Investigation of Novel Spectroscopic Features in the Near Ultraviolet Region Arising from Non-Aromatic Amino Acids in Peptides and Proteins

Abstract

Absorption in the UV region has been dominated by aromatic amino acids in peptides and proteins due to the presence of a conjugated structure in their side chains. However, L-Lysine monohydrochloride displayed new near ultraviolet (270 nm) absorption spectra at high concentrations (0.5 M) in aqueous medium [1]. Similar spectral signatures have been reported for lysine rich proteins such as Human Serum Albumin [2]. The origin of these novel and intriguing spectral features in proteins can only be revealed in the absence of any aromatic moiety.We present here the experimental investigations on absorption spectra of amine containing non-aromatic compounds, lysine containing short peptides in aqueous medium and a lysine rich protein (Alpha3C) which lacks aromatic amino acids in order to elucidate the role of the NH2 moiety behind these spectral properties.We carried out excited state TDDFT electronic structure calculations on lysine residues sampled from classical atomistic molecular dynamics simulations of Alpha3C (PDB Code: 2LXY). The calculated UV-VIS spectra of lysine residues from Alpha3C indicate absorption features above 250 nm in agreement with experiments. The unique spectral signatures of lysine residues are shown to arise from charge transfer and pi-pi∗ transitions involving both, the NH3(+) head group and the peptide backbone. The long tail of the experimental absorption spectra above 250 nm is attributed to the sensitivity of the charge transfer and pi-pi∗ transitions to the lysine conformation and environment. Taken together, our joint experimental and theoretical investigation adds a new dimension towards the understanding of UV-VIS absorption in proteins and amino acids.