Rotational spectra of the biomimetic molecule, alanine dipeptide and the double N15(N215) isotopomer have been observed using a pulsed-molecular-beam Fourier transform microwave spectrometer. The spectra reveal 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 asymmetricrotor Hamiltonians. The “infinite-barrier-limit” rotational constants of the N214 isotopomer are A=1710.97(8) MHz, B=991.89(9) MHz, and C=716.12(6) MHz. The AE-states are analyzed independently using “high-barrier” torsion-rotation Hamiltonians, yielding observedminus-calculated standard deviations of <400 kHz. The fits improve substantially (>100-fold for the N215 isotopomer) when analyzed in a ρ-axis frame where ρb=ρc=0. 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 C7eq conformational form. The V3 barriers of the amide and acetyl methyl groups are 84.0(3) cm−1 and 98.4(2) cm−1 for the N214 and 84.1(1) cm−1 and 98.65(8) cm−1 for the N215 isotopomers, respectively. These results are in good agreement with prior geometry optimizations and with current V3 barrier calculations which predict the C7eq conformation as the lowest energy form in the gas phase. Under certain conditions, the spectrum is dominated by transitions from a thermal 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).
The rotational spectrum of N-acetyl alanine methyl ester, a derivative of the biomimetic, N-acetyl alanine N'-methyl amide or alanine dipeptide, has been measured using a mini Fourier transform spectrometer between 9 and 25 GHz as part of a project undertaken to determine the conformational structures of various peptide mimetics from the torsion-rotation parameters of low-barrier methyl tops. Torsion-rotation splittings from two of the three methyl tops capping the acetyl end of the -NH-C(=O)- and the methoxy end of -C(=O)-O- groups account for most of the observed lines. In addition to the AA state, two E states have been assigned and include an AE state having a torsional barrier of 396.45(7) cm(-1) (methoxy rotor) and an EA state having a barrier of 64.96(4) cm(-1) (acetyl rotor). The observed torsional barriers and rotational constants of alanine dipeptide and its methyl ester are compared with predictions from Möller-Plesset second-order perturbation theory (MP2) and density functional theory (DFT) in an effort to explore systematic errors at the two levels of theory. After accounting for zero-point energy differences, the torsional barriers at the MP2/cc-pVTZ level are in excellent agreement with experiment for the acetyl and methoxy groups while DFT predictions range from 8% to 80% too high or low. DFT is found to consistently overestimate the overall molecular size while MP2 methods give structures that are undersized. Structural discrepancies of similar magnitude are evident in previous DFT results of crystalline peptides.
Rotational spectra of two conformers of the peptide mimetic, ethyl-acetamidoacetate, were measured in a molecular beam using a Fourier-transform microwave spectrometer. In each conformer, internal rotation of the acetyl methyl group gives rise to observable splittings in the spectrum. From analysis of the torsion-rotation interactions, the methyl group’s orientation has been determined in the principal axis frame of each conformer and is shown to unambiguously identify its conformational form. One conformer exists in the all-trans configuration and belongs to CS point group and the second, higher-energy conformer has C1 symmetry. Two separate theoretical fitting procedures are applied to assess the reliability of the structural information and are shown to be essentially equivalent. For example, methyl torsional barriers are 63.7(1) cm−1 versus 67.1(1) cm−1 and 64.8(1) cm−1 versus 67.5(1) cm−1 for the CS and C1 conformers, respectively, and principal axis orientations of the methyl groups agree to ±0.1°. The small differences in the torsional barriers and rotor axis angles for the two conformers are a result of a change in the orientation of the ethyl group on the other end of the molecule. The predicted energy ordering of these two conformers at the MP2/6-311G(d,p) level of theory is in disagreement with experimental observations.
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