Chiral and Achiral Fundamental Conformational Building Units of β-Peptides: A Matrix Isolation Conformational Study on Ac-β-HGly-NHMe and Ac-β-HAla-NHMe

Abstract

The infrared spectra of two model beta-peptides, N-acetyl-3-aminopropionic acid-N'-methylamide (Ac-beta-HGly-NHMe) and N-acetyl-3-aminobutanoic acid-N'-methylamide (Ac-beta-HAla-NHMe), have been recorded in low-temperature Ar and Kr matrixes. The spectra were assigned by the help of electronic structure calculations. The analysis of spectra, in line with the theoretical predictions, revealed that both biocompatible peptide building blocks have a single dominant backbone conformer. Besides this prevalent conformer, which has a six-membered H-bonded pseudoring, conformers with eight-membered H-bonded rings are also observed but in a significantly smaller amount. The calculated conformer distribution is consistent with the experimental findings. The present work along with other recent results supports the concept that the backbone conformation of longer biopolymers, such as alpha- and beta-peptides, can be deciphered using the conformers of their structural building blocks. In this respect, our conformational study on the simplest models for beta-peptides both by IR spectroscopic experiments and quantum chemical studies has significance for the better design and understanding of the backbone conformations of larger beta-peptides with biomedical potential. The present conformational analysis suggests that although beta-peptides, having an "extra" backbone torsion and hence more conformational freedom, should be more flexible than alpha-peptides, fewer backbone conformers are viable based on their relative energies. Thus, from a larger conformational arsenal, only a lower number of backbone conformers can emerge, which possibly had a fundamental effect on their applicability during prebiotic evolution.