Classification of protein solubilizing solutes by fluorescence assay

Aromatic interaction plays a crucial role in controlling protein interaction by additives. Here we investigated the interaction of protein salting-in (solubilizing) additives with tryptophan (Trp), tyrosine (Tyr), indole, and proteins based on their fluorescence spectra. Five salting-in additives, i.e., arginine (Arg), urea, guanidine (Gdn), ethylene glycol (EG), and magnesium chloride (MgCl₂), showed different effects on the fluorescence properties of Trp and Tyr. Arg significantly reduced fluorescence intensity of Trp and Tyr, as was the case for glycine to a lesser extent. MgCl₂ and calcium chloride (CaCl₂) showed little effect on the aromatic fluorescence spectra. Gdn also showed little effect on the aromatic fluorescence spectra of Trp and Tyr even at high concentrations. EG increased the aromatic fluorescence intensity of Trp and Tyr with blue-shifted emission wavelength. Urea enhanced fluorescence of Trp and Tyr without altering emission wavelength. These results indicate that the protein solubilizing additives interact with aromatic groups differently.