Single-Conformation Spectroscopy of Capped Aminoisobutryic Acid Dipeptides: The Effect of C-Terminal Cap Chromophore on Conformation.

08:00 EDT 11th April 2019 | BioPortfolio

Summary of "Single-Conformation Spectroscopy of Capped Aminoisobutryic Acid Dipeptides: The Effect of C-Terminal Cap Chromophore on Conformation."

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.


Journal Details

This article was published in the following journal.

Name: The journal of physical chemistry. A
ISSN: 1520-5215


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