Disruption of Spatial Proximities among Charged Groups in Equilibrium-Denatured States of Proteins Tracked Using Protein Charge Transfer Spectra.

The absorption and luminescence originating from protein charge transfer spectra (ProCharTS) depend on the proximity between multiple charged groups in a protein. This makes ProCharTS absorbance/luminescence intensity a sensitive probe for detecting changes in the protein structure, which alter the proximity among charged groups in the protein. In this work, ProCharTS absorbance of charge-rich proteins like human serum albumin (HSA), αC, and αW was used to monitor structural changes upon chemical denaturant-induced protein unfolding under equilibrium conditions. The denaturation midpoints were estimated using nonlinear regression analysis. For HSA, absorbance at 325 and 340 nm estimated the GdnHCl-induced denaturation midpoints to be 0.80 and 0.61 M, respectively. A similar analysis of αC and αW ProCharTS absorbance yielded denaturation midpoints of 0.88 and 0.86 M at 325 nm and 0.96 and 0.66 M at 340 nm, respectively. A previously reported molten globule-like state in the GdnHCl-induced HSA unfolding pathway was detected by the increase in HSA ProCharTS absorbance at 0.5 M GdnHCl. To validate the above results, protein unfolding was additionally monitored using conventional methods like circular dichroism (CD), Trp, and dansyl fluorescence. Our results suggest that disruption of charged amino acid sidechain contacts as revealed by ProCharTS occurs at lower denaturant concentrations compared to the loss of secondary/folded structure monitored by CD and fluorescence. Further, HSA ProCharTS absorbance at 315-340 nm revealed that tertiary contacts among charged residues were disrupted at lower GdnHCl concentrations compared to sequence adjacent contacts. Our data underscore the utility of ProCharTS as a novel label-free tool to track unfolding in charge-rich proteins.