Charge Transfer Transitions Originating from Charged Amino Acids Account for 300-800 nm UV-Visible Electronic Absorption Spectra in Proteins

Abstract

The electronic absorption spectra of the protein folds are primarily characterized over the ultraviolet region (180 nm to 320 nm) of the electromagnetic spectrum with distinctive absorption features attributed to peptide bonds (∼ 190 nm) and aromatic amino acids (∼270 nm). While previous studies have reported absorption beyond 320 nm in Lys-rich proteins, the exact origin/nature of such transitions have remained elusive. Here, in a joint theoretical and experimental study, we report the distinctive UV-Vis absorption profile of a synthetic 67 residue protein (α3C) devoid of aromatic amino acids upto 800 nm. Systematic control studies on high concentration non-aromatic amino acid solutions revealed significant absorption beyond 320 nm for charged amino acids which constitute over 50% of the sequence composition in α3C. Excited state electronic structure calculations on charged amino acid residues sampled from classical atomistic molecular dynamics simulations of α3C reveal that the novel absorption features of α3C arise from charge transfer (CT) transitions involving Lys/Glu amino acids. The charged amino (NH3+)/carboxylate (COO-) headgroups of Lys/Glu side chains act as photo-induced charge acceptors/donors for electron transfer from/to the polypeptide backbone. We further show that the sensitivity of the CT transitions to specific associations between charged amino acids conferred by the 3D protein fold, amino acid conformation, and the solvation of the Lys amino and Glu carboxylate groups leads to the long tail of the α3C absorption profile (320-800 nm). We experimentally demonstrate sensitivity of the absorption spectra to temperature induced perturbations which presumably alter the interactions within the protein tertiary structure while keeping the secondary structure intact. Taken together, our joint experimental and theoretical investigation adds Protein Charge Transfer Spectra (ProCharTS), a new optical UV-Vis spectral window for probing protein structure and dynamics.