UV–Visible Lysine–Glutamate Dimer Excitations in Protein Charge Transfer Spectra: TDDFT Descriptions Using an Optimally Tuned CAM-B3LYP Functional

Recent reports of distinctive UV–vis absorption profiles for monomeric proteins rich in charged amino acids that span 250–800 nm have opened up a new label-free optical spectral window for probing biomolecular structure and interactions. Combined experimental-computational studies have revealed that such broad absorption profiles of these proteins arise from photoexcited charge transfer (CT) transitions in spatially proximal charged amino acids such as lysine (Lys) and glutamate (Glu). Here, using time-dependent density functional theory (TDDFT) with an optimally tuned CAM-B3LYP functional, we refine the computed UV–vis spectra for Lys-Glu dimers within protein folds and quantify the percentage CT character of the constituent transitions. The optimally tuned functionals are derived through a careful analysis of the CAM-B3LYP parameter space for Lys-Glu dimers as a function of amino-acid conformation and side chain separation. Our studies reveal that the tuned Lys-Glu dimer spectrum spans 150–650 nm and possesses 5 specific types of CT excitations with diverse and large spatial charge separation length scales of 2–10 Å. These include inter-/intra-residue peptide backbone to peptide backbone (BB-CT) excitations spanning 160–210 nm, inter-/intra-residue peptide backbone to side chain (BS-CT) excitations spanning 160–260 nm, and side chain to side chain (SS-CT) excitations, which show the broadest absorption range spanning 260–650 nm.