Natural fluorescence properties of brome mosaic virus protein

Some of the fluorescence properties of brome mosaic virus protein in different states of aggregation (dimer, capsid) have been studied, in particular the emission spectra, fluorescence quantum yields and lifetimes, as well as the effect of external quenchers and of temperature on the fluorescence. Brome mosaic virus protein, which contains two tryptophan and five tyrosine residues per monomer, displayed an unusual fluorescence spectrum maximum of 308 nm at pH 7.4 when excited at 280 nm. The emission maximum was shifted to 327 nm when excited at 295 nm. Analysis of the results showed that the tyrosine emission is characterized by a high value for the quantum yield (phi = 0.07), which is consistent with a location of most of these residues in helical regions of the protein, while the tryptophan emission is strongly quenched (phi=0.035). The effects of external quenchers suggested that two kinds of tryptophan residues might exist, one buried (phi=0.056) and one exposed, the quantum yield of the latter being particularly low (phi=0.014). The tryptophan fluorescence quenching is partially removed at pH 8.4 and totally eliminated after chemical (guanidinium chloride) or thermal denaturation of the protein. Formation of capsid induced an additional quenching of the exposed tryptophan residue while interaction with the RNA in the virus did not modify the emission parameters of the protein.