Ultrafast Carbonyl Motion of the Photoactive Yellow Protein Chromophore Probed by Femtosecond Circular Dichroism

Motions of the trans-p-coumaric acid carbonyl group following the photoexcitation of the R52Q mutant of photoactive yellow protein (PYP) are investigated, for the first time, by ultrafast time-resolved circular dichroism (TRCD) spectroscopy. TRCD is monitored in the near-ultraviolet, over a time scale of 10 ps. Immediately after excitation, TRCD is found to exhibit a large negative peak, which decays within a few picoseconds. A quantitative analysis of the signals shows that, upon excitation, the carbonyl group undergoes a fast (<<0.8 ps) and unidirectional flipping motion in the excited state with an angle of ca. 17-53 degrees. For the subset of proteins that do not enter the signaling photocycle, TRCD provides strong evidence that the carbonyl group moves back to its initial position, leading to the formation of a nonreactive ground state intermediate of trans conformation. The initial ground state is then restored within ca. 3 ps. Comparative study of R52Q and wild type PYP provides direct evidence that the absence of Arg52 has no effect on the conformational changes of the chromophore during those steps.