MAGNESIUM-DNA INTERACTIONS FROM INTERPRETATION OF MG-25-NMR RELAXATION RATES - FIELD AND COLON DEPENDENCE

We have used Mg-25-nmr to investigate the binding of magnesium ions to double-stranded DNA. We have measured line shapes for Mg-25 in the pr esence of monodisperse calf thymus DNA (160 base pairs; b.p.) (magnesi um: phosphate = 2.0) at two different field strengths, 11.75 T and 7.0 5 T, and used the isotropic model of two-site exchange developed by We stlund and Wennerstrom to simultaneously fit the line shapes at both f ield strengths. This model does not reproduce the observed field depen dence. This is in contrast to a previous study [E. Berggren, L. Norden skiold, and W. H. Braunlin (1992), Biopolymers, Vol. 32, pp. 1339-1350 ] in which a similar model of isotropic two-site exchange qualitativel y reproduced the temperature dependence of the line widths. Relaxation rates were also measured as a function of magnesium : phosphate ratio and colon type. These measurements were used to assess the sensitivit y of magnesium relaxation measurements to small changes in DNA structu re induced by changes in the solvent environment. The temperature depe ndence of the line shape varies with the type of colon (chloride or su lfate) present. This colon dependence of the line shape is consistent with the colon dependence of the aggregation midpoint temperature repo rted by Bloomfield and co-workers [O. A. Knell, M. G. Fried, and V. A. Bloomfield (1988) in Structure and Expres sion, Vol. 2, R. H. Sarma a nd M. H. Sarma, Eds., Adenine Press, New York] and attributed to a lyo tropic effect. These results suggest that even at low magnesium : phos phate ratios, relaxation parameters are specific to each magnesium-coi on-DNA system. (C) 1994 John Wiley & Sons, Inc.