The role of the heme group in the folding of cytochrome c has been extensively studied by Dr. Denis Rousseau and others. However, much less work has been done on probing the secondary structure of the protein during the folding process. Using synchrotron FTIR, we are probing the secondary structure of cytochrome c during the folding process on a sub-millisecond time scale. The deconvolution of the Amide I band (1600 - 1700 cm-1), provides details of the percent of the protein structure that is ?-helical, ?-sheet, ?-turn, or extended coil. Native cytochrome-c examined by deconvolution of the infrared spectra reveals a structure that contains 51% ?-helical, 12% ?-turn, and about 34% extended coil. This is in good agreement with crystallography and NMR data. In our experiments, cytochrome c is unfolded with pH and refolded by adding salt (KCl). Before examining the folding intermediates in a time-resolved fashion with the rapid-mixer (see Subproject 34), first we have determined the equilibrium folding and unfolding conditions. At a concentration of 0.9 mM in D2O, cytochrome c is completely unfolded at pH 2 (using Cl) and completely refolded by adding 300 mM KCl. We have combined UV-visible, fluorescence, and infrared spectroscopies to confirm the endpoints of the reaction. Optical spectroscopy (uv-vis) probes the Soret absorption band of the heme during the folding. Fluorescence spectroscopy probes the environment of the single tryptophan in cytochrome c. Using FTIR, we find a decrease in the random coil content and an increase in helical content as the protein folds.
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