On the temperature dependence of amide I frequencies of peptides in solution.
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.
Amunson
Krista E
KE
Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071, USA.
Kubelka
Jan
J
D000081:Acetamides; D000577:Amides / Q000737:chemistry*; D017666:Deuterium Oxide / Q000737:chemistry; D004563:Electrochemistry; D006860:Hydrogen Bonding; D015233:Models, Statistical; D010455:Peptides / Q000737:chemistry*; D012996:Solutions; D012997:Solvents; D017550:Spectroscopy, Fourier Transform Infrared; D013696:Temperature
D000081:Acetamides; D000577:Amides; D010455:Peptides; D012996:Solutions; D012997:Solvents; D017666:Deuterium Oxide; C018595:N-methylacetamide