SynopsisThe distribution of bound Mn2+ ions about poly(rI).poly(rC) has been studied by measuring the effect of this paramagnetic metal ion on the relaxation behavior of poly(rI).poly(rC) protons. By combining selective spin -lattice and spin -spin relaxation rates for various protons, some of the principle regions of ion association can be identified. The relaxation data on the CH6 proton are consistent with a < 10% occupancy of phosphate inner-sphere binding sites. The broadening of the irnino proton resonance requires a substantial occupancy of sites located in the major groove, possibly near IN7. This would also be consistent with the observation that IH8 resonance is the proton most susceptible to relaxation by Mn2+. The relaxation data for the IH2 proton indicate a relatively low occupancy of minor-groove binding sites (e.g., IN3).