Nonactin,monactin, dinactin,trinactin, and tetranactin. A Raman spectroscopic study

Raman spectra are reported for crystalline nonactin, monactin, dinactin, trinactin, and tetranactin and their solutions in CCl₄, CHCl₃, CH₃OH, and 4:1 (v/v) CH₃OH:CHCl₃. The macrotetrolide nactins selectively bind a wide variety of cations, and are important model compounds for the study of ion complexation. The conformations of nonactin, monactin, and dinactin in solution are similar. Their conformations are found to be sufficiently open to permit the ester carbonyl groups to form hydrogen bonds with CH₃OH; this gives rise to characteristic changes in the vibration frequencies associated with the ester groups. Nonactin, which is the least soluble of the nactins in CH₃OH, is also the least effective at forming hydrogen bonds with CH₃OH. The greater ability of the higher nactins to form hydrogen bonds with CH₃OH may be due to the increased inductive effect of ethyl over methyl side chains, which may increase the dipole moment of the ester carbonyl groups. Spectra of crystalline nonactin, monactin, and tetranactin are fairly similar, while the spectra of dinactin and trinactin comprise a second, distinct family. This is consistent with X-ray crystallographic studies, which show that nonactin and tetranactin form monoclinic crystals, while trinactin is triclinic.