Phosphorescence and optical detection of magnetic resonance of cowpea chlorotic mottle virus

Phosphorescence spectra of the tryptophan residues in cowpea chlorotic mottle virus were recorded at 77 K and the influence of the quaternary structure on the emission characteristics was investigated. The position of the phosphorescence maxima appeared to be invariant under changes in the aggregation state of the virus particle. In contrast to the results of fluorescence experiments, the phosphorescence probably originates from tryptophan residues, buried in the hydrophobic interior of the virus. Optical detection of magnetic resonance on the triplet state of the tryptophan residues at 1.2 K shows a slight shift in the zero-field transitions, when the interaction between the protein and the RNA is abolished. This shift is discussed in relation with changes in polarity and in polarizability of the environment of the phosphorescing tryptophan residues when the interaction between RNA and the protein subunits decreases. The zero-field transitions in the virus are further characterized by a large linewidth, when comparisons are made with similar transitions observed in other proteins. This shows great heterogeneity in the environment of tryptophan residues, and makes the recognition and interpretation of changes in the transitions very complicated.