Nonlinear laser spectroscopy for probing reaction dynamics and energetics in biological macromolecules (proteins).

Mechanical relaxation (loss of energy) in proteins can be investigated by nonlinear optical spectroscopies. In one such technique -- called "pump/probe spectroscopy" -- photo-excitable molecules are dissociated by an intense laser pulse to transiently form two unstable subspecies. The ensuing dynamics of recombination of the two subspecies, which usually occurs on nanosecond through millisecond timescale, is probed by monitoring changes in the absorption of a second, less intense laser beam having an appropriate wavelength. In experiments on glass-embedded Myoglobin (Mb) (a biological carrier of oxygen that can also bind CO), Raicu and co-investigators observed that the time-course of Mb and CO recombination following laser-induced dissociation of MbCO deviated from a classical exponential relaxation law. Instead, it resembled the time-domain behavior of the general dispersion function introduced in dielectric studies (see above). This kind of investigation is important for the basic understanding of relaxation processes in complex systems, and, ultimately, for our understanding of protein functions.