Advances in grating-based photoacoustic spectroscopy for the study of protein dynamics

Recent advances in transient grating spectroscopy are described in relation to extracting photoacoustic parameters. The time resolution for measure me nt of dynamically driven acoustics has been extended to the picosecond leve l. This improvement was achieved by adopting counter-propagating beam geome tries and resolving the acoustic phase shift in analogy to phase modulation spectroscopy. At the other extreme, the dynamic range of this technique ha s been extended to milliseconds in order to follow dynamical processes cent ral to biological functions. In addition, diffractive optics were used for generating the necessary excitation and probe beam geometries. A novel opti cal setup was developed which permits both the rapid exploration of fringe spacing dependencies in separating thermal from nonthermal contributions to the observed signal as well as heterodyne detection without active feedbac k. The latter capability significantly increases the signal-to-noise ratio and permits separation of the real and imaginary components from the nonlin ear four-wave-mixing signal. Applications of these new methods are demonstr ated by following the functionally relevant structural relaxation processes of heme proteins over 10 decades in time.