Conformational analysis of the jet-cooled peptide mimetic ethylacetamidoacetate from torsion-rotation spectra

Abstract

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.

Analysis and fit of the Fourier-transform microwave spectrum of the two-top molecule N-methylacetamide

“…These values for the barrier heights of the two methyl tops are close to those of peptide molecules studied recently by other groups, e.g., 80.06487 (14) cm À1 for NACH 3 of Nmethylpropionamide [27], and 98.4(2) and 84.0(3) cm À1 for the acetyl and amide methyl groups, respectively, of N-acetyl-alanine-N 0 -methylamide [28].…”

Methyl torsional state analysis of the jet-cooled microwave spectrum of N-acetylglycine

“…One possible structure includes a hydrogen bond between the carbonyl group of the acid and the amide proton of the peptide bond, defining a five membered ring. This configuration was found to be lowest energy form of N-acetylglycineethylester (ethylacetamidoacetate) [24]. A second set of favorable configurations includes a hydrogen bond linking the carbonyl or amide groups of the peptide bond to the hydroxyl group, acting as either proton donor or acceptor.…”

“…6 06 ) 5 05 11007.307(4) )1 6 15 ) 5 14 11557.311(4) 0 5 23 ) 5 14 12343.041(4) )2 7 17 ) 6 16 12421.876(4) 1 5 15 ) 4 04 12625.871(4) )2 7 07 ) 6 06 12788.466(4) )2 7 26 ) 6 25 12965.032(4) )1 7 34 ) 6 33 13030.307(4) )1 2 20 ) 2 11 13100.450(4) 2 7 25 ) 6 24 13171.221(4) 1 7 16 ) 6 15 13467.149(4) )2 3 22 ) 3 13 13775.838(4) 1 9 09 ) 8 18 13954.651(4) )1 6 16 ) 5 05 14077.784(4) 0 8 18 ) 7 17 14180.425(4) 1 8 08 ) 7 07 14549.057(4) 1 8 27 ) 7 26 14804.498(4) 1 8 36 ) 7 35 14889.506(4) )4 8 35 ) 7 34 14904.651(4) 1 6 25 ) 6 16 14927.681(4) 0 8 26 ) 7 25 15106.296(4) 0 8 17 ) 7 16 15368.561(4) )1 7 17 ) 6 06 15492.352(4) 2 9 19 ) 8 18 15933.652(4) )1 10 0;10 ) 9 19 16034.386(4) 1 9 09 ) 8 08 16289.905(4) 1 9 28 ) 8 27 16638.941(4) 2 2 21 ) 1 10 16959.839(4) 1 9 27 ) 8 26 17055.888(4) 4 2 20 ) 1 11 17114.071(4) )2 9 18 ) 8 17 17259.735(4) 1 10 1;10 ) 9 19 17681.499(4) 1 10 0;10 ) 9 09 18013.388(4) 1 10 3;8 ) 9 37 18627.156(4) 3 3 22 ) 2 11 18665.808(4) )1 10 3;7 ) 9 36 18674.055(4) )2 3 21 ) 2 12 19136.137(4) 1 10 19 ) 9 18 19138.649(4) )2 11 1;11 ) 10 1;10 19424.048(4) 0 11 0;11 ) 10 0;10 19723.023(4) 1 11 1;10 ) 10 19 21003.123(4) )1 12 0;12 ) 11 0;11 21422.817(4) 0 13 0;13 ) 12 0;12 23116.594(4) )1 r (A-state) 1.7 a The expanded uncertainties shown in parentheses are Type B with coverage factor k ¼ 2 or two standard deviations [21]. intramolecular hydrogen bond.…”

“…6 06 ) 5 05 10890.613(4) )1 7 17 ) 6 16 12555.273(4) )2 7 07 ) 6 06 12694.006(4) 1 7 25 ) 6 24 13054.912(4) 0 7 26 ) 6 25 13133.287(4) 1 7 16 ) 6 15 13221.110(4) )1 6 16 ) 5 05 13398.102(4) )2 8 18 ) 7 17 14274.538(4) )3 8 08 ) 7 07 14478.877(4) 3 7 25 ) 7 16 14862.223(4) )5 8 35 ) 7 34 14875.433(4) )3 8 36 ) 7 35 14904.849(4) )3 8 26 ) 7 25 14943.634(4) 0 6 24 ) 6 15 15028.422(4) )3 5 23 ) 5 14 15182.938(4) 1 4 22 ) 4 13 15305.715(4) 2 3 21 ) 3 12 15388.169(4) 5 9 19 ) 8 18 15996.445(4) 3 9 09 ) 8 08 16242.797(4) 3 9 37 ) 8 36 16785.107(4) 3 9 27 ) 8 26 16843.541(4) 6 9 18 ) 8 17 16958.902(4) )1 10 0;10 ) 9 09 17985.644(4) 2 10 4;7 ) 9 46 18607.901(4) )5 10 3;7 ) 9 36 18619.463(4) 1 10 3;8 ) 9 37 18672.142(4) 1 10 1;9 ) 9 18 18815.637(4) )5 r (kHz b ) 3.0 a The expanded uncertainties shown in parentheses are Type B with coverage factor k ¼ 2 or two standard deviations [21]. b E-state standard deviation from global fit: A-state standard deviation ¼ 2.1 kHz.…”

A new scheme of K-labeling for torsion–rotation energy levels in low-barrier molecules

“…These test molecules represent the cases of various barrier heights (from 407 cm À1 in acetaldehyde to 25 cm À1 in acetamide), various couplings between internal rotation and global rotation (q varies from 0.8 in methanol to 0.022 in ethyl acetamidoacetate), and both prolate and oblate character of the overall rotation. Although we believe that the proposed approach is applicable to different computation schemes (e.g., principal axis method or internal axis method), we limit our consideration to the q-axis-method, which was extensively applied in the last decade for spectral investigation of the molecules mentioned above [1][2][3]13,14].…”