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Vibrational circular dichroism and IR spectral analysis as a test of theoretical conformational modeling for a cyclic hexapeptide
Author(s) -
Bour Petr,
Kim Joohyun,
Kapitan Josef,
Hammer Robert P.,
Huang Rong,
Wu Ling,
Keiderling Timothy A.
Publication year - 2008
Publication title -
chirality
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.43
H-Index - 77
eISSN - 1520-636X
pISSN - 0899-0042
DOI - 10.1002/chir.20560
Subject(s) - chemistry , molecular dynamics , vibrational circular dichroism , side chain , hydrogen bond , protonation , spectral line , computational chemistry , simulated annealing , density functional theory , crystallography , basis set , circular dichroism , molecule , peptide , stereochemistry , organic chemistry , ion , biochemistry , physics , algorithm , astronomy , computer science , polymer
A model cyclohexapeptide, cyclo‐(Phe‐ D Pro‐Gly‐Arg‐Gly‐Asp) was synthesized and its IR and VCD spectra were used as a test of density functional theory (DFT) level predictions of spectral intensities for a peptide with a nonrepeating but partially constricted conformation. Peptide structure and flexibility was estimated by molecular dynamics (MD) simulations and the spectra were simulated using full quantum mechanical (QM) approaches for the complete peptide and for simplified models with truncated side chains. After simulated annealing, the backbone conformation of the ring structure is relatively stable, consisting of a normal β‐turn and a tight loop (no H‐bond) which does not vary over short trajectories. Only in quite long MD runs at high temperatures do other conformations appear. MD simulations were carried out for the cyclic peptide in water and in TFE, which match experimental solvents, as well as with and without protonation of the Asp carboxyl group. DFT spectral simulations were made using the annealed structure and were extended to include basis set variation, to determine an optimal computational approach, and solvent simulation with a polarized continuum model (PCM). Stepwise full DFT simulation of spectra was done for various sequences with the same backbone geometry but based on (1) solely Gly residues, (2) Ala substitution except Gly and Pro, and (3) complete sequences with side chains. Additionally, a selection of structures was used to compute IR and VCD spectra with the optimal method to determine structural variation effects. The side chains, especially the AspCOOH and ArgNH 2 transitions, had an impact on the computed amide frequencies, IR intensities and VCD pattern. Since experimentally these groups would have little chirality, due to conformational variation, they do not impact the observed VCD spectra. Correcting for frequency shifts, the Ala model for the cyclopeptide gives the clearest representation of the amide VCD. The experimental sign pattern for the amide I' band in D 2 O and also the sharper, more intense amide I VCD band in TFE was seen to some degree in one conformer with Type II′ turns, but the data favor a mix of structures. Chirality, 2008. © 2008 Wiley‐Liss, Inc.