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Aminoisobutyric acid (Aib) oligomers are known to form racemic mixtures of enantiomeric left- and right-handed structures. The introduction of a chiral cap converts the enantiomeric structures into diastereomers that, in principle, afford spectroscopic differentiation. Here, we screen different C-terminal caps based on a model Aib dipeptide using double-resonance laser spectroscopy in the gas phase to record IR and UV spectra of individual conformations present in the supersonic expansion: NH-benzyl (NHBn) as a reference structure due to its common use as a fluorophore in similar studies, NH-p-fluorobenzyl (NHBn-F), and alpha-methylbenzylamine (AMBA). For both the NHBn and NHBn-F caps, a single conformer is observed, with infrared spectra assignable to an enantiomeric pair of Type II/II' β-turns in these molecules lacking a chiral center. The higher oscillator strength of the NHBn-F cap enabled UV-UV-holeburning, not readily accomplished with the NHBn cap. The AMBA capped structure, with its chiral center, produced two unique conformers, one of which was a nearly identical left-handed Type II β-turn, while the minor conformer is assigned to a C7-C7 sequential double ring which is an emergent form of a 27-ribbon. While not observed, the Type II' β-turn diastereomer, with opposite handedness, is calculated to be 11 kJ/mol higher in energy, a surprisingly large difference. This destabilization is attributed primarily to steric interference between the C-terminal acyl oxygen of the peptide and the chirality-inducing methyl of the AMBA group. Lastly, computational evidence indicates that the use of an N-terminal aromatic cap hinders formation of a 3¬10-helix in Ac-Aib2 dipeptides.
This article was published in the following journal.
Name: The journal of physical chemistry. A
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Peptides composed of two amino acid units.
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A synthetic PGE2 analog that has an inhibitory effect on gastric acid secretion, a mucoprotective effect, and a postprandial lowering effect on gastrin. It has been shown to be efficient and safe in the treatment of gastroduodenal ulcers.
Spectroscopy technique which measures changes in organic compounds by tracking the spectral energy of absorption of HYDROGEN atoms.