highly thermal stable. The maximum wavelength of the protein fluorescence emission only shifted from 340 nm to 346 nm at 100 degrees C. More interestingly, as temperature increased, the characteristic absorption peak of bRb at 605 nm decreased and a new absorption peak at 380 nm formed. The transition occurred at a narrow temperature range (65 degrees C-70 degrees C). These facts indicated that an intermediate can be induced by high temperature. This phenomenon has not been reported before.