Tobacco mosaic virus and its protein exhibit pH-dependent changes in their wavelength and intensity of maximum emission, λmax,pro and Ipro, respectively. The binding of 2-p-toluidinylnaphthalene-6-sulfonate (TNS) to tobacco mosaic virus protein or tobacco mosaic virus produces a strong increase in its fluorescence intensity, ITNS. This binding is pH dependent. The protein is severely denatured at pH >; 10.5 or < 3 as indicated by large increases in λmax,pro. Strong increases in ITNS occur concomitantly with these increases in λmax,pro, indicating increased binding of TNS upon denaturation. Presumably hydrophobic sites, exposed on denaturation, provide effective binding sites for TNS. Heat denaturation of tobacco mosaic virus protein at pH 7 led to similar increases in λmax,pro and ITNS. In the pH range where aggregation of the protein occurs, pH 6-7, λmax,pro remains constant and only minor changes in Ipro and ITNS occur. Major conformational changes do not appear to accompany aggregation. No significant changes in λmax,pro, Ipro and ITNS occur between pH 6 and 7 when the native virus is titrated. The changes in Ipro and ITNS for tobacco mosaic virus protein in this pH range are apparently associated with the aggregation of the protein. TNS is bound more weakly to the virus than to the protein at all pH values.
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