Optical and hydrodynamic studies of the structure of bacteriophage f2. II. Fluorescence of the capsid.

Fluorescence properties of the icosahedral RNA virus bacteriophage f2 and its empty capsid are reported. Emission is dominated by tryptophan with a maximum wavelength of 320 nm for f2 and its empty capsid. In addition to this short wavelength maximum, perturbation and denaturation studies indicate the inaccessibility of the tryptophan residues. However a high degree of thermal quenching and a red shift in fluorescence emission on heating suggest a noncooperative structural transition, not a denaturation, which allows buried tryptophans to become exposed to solvent. Therefore the tryptophan residues may be located between subunits. Fluorescence from tyrosine is detected near 315 nm for both f2 and its empty capsid, and may indicate an unusual tyrosine environment. Sensitization of tryptophan fluorescence by tyrosine absorption and low values of polarization indicate tyrosine leads to tryptophan and tryptophan leads to tryptophan energy transfer. The presence of RNA in f2 decreases the efficiency of these transfer processes, but does not significantly affect the other reported fluorescence properties.