Ultrafast Transient Absorption Spectroscopy for Probing Primary Photochemical Reaction of Proteins

Photochemical reactions of proteins are playing important roles for all life on earth; retinal proteins in retina work as light sensor to provide vision, phytochromes in plants regulate the germination of seeds, photoprotein in fireflies and jelly fishes generates bioluminescence, and chlorophyll triggers carbonic acid assimilation by photosynthesis reaction. Exploration of materials which have better efficiency for those photochemical reactions cannot be performed efficiently by haphazard approach. Elucidation of the primary reaction process is expected to provide key information for the development of future materials. The primary reaction could be studied in detail by performing ultrafast spectroscopy which visualizes spectral change during ultrafast transition between electronic states after photoexcitation. Using ultrashort laser pulse whose duration is shorter than the molecular vibration period, the observed transient absorption signal shows modulation reflecting the real-time motion of the molecular vibration in time domain. Time-gated Fourier analysis of the signal elucidates the time development of the molecular vibration frequency which elucidates molecular structure change during the photo-reaction. Ultrashort pulse lasers in visible and ultraviolet spectral region were developed to be applied for ultrafast spectroscopy of the protein samples to elucidate their primary reactions. For the study of molecular vibrational dynamics, signal fluctuation of the transient absorption should be suppressed to observe fine signal modulation caused by the molecular vibration. However, irradiation of the ultrashort laser pulse required for the ultrafast spectroscopy gives serious damage accumulation especially in the protein samples, which degrades signal quality of the transient absorption. Developing fastscan ultrafast spectroscopy system, we have succeeded to improve the signal-to-noise ratio of the transient absorption signal able to study molecular vibrational dynamics. The ultrashort laser pulse and fastscan ultrafast spectroscopy system to study the primary reaction dynamics of the protein samples. Here we describe some of those works for samples of a light-driven proton pump (bacteriorhodopsin) and a heme protein (nitric oxide synthase).