Time-resolved absorption and fluorescence spectroscopy are used to study the dynamics of protein structural changes subsequent to rapid mixing or excitation with short laser pulses. Molecular models for the protein dynamics are used to fit and interpret the measured data. A. We have succeed in implementing two new techniques which can be used to study the kinetics of a wide variety of processes, a rapid mixing technique capable of initiating reactions within 80 microseconds and a laser temperature jump experiment in which water is heated 10-20 K in less than 10 nanoseconds by an infrared laser pulse. B. The dynamics of short-lived states on the folding pathway of cytochrome c have been investigated using continuous-flow mixing and fluorescence quenching to monitor the tryptophan-heme distance. In the presence of imazole, which blocks binding of ligands to the sixth coordination site of the heme iron, mixing to low guanidine HCl produces a rapid collapse of the denatured state, occurring in less than 50 microseconds, followed by rapid exponential (< 1 millisecond) folding to the native conformation. Under conditions where non-native heme ligands can bind, rapid collapse is still observed, but the folding phase becomes at least biphasic as a result of 'trapping' of the denatured states by incorrect heme-ligand interactions. C. Laser temperature jump studies have been carried out on an 19- residue alanine peptide labeled at the n-terminus by methyl-amino benzoic acid. The fluorescence of this label appears to be highly dependent on hydrogen bonding of the carbonyl oxygen. We observe rapid melting of the helix with a maximum relaxation time of about 20 nanoseconds at the midpoint of the folding curve (35 C), while time- resolved infrared studies have found a relaxation time of about 100 nanoseconds under similar conditions. We attribute the faster relaxation to the fact that our probe monitors melting of the first turn of the helix, while the infrared measures the melting of the entire peptide.
