On the absorbance changes in the photocycle of the photoactive yellow protein: A quantum-chemical analysis

Spectral changes in the photocycle of the photoactive yellow protein (PYP) are investigated by using ab initio multiconfigurational second-order pertu rbation theory at the available structures experimentally determined. Using the dark ground-state crystal structure [Genick, U. K., Soltis, S. M., Kuh n, P., Canestrelli, I. L. & Getzoff, E. D. (1998) Nature (London) 392, 206- 209], the pi pi* transition to the lowest excited state is related to the t ypical blue-light absorption observed at 446 nm. The different nature of th e second excited state (n pi*) is consistent with the alternative route det ected at 395-nm excitation. The results suggest the low-temperature photopr oduct PYPHL as the most plausible candidate for the assignment of the cryog enically trapped early intermediate (Genick et al.). We cannot establish, h owever, a successful correspondence between the theoretical spectrum for th e nanosecond time-resolved x-ray structure [Perman, B., Srajer, V., pen, I. , Teng, T., Pradervand, C., et al. (1998) Science 279, 1946-1950] and any o f the spectroscopic: photoproducts known up to date. It is fully confirmed that the colorless light-activated intermediate recorded by millisecond tim e-resolved crystallography [Genick, U. K., Borgstahl, G. E. O., Ng, K., Ren , Z., Pradervand, C., et al. (1997) Science 275, 1471-1475] is protonated, nicely matching the spectroscopic features of the photoproduct PYPM. The ov erall contribution demonstrates that a combined analysis of high-level theo retical results and experimental data can be of great value to perform assi gnments of detected intermediates in a photocycle.