13C NMR studies on fluorescent probes: 13C chemical shifts and longitudinal relaxation times of n-hydroxy-fatty (n=2,6,9,12) acids and n-(9-anthroyloxy)-stearic (n=6,12) acids

¹³C NMR has been used to confirm the structure of two fluorescent probes, n-(9-anthroyloxy)-stearic acids (n=6,12), and the series of n-hydroxy-fatty acids (n=2,6,9,12) from which the set of fluorescent fatty acids may be synthesised. ¹³C longitudinal relaxation times and correlation times of the individual carbon atoms in 12-hydroxy- and 6- and 12-(9-anthroyloxy)-stearic acids show differences in motional properties between these derivatives and the parent stearic acid in chloroform(d) solution. The correlation times of the substituted carbons in 6-, 9-, and 12-hydroxy-stearic acids are longer than the corresponding carbons in stearic acid. The change in correlation times at the substituted carbons reflects the increase in motion along the acyl chain. Attachment of the bulky anthracene ring causes greater restriction of motion at the substituted carbon atom but the gradient of motion along the chain is preserved. These results are discussed in terms of the types of motion which lead to fluorescence depolarization when the fluorescent fatty acids are used as fluidity probes in biomembranes.