Conformers of gaseous threonine

Abstract

Following an extensive search on the potential energy surfaces (PES) of the natural amino acid L-threonine (Thr) and its allotropic form L-allo-threonine (aThr), 56 and 61 conformers of Thr and aThr, respectively, have been located with the help of density functional theory (DFT). Accurate structures, relative energies, rotational as well as quartic and sextic centrifugal distortion constants, dipole moments, ¹⁴N nuclear quadrupole coupling constants, anharmonic vibrational frequencies and double-harmonic infrared intensities have been determined from ab initio electronic structure calculations for the five most stable Thr and aThr conformers. The global minimum, Thr-I, has a cyclic triple H-bond motif with strong OH ··· N, C=O ··· HO, and a weaker NH ··· OH H-bond, where the latter two involves the side chain OH, and an energetically unfavourable trans-COOH arrangement. The best relative energies of the conformers, accurate within ±1 kJ mol⁻¹, have been determined through the first-principles composite focal-point analysis (FPA) approach. There are four and three conformers of Thr and aThr, respectively, within a relative energy of 5 kJ mol⁻¹. Similarly to other amino acids investigated, lower levels of electronic structure theory, especially the Hartree–Fock level, are unable to determine the correct relative energies of the conformers. The rotational, the quartic and sextic centrifugal distortion, and the ¹⁴N nuclear quadrupole coupling constants as well as the anharmonic vibrational fundamentals and double-harmonic infrared intensities, all determined using DFT, should aid identification and characterization of the conformers of threonine and allo-threonine by rotational and vibrational spectroscopies, respectively.

†This paper is dedicated to Professor Fritz Schaefer on the occasion of his 65th birthday.

Keywords:

• neutral L-threonine
• neutral L-allo-threonine
• amino acids
• ab initio calculations
• conformational analysis
• structure elucidation
• focal-point analysis

Acknowledgements

The work described was supported by the Hungarian Scientific Research Fund (OTKA, T72885). The authors are grateful to Ms. Edit Mátyus for her help with some of the electronic structure computations and to Mr. Iván Solt for discussions concerning conformational searches.

Notes

†This paper is dedicated to Professor Fritz Schaefer on the occasion of his 65th birthday.