Not All β-Sheets Are the Same: Amyloid Infrared Spectra, Transition Dipole Strengths, and Couplings Investigated by 2D IR Spectroscopy

We report the transition dipole strengths and frequencies of the amyloid beta-sheet amide I mode for the aggregated proteins amyloid-beta(140), calcitonin, alpha-synuclein, and glucagon. According to standard vibrational coupling models for proteins, the frequencies of canonical beta-sheets are set by their size and structural and environmental disorder, which determines the delocalization length of the vibrational excitons. The larger the delocalization the lower the frequency of the main infrared-allowed transition, A(1). The models also predict an accompanying increase in transition dipole strength. For the proteins measured here, we find no correlation between transition dipole strengths and amyloid beta-sheet transition frequency. To understand this observation, we have extracted from the protein data bank crystal structures of amyloid peptides from which we calculate the amide I vibrational couplings, and we use these in a model beta-sheet Hamiltonian to simulate amyloid vibrational spectra. We find that the variations in amyloid beta-sheet structures (e.g., dihedral angles, interstrand distances, and orientations) create significant differences in the average values for interstrand and nearest neighbor couplings, and that those variations encompass the variation in measured A(1) frequencies. We also find that off-diagonal disorder about the average values explains the range of transition dipole strengths observed experimentally. Thus, we conclude that the lack of correlation between transition dipole-strength and frequency is caused by variations in amyloid beta-sheet structure. Taken together, these results indicate that the amide I frequency is very sensitive to amyloid beta-sheet structure, the beta-sheets of these 4 proteins are not identical, and the assumption that frequency of amyloids scales with beta-sheet size cannot be adopted without an accompanying measurement of transition dipole strengths.