FLUORESCENCE-SPECTRA OF BACTERIORHODOPSIN AND THE INTERMEDIATE-O AND INTERMEDIATE-Q AT ROOM-TEMPERATURE

An unequivocal answer is given to the question of why the reported flu orescence spectra of bacteriorhodopsin (bR(568)) have been different f rom one another. The inconsistency is shown to arise from the accumula tion of the fluorescent intermediates O and Q (K-N) by cw excitation l ight. Their fractions in the photo-stationary states depend on the exc itation power and the suspension pH. We report the intermediate-free f luorescence spectrum of bR(568) obtained with a weak excitation source (632.8 nm, 5.3 x 10(15) - 1.9 x 10(16) photons cm(-2).s(-1)) and a ne ar-IR sensitive intensified photodiode array system. The fluorescence maxima of the spectra, F(lambda) and f(nu), are located at 755 +/- 10 nm and 12700 +/- 200 cm(-1), respectively. The spectrum of O is identi cal to that of the deionized purple membrane bR(605) (F-max = 750 +/- 5 nm, f(max) = 13,000 +/- 100 cm(-1)). Q (K-N) exhibits a blue-shifted spectrum more than that of bR(568) (F-max < 720 nm, f(max) > 13,400 c m(-1)).