Near UV-Visible electronic absorption originating from charged amino acids in a monomeric protein

Electronic absorption spectra of proteins are primarily characterized over the ultraviolet region (185-320 nm) of the electromagnetic spectrum. While recent studies on peptide aggregates have revealed absorption beyond 350 nm, monomeric proteins lacking aromatic amino acids, disulphide bonds, and active site prosthetic groups are expected to remain optically silent beyond 250 nm. Here, in a joint theoretical and experimental investigation, we report the distinctive UV-Vis absorption spectrum between 250 nm [ε = 7338 M^-1 cm^-1] and 800 nm [ε = 501 M^-1 cm^-1] in a synthetic 67 residue protein (α₃C), in monomeric form, devoid of aromatic amino acids. Systematic control studies with high concentration non-aromatic amino acid solutions revealed significant absorption beyond 250 nm for charged amino acids which constitute over 50% of the sequence composition in α₃C. Classical atomistic molecular dynamics (MD) simulations of α₃C reveal dynamic interactions between multiple charged sidechains of Lys and Glu residues present in α₃C. Time-dependent density functional theory calculations on charged amino acid residues sampled from the MD trajectories of α₃C reveal that the distinctive absorption features of α₃C may arise from two different types of charge transfer (CT) transitions involving spatially proximal Lys/Glu amino acids. Specifically, we show that the charged amino (NH₃⁺)/carboxylate (COO^-) groups of Lys/Glu sidechains act as electronic charge acceptors/donors for photoinduced electron transfer either from/to the polypeptide backbone or to each other. Further, the sensitivity of the CT spectra to close/far/intermediate range of encounters between sidechains of Lys/Glu owing to the three dimensional protein fold can create the long tail in the α₃C absorption profile between 300 and 800 nm. Finally, we experimentally demonstrate the sensitivity of α₃C absorption spectrum to temperature and pH-induced changes in protein structure. Taken together, our investigation significantly expands the pool of spectroscopically active biomolecular chromophores and adds an optical 250-800 nm spectral window, which we term ProCharTS (Protein Charge Transfer Spectra), for label free probes of biomolecular structure and dynamics.