title: The influence of the peptide bond on the conformation of amino acids: a theoretical and FT-IR matrix-isolation study of N-acetylproline.
abstract: A combined experimental matrix-isolation FT-IR and theoretical study has been performed to investigate the conformational behavior of N-acetylproline. The conformational landscape of N-acetylproline was explored using successively higher computational methods, i.e. HF, DFT(B3LYP) and finally MP2. The exploration resulted in 10 conformations with a relative energy difference smaller than 22 kJ.mol(-1) at the HF/3-21G level of theory. These conformations led to six different conformations after DFT(B3LYP) optimizations. Further optimization at the MP2/6-31++G** level of theory resulted in the same six conformations, all of them with an energy difference smaller than 11.5kJ.mol(-1). One conformation with an intramolecular H-bond was found which was energetically the most favorable conformation. The vibrational and thermodynamical features were calculated using the DFT and MP2 methodologies. In the experimental matrix-isolation FT-IR spectrum, the most stable conformation was dominant and at least two non-H-bonded conformations could be identified. An experimental rotamerization constant between the H-bonded and the other non-H-bonded conformations was estimated and appeared to agree reasonably well with the theoretical MP2 predictions. Some new spectral features of N-acetylproline compared to proline were discovered which might be used to discriminate between the acetylated and non-acetylated form.
authors.affiliation: Department of Chemistry, University of Leuven, Celestijnenlaan 200 F, B-3001 Leuven, Belgium. boeckx@hotmail.com
authors.forename: Bram
authors.initials: B
authors.lastname: Boeckx
authors.forename: Riet
authors.initials: R
authors.lastname: Ramaekers
authors.forename: Guido
authors.initials: G
authors.lastname: Maes
mesh_terms: D008958:Models, Molecular; D008968:Molecular Conformation; D010455:Peptides / Q000737:chemistry*; D011392:Proline / Q000031:analogs & derivatives* / Q000737:chemistry; D011789:Quantum Theory; D017550:Spectroscopy, Fourier Transform Infrared
chemical_list: D010455:Peptides; D011392:Proline; C055247:N-acetylproline