We have studied intramolecular hydrogen bonding in a homologous series of diamides (compounds 1-6) in methylene chloride, 9: 1 carbon tetrachloride/benzene, and acetonitrile. By correlating variable-temperature 'H NMR and 1R measurements, we have shown that the temperature dependence of the amide proton NMR chemical shift (Ab/AT) can provide qualitative (and in some cases quantitative) information on the thermodynamic relationship between the intramolecularly hydrogen bonded and non-hydrogen-bonded states of flexible molecules. Among the hydrogen-bonded ring sizes represented in the diamide series, the intramolecular interaction is particularly enthalpically favorable in the nine-membered hydrogen-bonded ring (compound 4). Variable-temperature IR and NMR data indicate that the internally hydrogen bonded state of diamide 4 is 1.4-1.6 kcal/mol more favorable enthalpically than the non-hydrogen-bonded state, in methylene chloride solution; the non-hydrogen-bonded state is 6.8-8.3 eu more favorable entropically in this solvent. In contrast, there appear to be much smaller enthalpy differences between the internally hydrogen bonded and non-hydrogen-bonded states of diamides 2 and 3. Our findings are important methodologically because the temperature dependences of amide proton chemical shifts are commonly used to elucidate peptide conformation in solution. Our results show that previous "rules" for the interpretation of such data are incomplete. In non-hydrogen-bonding solvents, small amide proton Ab/AT values have been taken to mean that the proton is either entirely free of hydrogen bonding or completely locked in an intramolecular hydrogen bond over the temperature range studied. We demonstrate that an amide proton can be equilibrating between intramolecularly hydrogen bonded and non-hydrogen-bonded states and still manifest a small chemical shift temperature dependence (implying that the hydrogen-bonded and non-hydrogen-bonded states are of similar enthalpy).
We describe the conformational behavior of triamide 1' in methylene chloride solution. Triamide 1 has several alternative folding patterns available to it, but at low temperatures in this relatively non-interactive solvent, one highly ordered conformation predominates. The non-covalent forces at work in 1 include amide-amide hydrogen bonding, one of the principal conformation-specifying factors in proteins. The examination of oligoamides under conditions in which conformation is determined by a relatively small number of internal non-covalent interactions is the basis of our effort to understand the interplay of the non-covalent forces that influence protein secondary and tertiary structural stability.2 R R CH, n. 0 1 Insight on the conformational behavior of triamide 1 in CD2C12 is provided by the temperature dependences of the amide proton NMR chemical shifts ( Figure 1). The data for 1 are juxtaposed with the data for diamides 2 and 3,' which model respectively the six-and seven-membered-ring hydrogen bonds available to the triamide. Also shown are the data for a 1:l mixture of Nmethylacetamide and N,N-dimethylacetamide, indicating the inherent chemical shift temperature dependence for a non-hydrogen bonded amide proton (intermolecular hydrogen bonding is minimal under these conditions)). It is immediately clear that the folding patterns experienced by 1 in CD2C12 include conformations other than those involving the six-and seven-membered-ring amide-amide hydrogen bonds.In a very weakly hydrogen bonding solvent, both the absolute value and the temperature dependence of the amide proton chemical shift provide structural i n f~r m a t i o n .~,~ Under these conditions, the rate at which an amide proton exchanges among its various hydrogen bonded and non-hydrogen bonded states is usually rapid on the NMR time scale. Therefore, the lone signal observed for a given amide proton represents a weighted average of the signals for that proton in its alternative environments, with more time in a hydrogen bonded state moving the signal further downfield.5 The substantial difference between the observed amide proton chemical shifts for diamides 2 and 3 at all temperatures indicates that much more intramolecular hydrogen bonding occurs in 2 than in 3. This qualitative conclusion is confirmed by the infrared spectra of these two diamides in dilute CH2C12 solution at room temperature: the N-H stretch region for diamide 2 is dominated by a broad hydrogen bonded N-H (1) All new amides were prepared by standard methods, which will be presented in a full paper. The NMR, IR, and high-resolution mass spectroscopic data for each new compound are consistent with the proposed structure.(2) Gellman, S. H.; Adams, B. R. Tetrahedron Leu. 1989, 30, 3381.(3) The chemical shift temperature dependence data shown in Figure 1 were obtained with 1 mM amide solutions. When a 5 mM solution of 1 was examined, little variation was observed in the behavior of Hb, relative to the I mM sample, over the entire temperature range. Below 253 K, however, t...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.