Title

Conformations of protonated AlaDap and DapAla characterized by IRMPD spectroscopy and molecular modeling

Document Type

Article

Publication Title

Journal of Physical Chemistry B

Department

Chemistry

ISSN

1520-6106

Volume

122

Issue

8

DOI

10.1021/acs.jpcb.7b10435

First Page

2191

Last Page

2202

Publication Date

1-29-2018

Abstract

Oligopeptides containing 2,3-diaminopropionic acid (Dap) serve as a unique model to study conformational effects on the ionizability of a side-chain group. In this study, conformations of acetylated isomeric dipeptide ions containing alanine (Ala) and Dap, AlaDapH+ and DapAlaH+, are studied by infrared multiple photon dissociation (IRMPD) spectroscopy and computation. The IRMPD spectra are characterized in detail by comparing them with theoretical IR spectra of a set of low-energy conformations calculated at the ωB97X-D/6-311+G(d) level of theory. The averaged IR spectra according to the Boltzmann distribution of the set of conformations have a good match to the IRMPD spectra. The characteristic amide I band of AlaDapH+ appears to be downshifted compared to that of DapAlaH+. The relative positions of the amide band suggest a stronger hydrogen-bonding interaction between the charged side-chain amino group and the amide carbonyl groups in AlaDapH+ than in DapAlaH+. The stronger hydrogen bonding in the former is likely due to a better alignment of the N–H and O═C bonds, which enables an effective sequestering of the positive charge at the amino group. The effect results in a higher proton affinity of acetylated dipeptides with the Dap residue at the C-terminus.

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