Nanosecond dynamics of tryptophans in different conformational states of apomyoglobin proteins

Fluorescence anisotropy kinetics were employed to quantify the nanosecond mobility of tryptophan residues in different conformational states (native, molten globule, unfolded) of apomyoglobins. Of particular interest is the similarity between the fluorescence anisotropy decays of tryptophans in the native and molten globule states. We find that, in these compact states, tryptophan residues rotate rapidly within a cone of semiangle 22-25 degrees and a correlation time of 0.5 ns, in addition to rotating together with the whole protein with a correlation time of 7-11 ns. The similar nanosecond dynamics of tryptophan residues in both states suggests that the conformation changes that distinguish the molten globule and native states of apomyoglobins originate from either subtle, slow rearrangements or fast changes distant from these tryptophans.