On the Temperature Dependence of Amide I Frequencies of Peptides in Solution

Abstract

The temperature dependence of the amide I vibrational frequencies of peptides in solution was investigated. In D2O, the amide I' bands of both an alpha-helical oligopeptide, the random-coil poly(L-lysine), and the simplest amide, N-methyl acetamide (NMA), exhibit linear frequency shifts of approximately 0.07 cm(-1)/degrees C with increasing temperature. Similar amide I frequency shifts are also observed for NMA in both polar (acetonitrile and DMSO) and nonpolar (1,4-dioxane) organic solvents, thus ruling out hydrogen-bonding strength as the cause of these effects. The experimental NMA amide I frequencies in the organic solvents can be accurately described by a simple theory based on the Onsager reaction field with temperature-dependent solvent dielectric properties and a solute molecular cavity. DFT-level calculations (BPW91/cc-pVDZ) for NMA with an Onsager reaction field confirm the significant contribution of the molecular cavity to the predicted amide I frequencies. Comparison of the computations to experimental data shows that the frequency-dependent response of the reaction field, taken into account by the index of refraction, is crucial for describing the amide I frequencies in polar solvents. The poor predictions of the model for the NMA amide I band in D2O might be due, in part, to the unknown temperature dependence of the refractive index of D2O in the mid-IR range, which was approximated by the available values in the visible region.

Transient two-dimensional spectroscopy with linear absorption corrections applied to temperature-jump two-dimensional infrared

“…The amide I' peak difference spectrum (upper right corner) appears to be the result of two loss features (marked with vertical lines), but the observed doublet can be more consistently described by loss of the equilibrium peak and gain of a narrower center peak, which results in the low amplitude, broad off-diagonal gain intensity. The dominance of loss features over gain is consistent with the transient absorption spectrum and may be the result of non-Condon effects, like those observed in temperature-dependent FTIR studies of NMA [39,40] or shifting of oscillator strength to other coupled vibrational modes that lie outside of our spectral window.…”

Folding of a heterogeneous β-hairpin peptide from temperature-jump 2D IR spectroscopy

“…On heating, the UL β-sheet peaks decrease and broaden concomitant with the increase of intensity at 1,660 cm −1 , spectral changes that can be assigned to full disordering or partial fraying of the structure, which leaves residual β-turns (9, 10). Broadening and blue-shifting of peaks were also observed and are expected based on the general amide I′ response to temperature (23,33). Although both K8 turn configurations are lost with increasing temperature due to net conversion to a disordered or partially disordered state, the native type I′ β-turn configuration (K8-2) is retained at a higher population relative to the bulged turn population (K8-1) (10).…”

The non-uniform early structural response of globular proteins to cold denaturing conditions: A case study with Yfh1

“…Other factors that might cause spectral red-shifts of the amide-I modes upon lowering of temperature are increase in hydrogen bond strength 72,73 and temperature dependence of the solvent dielectric properties. 74,75 Figure 4 suggests greater solvent accessibility for the helical domains than that of the beta sheet regions at T s . If increasing hydrogen bond strength at T c was the dominant factor resulting in the spectral red-shifts, a larger red-shift would be expected for the helical domains.…”