![]() Due to coupling of the O–H stretch vibrations to low-frequency hydrogen bond modes, the absorption spectrum contains a series of subbands 14, 15. For N-acetylproline dissolved in water, the spectrum at frequencies >3000 cm −1 is not accessible due to strong absorption of water molecules. When N-acetylproline is dissolved in DMSO or water, a strong absorption is observed in the frequency region 2400–2900 cm −1 due to strongly hydrogen-bonded acidic O–H groups. The observed broad absorption spectrum can be subdivided into a broad continuous absorption below 2900 cm −1 and a broad band centered ~3250 cm −1. ![]() When N-acetylproline is dissolved in acetonitrile, the carboxyl O–H stretch spectrum extends to frequencies up to 3400 cm −1. The absorption spectrum of the carboxyl O–H stretch vibrations is also strongly solvent dependent. Nevertheless, in the presence of an intra-molecular hydrogen-bond, the two molecular groups are interacting, and, thus, the two molecular vibrations can be coupled leading to the observation of cross-peak signatures. The amide and hydroxyl vibration are usually not as strongly coupled as the carboxyl and hydroxyl vibrations since they are a few atoms apart. Hence, N-acetylproline represents an ideal system to study the competition of inter- and intramolecular interactions on the molecular conformation in solvents that resemble hydrophobic and hydrophilic environments. 1, it is seen that the trans-anti conformation offers the possibility of forming an intramolecular hydrogen bond between the C=O group of the amide group and the O–H group of the carboxyl group. We here do not consider the possible conformational isomers due to the rotation of the carboxyl group around the C–C bond since our experiments are not sensitive to this rotation. Figure 1 shows all the possible structures of N-acetylproline considering the conformational isomerism of both the amide (cis/trans) and the carboxyl group (syn/anti). Here we study the conformational isomerism of the carboxyl group of the model acetylated amino acid N-acetylproline in weakly and strongly interacting solvents with polarization-resolved two-dimensional infrared spectroscopy (2DIR). It has also been shown that the reactivity of the carboxyl group, and more general the strength of the intra- and intermolecular interactions, strongly depends on the conformation of the carboxyl group 12, 13. In the syn-conformation the hydroxyl group is oriented at an angle of 60° with respect to the carbonyl, and in the anti-conformation the hydroxyl group is oriented anti-parallel with respect to the carbonyl group 10, 11. Recently, we showed that the carboxyl group of simple carboxylic acids in room temperature solution adopts two distinct nearly planar conformations, syn- (70–80% fraction) and anti- (20–30%) 9. However, under specific conditions, like the nonpolar microenvironments of polypeptides, the side-chain carboxyl groups of aspartic and glutamic acids, and the carboxyl groups of C-terminal amino acids can exist in their protonated form 4, 5, 6, and can participate in enzymatic processes or in the stabilization of protein tertiary structure 7, 8. In view of its low pK a values of ~2 in aqueous solution, the carboxyl group of the amino acid will usually be deprotonated. Similar content being viewed by othersĪmino acids fulfill diverse roles in living systems as protein building blocks, neurotransmitters, and metabolic intermediates, thereby making them one of the most important classes of organic molecules 1, 2, 3. ![]() However, when N-acetylproline is dissolved in a weakly hydrogen-bond accepting solvent (acetonitrile), we observe the formation of a strong intramolecular hydrogen bond between the carboxyl group in the anti-conformation and the amide group, which stabilizes the anti-conformer, increasing its relative abundance to ~60%. In hydrogen-bond accepting solvents such as dimethyl sulfoxide or water, we observe that, similar to simple carboxylic acids, around 20% of the -COOH groups adopt an anti-conformation. In the syn-conformer the O–H group is oriented at ~60 ∘ with respect to the C=O and in the anti-conformer the O–H is anti-parallel to the C=O. We find that the carboxyl group of N-acetylproline adopts two distinct conformations, syn- and anti. Here, we study the conformational behaviour of the model amino acid N-acetylproline in solution at room temperature with two-dimensional infrared spectroscopy. The molecular conformation of the carboxyl group can be crucial for its chemical properties and intermolecular interactions, especially in complex molecular environments such as polypeptides.
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