31P and two-dimensional 31P/1H correlated NMR spectra of Duplex d(Ap[17O]Gp[18O]Cp[16O]T) and assignment of 31P signals in d(ApGpCpT)2-actinomycin D complex.

A solid-phase phosphoramidite method was used for the synthesis of unlabeled and phosphoryl-labeled d(Ap-[17O]Gp[18O]Cp[16O]T). The ability to label the phosphoryl oxygens of d(ApGpCpT) and thus assign the 31P signals, combined with a two-dimensional 31P/1H chemical shift correlated NMR spectral technique, provided a novel means for the ready assignment of the H5' and H3' protons coupled to the phosphates. Phosphoryl labeling has also allowed us to assign the 31P NMR signals in the actinomycin D-d(Ap-[17O]Gp[18O]Cp[16O]T)2 duplex complex and confirm that the drug intercalates between the GpC stacked base pairs.