STRUCTURE AND DYNAMICS OF THE ACIDIC COMPACT STATE OF APOMYOGLOBIN BYFREQUENCY-DOMAIN FLUOROMETRY

The conformational dynamic properties of tuna apomyoglobin, a single t ryptophan-containing protein, in the acidic compact state, as well as in the native and in the fully unfolded state, have been explored by f requency-domain fluorometry. Apomyoglobin at acidic pH in the presence of high salt concentration displays bimodal tryptophanyl lifetime dis tributions which may be related to the simultaneous presence of differ ent populations of structural states (compact and fully unfolded state s). The tryptophanyl anisotropy decay indicated that the acidic compac t state displays at least two rotational correlational times, suggesti ng that this state possesses a complex geometrical organization. 1-Ani lino-8-naphthalene sulfonate (ANS), bound both to native and compact p rotein forms, shows broad unimodal lifetime distributions. The small t ime dependence of the ANS emission spectra indicated that the solvent dipolar reorganization are either absent or they occur on a time scale much shorter than the lifetime of the excited ANS molecule bound to a pomyoglobin. The anisotropy decay data relative to the extrinsic fluor ophore (ANS) are consistent with the presence of a single rotational c orrelation time for both native (12.1 ns) and compact (6.2 ns) states.