Near Ultraviolet Absorption Arising from Lysine Residues in Close Proximity: A Probe to Monitor Protein Unfolding and Aggregation in Lysine-Rich Proteins

There is a need for an intrinsic spectral probe to monitor key events like protein unfolding and aggregation in a rapid and unambiguous manner. Protein aggregation is an important issue, but ironically there is a dearth of simple techniques to directly detect the presence of aggregates in solution. We report here the hitherto undiscovered electronic absorption around 300–350 nm in aqueous solutions (pH 7) of human serum albumin (HA), calf thymus histone, and poly-L-lysine. The above spectra were significantly absent in controls like subtilisin carlsberg, mutant barstar, and lysozyme. The possibilities that Rayleigh scattering or impurities could account for the above spectra were checked and ruled out. Based on the analysis of available three-dimensional structures from PDB and our earlier work on the lysine amino acid, an intramolecular interaction between lysine side chains in close spatial proximity was deduced to be the origin for the above spectra. The utility of Lys–Lys interaction in detecting protein unfolding and aggregation in a lysine-rich protein like calf thymus histone using near-ultraviolet absorption is demonstrated.