Transition Dipole Strength as a Quantitative Tool for Protein Secondary Structure Analysis.

Proteins adopt complex structures through dynamic folding processes that are challenging to capture experimentally. This study advances our understanding of the structural information that can be obtained by transition dipole strength (TDS) analysis, an innovative extension of two-dimensional infrared (2D IR) spectroscopy. By systematically characterizing the TDS of model α-helical peptides, we find a linear correlation between TDS values and helical length that can be used to extrapolate the maximum α-helical lengths in globular proteins, even when multiple helices are present. In contrast, the interpretation of TDS values for β-sheet structures is complicated by their increased structural diversity. While TDS is generally expected to increase with the number of β-strands, we find that actual values can vary greatly depending on higher order structural organization, including the three-dimensional folding of the peptide chain and the formation of protein complexes. This work demonstrates TDS analysis as a promising method for the elucidation of structural dynamics that cannot be obtained by other methods, especially in complex protein architectures, while highlighting the need for an increased understanding of the interplay of higher order structural organization with vibrational delocalization.