Experimental studies of peptide bonds:: Identification of the C7eq conformation of the alanine dipeptide analog N-acetyl-alanine N-methylamide from torsion-rotation interactions

Rotational spectra of the biomimetic molecule, alanine dipeptide and the double N-15(N-15(2)) 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 N-14(2) 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 N-15(2) isotopomer) when analyzed in a rho-axis frame where rho(b)=rho(c)=0. The best-fit torsion-rotation parameters provide accurate V-3 barriers and C-3 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 C-7(eq) conformational form. The V-3 barriers of the amide and acetyl methyl groups are 84.0(3) cm(-1) and 98.4(2) cm(-1) for the N-14(2) and 84.1(1) cm(-1) and 98.65(8) cm(-1) for the N-15(2) isotopomers, respectively. These results are in good agreement with prior geometry optimizations and with current V-3 barrier calculations which predict the C-7(eq) 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). (C) 2003 American Institute of Physics.