Protein dynamics studied by ultrafast visible and infrared laser spectroscopy

We have studied ultrafast dynamics in two systems: heme proteins and small de novo peptides. Heme relaxation after photoexcitation of both ligated and deoxy myoglobin (Mb) starts with two steps of electronic relaxation on sub-picosecond timescales and leads to a vibrationally hot heme in the ground electronic state. Subsequent nonexponential vibrational relaxation is completed by ca. 20 ps. Transient spectra of ground state vibrationally hot (possibly thermally non-equilibrated) deoxy Mb and calculated static difference spectra between hot and cold proteins are qualitatively similar. Ligand dynamics is investigated for wild type and mutants of iron and cobalt Mb. Ligand photodissociation is extremely rapid (\u3c50 fs). NO recombination is highly nonexponential and proceeds on timescales of tens of picoseconds to nanoseconds and varies between the myoglobins of different species. NO recombination at different wavelengths can be described by the same global set of fit parameters, contradicting the inhomogeneous model. Changes in the detailed structure of the heme pocket achieved by mutations of residue 68 cause much more drastic changes in recombination than metal substitution, indicating a primary role of the heme pocket structure in the dynamics and suggesting that proximal protein relaxation is not the limiting factor in geminate recombination. Dissipation processes proceed similarly in ligated and deoxy Mb. Neither dissipation nor ligand dynamics are strongly affected by the amount of excess energy supplied by a pump photon. This suggests that fast intramolecular energy redistribution is occurring before dissociation. Conformational relaxation of a peptide was photoinitiated by photolysis of the aryl disulfide chromophore, which constrains a peptide to a nonequilibrium form. Anisotropy measurements indicate that overall peptide rotation is on the time scale of 600 ps. By ca. 100 ps the amide I′ region absorption is bleached by a vibrational Stark effect. Absence of a significant shift in the amide I′ region suggests no appreciable helix formation up to 2 ns. Thiyl radicals recombine with a power law rate over the time range of picoseconds to microseconds, signaling an unusual type of scaled kinetics