Abstract
The total assignment of the carbon-13 and proton n.m.r. spectra of the polyether ionophore antibiotic Na monensin A are reported. These assignments were derived unambiguously by consideration of two-dimensional correlation and J-resolved spectra of Na monensin A. These data are essential for the correct analysis by n.m.r. spectroscopy of biosynthetic experiments using stable isotope labelling techniques. The incorporation of molecular oxygen, labelled with oxygen-18, into monensin A, has been used to show that four of the oxygen atoms in the antibiotic are derived from this source.