期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：1978
卷号：75
期号：9
页码：4286-4290
DOI：10.1073/pnas.75.9.4286
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：We report equilibrium, relaxation kinetic, and transient electric dichroism studies on the complex of the diamino steroid irehdiamine A with DNA. The results are consistent with a {beta}-kinked structure for the complex at saturation, with a kink in the DNA structure induced by a bound steroid every second base pair. The results that favor this hypothesis include an apparent length decrease of rod-like bacterial DNA molecules when only a small amount of drug is bound, followed by an apparent length increase at saturation. The limiting dichroism amplitude implies a substantial increase in the tilt of the bases relative to the orientation axis; at saturation the base UV transition moments are tilted about 31{degrees} from the plane perpendicular to the orientation axis. Because of the direction of polarization of the 260-nm transition moments, the results indicate that the tilt of the bases must be predominantly in the short rather than the long axis of the base pair. The large hyperchroism of the complex is consistent with loss of base stacking, as required by a kinked structure. The kinetic results imply a bimolecular reaction mechanism, with a temperature-dependent association rate constant of roughly 108 M-1 sec-1, and a dissociation rate constant of about 5 x 103 sec-1, nearly independent of temperature. The association activation energy and apparent reaction enthalpy vary from 12 to 22 kcal mol-1; heat is absorbed on complex formation as expected for loss of base-stacking interactions. An anomalous result of the experiments is the larger apparent length increase (13%) exhibited by two eukaryotic DNAs, compared to 6% for three prokaryotic DNAs. Differences were also observed in the kinetic properties of the complexes.
