Geometry, transition states, and vibrational spectra of boron isostere of N-methylacetamide by ab initio calculations

The basic unit of the peptide, the CONH bond, is responsible for imparting specific secondary structural features to peptides. Many modifications, such as isosteric/bioisosteric replacement of this basic unit, have been made to alter the properties of peptides. Isosteric replacement of the nitrogen atom by boron (boron peptides) is another plausible way of changing peptide characteristics. Like N-methylacetamide (NMA), acetylmethylborane, or BMA, is a good model for studying boron peptides. The potential energy surface (PES) of BMA has been studied by Hartree-Fock (HF), density functional theory (DFT), and post-HF methods. The PES of BMA is an inverted form of the NMA hypersurface. Two minima and two saddle points of index 1 have been identified on this PES and fully optimized at the HF, Becke's three-parameter exchange functional and the gradient-corrected functional of Lee, Yang, and Paar, second-order Møller-Plesset (full), and quadratic configuration interaction method with single and double substitutions followed by a perturbative treatment of triple substitutions (QCISD) levels of theory. The minima correspond to structures with omega values close to 90° and 270°, and the transition states have omega values around 0° and 180°. The energy barrier for rotation around the omega bond is found to be 4.3 kcal/mol at the QCISD/6-31G* level, which is much lower than the 16-23 kcal/mol barrier in NMA. The unique shape of the PES of BMA and the low barrier to rotation can be well explained by second-order orbital interaction studies. Normal-mode analysis reveals a significant shift in the BH stretching frequency of BMA from that in alkyl boranes and borane-phosphine complexes. The replacement of nitrogen by boron bestows on such peptides greater proteolytic stability, more flexibility around the omega angle, and a unique preference for positive angles, all of which are absent in peptides of natural origin