QUANTUM YIELDS OF THE PHOTOCHROMIC EQUILIBRIUM BETWEEN BACTERIORHODOPSIN AND ITS BATHOINTERMEDIATE-K - FEMTOSECOND AND NANOSECOND OPTOACOUSTIC SPECTROSCOPY

The primary photochemistry of bacteriorhodopsin (BR) was investigated by laser-induced optoacoustic spectroscopy. Excitation of light adapted BR by femtosecond flashes of 585 nm (<500 fs flash duration) produced the first intermediates of the BR photocycle, J and K, without initiating any additional photochemistry, whereas excitation by 8-ns flashes at various wavelengths established photoequilibria between BR and K. The extent of K formation was determined optoacoustically from the heat stored after excitation. On thc basis of the results from femtosecond and nanosecond measurements, a mathematical description of the system BR half arrow right over half arrow left K --> L was developed which yields an energy content of 160 kJ/mol for the first intermediate K. This value corresponds to an energy difference of 30 kJ/mol between the energy levels of K and excited BR. The energy content of excited BR was taken as 190 kJ/mol corresponding to the 0-0) band of BR at 628 nm. A quantum yield, PHI(BR-->K) = 0.6, for the BR --> K photoreaction, the lifetime of excited K, tau(K*) = 4-10 ps, and a quantum yield for the back photoisomerization, PHI(k-->BR) = 0.6, result. The determination of the quantum yield by optoacoustics does not depend on assumptions on wavelength maximum and shape of the K-spectrum which are required for the interpretation of optical data. Thus, the results presented here give evidence for a high value (>0.5) for the quantum yield of the BR photochemistry which in the literature was controversially discussed as being, 0.3 or 0.6