Wydział Chemii

Robert Wieczorek

J. J. Dannenberg

2008

Journal of Physical Chemistry B

112

1320-1328

10.1021/jp077527j

Naukowa

Angielski

Artykuł

We present ONIOM and pure DFT calculations on infrared spectra of α-helical-capped polyalanines. The calculations used two-layer ONIOM (B3LYP/D95**:AM1) calculations of the amide I vibrational frequencies for acetyl(ala)_NNH₂ (N = 8, 10, 12−18) whose structures have been previously completely optimized by the same method. These are the first such calculations based upon structures of α-helical peptides that are completely optimized using DFT or molecular orbital methods. As the peptide becomes longer, the amide I band becomes both more intense and more red shifted. However, the individual absorptions that contribute most to the band vary between three patterns:  one very intense absorption, two absorptions of similar intensity, and two strong absorptions where one is roughly twice as intense as the other. This pattern appears to be related to the relative number of H bonds in the individual H-bonding chains; however, there is one exception. Using ¹⁴CO's to selectively decouple specific CO's, we found that the couplings between the CO's within each of the three individual H-bonding chains within the helices follow the same pattern previously reported for planar H-bonding chains of formamides. The coupling between the H-bonding chains appears to involve through-space coupling between the H-bonding chains. While decoupling individual CO's always decreases the intensity of the amide I band, it leads to complex changes in the individual amide I absorptions that contribute to the band. Depending upon the position of the ¹⁴CO, the amide I band can either red or blue shift. Moreover, the individual absorptions that contribute to the band can increase or decrease in intensity as well as shift. The patterns of the individual absorptions (mentioned above) also change. Using the CO stretch of acetamide as a reference, we calculate the red shifts for the most intense absorptions to be much greater than predicted by the transition dipole method.

Absorption, Amides, Coupling reactions, Molecular structure, Peptides and proteins

https://doi.org/10.1021/jp077527j

https://www.acs.org/content/acs/en.html