An apparent general solution for the kinetic models of the bacteriorhodopsin photocycles.

For the past decade, the field of Bacteriorhodopsin (BR) research has been influenced by a kinetic view of the photocycle as a reversible, homogeneous, model (RHM) with a linear sequence of intermediates. More recently, we proposed a much different model which consists of essentially unidirectional, parallel (i.e., heterogeneous) cycles (UPM) (Hendler, R. W.; Shrager, R. I.; Bose, S. J. Phys. Chem. B 2001, 105, 3319-3328). It is important to try to resolve which of the two models is more likely to be correct, because models influence and provide a basis for further experimentation. Therefore, in this communication, we reexamine the basis for the RHM with a focus on the most recent and complete description of this model (van Stokkum, I., H., M.; Lozier, R. J. Phys. Chem. B 2002, 106, 3477-3485) vis a vis the UPM in an in-depth study. We show that (i) the tested RHM does not really work for the data of van Stokkum and Lozier nor ours; (ii) no previously published RHM model has been shown to work for data under any conditions; (iii) there are many published observations that are difficult if not impossible to explain by RHM, but are readily explained by parallel cycles. It is also shown that either a UPM or a parallel cycle model with limited reversibility correctly describes photocycle data collected at pH 5, 7, and 9 and at 10, 20, and 30 degrees and is consistent with all known experimental observations.