Low Energy Optical Excitations as an Indicator of Structural Changes Initiated at the Termini of Amyloid Protein
There is a growing body of experimental work showing that protein aggregates associated with amyloid fibrils feature an intrinsic fluorescence. In order to understand the microscopic origin of this behavior observed in non-aromatic aggregates of peptides and proteins, we conducted a combined experimental and computational study on the optical properties of amyloid-derived oligopeptides in the near-UV region. We have focused on a few model systems having charged termini (zwitterionic) or acetylated termini. For the zwitterionic system, we were able to simulate the longer tail absorption in the near UV (250-350 nm), supporting the experimental results in term of excitation spectra. We analyzed optical excitations responsible for the low-energy absorption and found a large role played by charge-transfer states around the termini. These charge-transfer excitations are very sensitive to the conformation of the peptide and in the realistic fibrils may involve inter and intra chain charge reorganization.