On the temperature dependence of amide I intensities of peptides in solution

Temperature dependence of the amide I/I′ spectral intensities is investigated for N-methylacetamide (NMA) as a model compound for the peptide bond in D2O and three organic solvents with different polarities (dimethyl sulfoxide (DMSO), acetonitrile and 1,4-dioxane). The total amide I/I′ intensity (dipole strength) systematically decreases in less polar solvents as well as with the increasing temperature. Decreased solvent polarity results in the narrowing of the amide I bandwidths, while increasing temperature predominantly reduces the peak absorbance, with only a small effect on the spectral width. In D2O, the NMA amide I′ dipole strength decreases by 1.7 × 10−4 Debye2/deg, in DMSO, acetonitrile and 1,4-dioxane by 1.0 × 10−4 Debye2/deg. The amide I/I′ intensity variations in the non-protic solvents rule out hydrogen bonding as the sole source of these effects. The experimental NMA amide I dipole strengths in the organic solvents are accurately described by a simple theory based on the Onsager reaction field with temperature-dependent solvent dielectric constant, refractive index and the solute molecular cavity, which can be approximated using NMA density. Experimental results are compared to density functional theory (DFT) BPW91/cc-pVDZ/Onsager calculations. The computations significantly overestimate the absolute experimental amide I intensities, but comparison of the relative values underscores the importance of the temperature-dependent molecular cavity dimension (density) as well as the frequency-dependent response of the reaction field (index of refraction) for describing the amide I spectral intensities in polar solvents. Correlations between temperature-dependent amide I frequencies and intensities, and their possible utility for analyses of the temperature-dependent peptide and protein infrared spectroscopy (IR) spectra are discussed.