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Structure and IR Spectrum of Phenylalanyl–Glycyl–Glycine Tripetide in the Gas‐Phase: IR/UV Experiments, Ab Initio Quantum Chemical Calculations, and Molecular Dynamic Simulations
Author(s) -
Řeha D.,
Valdés H.,
Vondrášek J.,
Hobza P.,
AbuRiziq Ali,
Crews Bridgit,
de Vries Mattanjah S.
Publication year - 2005
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200500465
Subject(s) - ab initio , potential energy surface , chemistry , basis set , intramolecular force , potential energy , ab initio quantum chemistry methods , computational chemistry , rigid rotor , molecule , density functional theory , atomic physics , physics , quantum mechanics , organic chemistry , rotor (electric) , stereochemistry
We investigated the potential‐energy surface (PES) of the phenylalanyl–glycyl–glycine tripeptide in the gas phase by means of IR/UV double‐resonance spectroscopy, and quantum chemical and statistical thermodynamic calculations. Experimentally, we observed four conformational structures and we recorded their IR spectra in the spectral region of 3000–4000 cm −1 . Computationally, we investigated the PES by a combination of molecular dynamics/quenching procedures with high‐level correlated ab initio calculations. We found that neither empirical potentials nor various DFT functionals provide satisfactory results. On the other hand, the approximative DFT method covering the dispersion energy yields a reliable set of the most stable structures, which we subsequently investigated with an accurate, correlated ab initio treatment. The global minimum contains three moderately strong intramolecular hydrogen bonds and is mainly stabilized by London dispersion forces between the phenyl ring, the carboxylic acid group, and various peptide bonds. A proper description of the last type of interaction requires accurate correlated ab initio calculations, including the complete basis set limit of the MP2 method and CCSD(T) correction terms. Since in our beam experiments the conformations are frozen by cooling from a higher temperature, it is necessary to localize the most stable structures on the free‐energy surface rather than on the PES. We used two different procedures (rigid rotor/harmonic oscillator/ideal gas approximation based on ab initio characteristics and evaluation of relative populations from the molecular dynamic simulations using the AMBER potential) and both yield four structures, the global minimum and three local minima. These four structures were among the 15 most energetically stable structures obtained from accurate ab initio optimization. The calculated IR spectra for these four structures agree well with the experimental frequencies, which validates the localization procedure.