Temperature dependence of peptide carboxylic group IR spectra in the amide I′ region at neutral and acidic pH

Analysis of the temperature-dependent amide I′ bands of peptides and proteins can be complicated by their overlap with other IR bands, particularly those of carboxylic groups of amino acid side-chains and the C-terminus. Previously, we reported IR spectra of charged carboxylic side-chains in Asp and Glu amino acids, and C-terminal groups of several amino acids and dipeptides at neutral pH. To complement these results, here we investigate the IR absorptions of Asp and Glu side-chains in capped amino acids (AcAspNMe and AcGluNMe), at both neutral and acidic pH. Spectra of protonated (acidic pH) C-terminal group absorptions are also investigated, using three dipeptides (AlaGly GlyAla and GlyGly) as model compounds. Sets of temperature-dependent experimental IR spectra were analyzed using pseudo-Voigt lineshape profiles. We find that the temperature-dependent behavior of the IR bands of deprotonated (neutral pH) side-chains in AcAspNMe and AcGluNMe dipeptides are generally similar to those reported previously for Asp and Glu. Protonated carboxylic group (acidic pH) IR bands behave uniformly with respect to temperature, showing very similar magnitude frequency shifts and intensity changes. Implications for analyses of amide I′ bands of peptides and proteins are discussed.