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 which 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 in 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 sidechain separations. Our studies reveal that the tuned Lys-Glu dimer spectrum spans 150-650 nm and p...