Hydrogen-1 and carbon-13 magnetic resonance studies of nonactin-calcium complex

For an understanding of the cation selectivity and general binding characteristics of macrotetralide antibiotic nonactin (NA) with ions of different sizes and charges, the nature of binding of divalent caption, Ca2+, to NA and conformation of the NA-Ca2+ complex have been studied by use of 270-MHz proton nuclear magnetic resonance (1H NMR) and carbon-13 nuclear magnetic resonance (13C NMR). The calcium ion induced significantly large changes in chemical shifts for H7, H2, H3, and H5 protons of NA and relatively small changes for H18 and H21 protons. Changes in 13C chemical shift were quite large for carbonyl carbon, C1; it is noteworthy that in the NA-K+ complex, H2 and H21 protons practically do not shown any change during complexation and carbonyl carbon shows a much smaller chemical shift change. From an analysis of calcium ion titration data, it is concluded tht NA forms an equimolar complex with Ca2+. The study of complexation of NA and dry acetone-d6 and in acetone-d6 containing 0.1 mol fraction of D2O indicated that only unhydrated Ca2+ is bound to the NA, similar to complexes of NA with other cations. The binding constant for NA-Ca2+ in dry acetone is of the order of 10(4) (in mole fraction units); this gets marginally reduced to 7.5 X 10(3) in the case of wet acetone. The study further indicated that the overall conformation for NA-Ca2+ is similar to that of NA-K+ reported earlier, with significant local conformational changes occurring in order to accommodate the doubly charged, smaller sized calcium ion.