NMR and molecular orbital studies of isomerism and tautomerism in oximes of 2-acyl cyclic 1,3-diones.

Molecular orbital calculations and 13C and 15N NMR experiments have been performed on the O-methyl oximes of two types of 2-acyl cyclic 1,3-diones. One (III; X = CH2) was based on cyclohexane, and the other (X = O) on pyran. The data indicate that both the cyclohexane and pyran compounds prefer to exist as the oxime, rather than the enamine isomer. Two equivalent, interconverting, keto-enol tautomers exist in the cyclohexane compound. The pyran compound exists as the lactone-enol tautomer. Our results have implications in the design of herbicidal compounds and drugs containing similar tautomeric systems.

Preparation and comparison of model bilayer systems from chloroplasts thylakoid membrane lipids for 13C-NMR studies.

Model bilayer systems from individual purified chloroplast thylakoid membrane lipids, from reconstituted mixtures of these purified lipids, and from leaf total polar lipid extracts have been prepared in water, and the longitudinal relaxation times (T1's) of the individual carbon atoms of the fatty acyl chains measured by 13C-NMR spectroscopy. The T1's increase with increasing distance of the carbon atoms from the polar headgroups in all cases, and as the results from each of the preparations are similar, all can be used as models of chloroplast membrane bilayers. Relaxation time measurements on intact chloroplast thylakoid membranes indicate the presence of chlorophyll resonances in the 13C-NMR spectrum of the membrane.

13C nuclear magnetic resonance studies of the composition and fluidity of several chloroplast monogalactosyldiacylglycerols.

The motional properties of four monogalactosyldiacylglycerols isolated from photosynthetic membranes, and containing different fatty acid chain lengths and degrees of unsaturation, have been determined using 13C nuclear magnetic resonance. These properties have been compared with those of a lipid containing only saturated fatty acids. The 13C longitudinal relaxation times (T1) of the carbon atoms of the acyl chains in [2H4] methanol were measured as an index of the rates of motion of the lipid molecules and used to compare the relative fluidity of the acyl chains. The T1 values of the glyceryl and galactosyl carbon atoms in each monogalactosyldiacylglycerol are essentially constant, when allowance is made for concentration differences and the presence of two hydrogens on a methylene carbon versus one on a methine carbon. These results indicate similar rates of motion for the headgroup carbons in each lipid. However, for the acyl chains, the T1 values increase with the introduction of a double bond and increase further with additional unsaturation. This increase in the rate of motion only occurs at carbon atoms beyond the first double bond in an acyl chain. These results differ to those reported for monolayer experiments where changes in packing characteristics are predominantly dependent on the introduction of the first double bond and then vary little between species.