Experimental Studies of Peptide Bonds: Identification of the C7eq Conformation of the Alanine Dipeptide Analog N-Acetyl-Alanine N'-Methylamide From Torsion-Rotation Interactions

Published

January 1, 2003

Author(s)

R J. Lavrich, David F. Plusquellic, R D. Suenram, Gerald T. Fraser, Angela R. Hight Walker, M J. Tubergen

Abstract

Rotational spectra of the bio-mimetic molecule, alanine dipepitide and the double ¹⁵N(¹⁵N₂) isotopomer have been observed using a pulsed-molecular-beam Fourier transform microwave spectrometer. The spectra reveal large tunneling splittings from the torsional mode structure of two of its three-methyl rotors. The torsional states assigned include one AA-state and two AE-states (i.e. AE and EA) for each isotopomer. The AA-states are well-fit to A-reduction asymmetric-rotor Hamiltonians. The infinite-barrier-limit rotational constants of the ¹⁵N₂ isotopomer are: A=1699.881(5).MHz, B = 986.388(5) MHz, and C = 711.644(5) MHz. The AE-states are analyzed independently using high-barrier torsion-rotation Hamiltonians, yielding observed-minus-calculated standard deviations of 100 fold for the ¹⁵N₂ isotopomer) when analyzed in non-principal axis frames. The best-fit torsion-rotation parameters provide accurate V₃ barriers and C₃ rotor axis angles for both methyl groups. The observed angles are shown to uniquely correlate with those calculated for the acetyl and amide methyl groups in the C₇^eq conformational form. The V₃ barriers of the amide and acetyl methyl groups are 86.3(3) and 98.6(3) cm^-1 for the ¹⁴N₂ and 86.3(1) and 98. 9(1) cm^-1 for the ¹⁵N₂ isotopomers, respectively. These results are in good agreement with prior geometry optimizations and with current V₃ barrier calculations which predict the C₇^eq conformation as the lowest energy form in the gas phase. Under certain conditions, the spectrum is dominated by transitions from a decomposition product formed by dehydration of alanine dipeptide. This molecule is tentatively identified as 3,5-dihydro-2,3,5-trimethyl-(9CI)4H imidazole-4-one (CAS registry #32023-93-1).

Citation

Journal of Chemical Physics

Volume

118

Issue

No. 3

Pub Type

Journals

Keywords

dipeptide, Fourier-Transform Spectroscopy, rotational spectrum, structure, torsional-rotation splitting

Citation

Lavrich, R. , Plusquellic, D. , Suenram, R. , Fraser, G. , Hight, A. and Tubergen, M. (2003), Experimental Studies of Peptide Bonds: Identification of the C₇^eq Conformation of the Alanine Dipeptide Analog N-Acetyl-Alanine N'-Methylamide From Torsion-Rotation Interactions, Journal of Chemical Physics (Accessed April 20, 2024)