Fast events in protein folding: structures and fast folding dynamics of the native and acid forms of apomyoglobin

We have studied the equilibrium structures and the fast folding dynamics of the native and acid forms of apomyoglobin (apoMb) using infrared and fluorescence spectroscopies. The fluorescence spectrum is sensitive to the solvation of the tryptophans, and hence the intactness of the AGH core, while the ]R spectrum is sensitive to secondary structure and solvation of the protein. Our results, combined with those of previous studies show that the structure of apoMb is heterogeneous, including native-like (tightly packed) and molten globule-like substructures which exhibit denaturation transitions under different conditions of pH and temperature and are largely independent. Progressive destabilization of the protein (by lowering pH and ionic strength) allows us to probe the stability and dynamics of these substructures. For example, fast relaation dynamics of the 'native' form of apoMb (N, pH 5.3) at 60 C following a 10 ns, 20 C laser-induced temperature jump exhibit two distinct phases on very different timescales (50 ns at 1633 cm-l; t30 #s at 1650 cm-l), corresponding to solvated and native helix, respectively. Results for the N, I (pH 3, high salt) and E (pH 3, low salt) states suggest that the folding apoMb proceeds via two independent or quasi-independent 'nucleation' events whereby native-like contacts are formed. One of these events, which seems to involve AGH 'core' formation, appears to occur very early in the folding process, even b,fore significant hydrophobic collapse in the rest of the protein molecule.