HENRY M. SOBELL

PHYSICAL EVIDENCE CONCERNING THE BINDING OF IREHDIAMINE AND OTHER STEROIDAL DIAMINES TO DNA.

NUCLEIC ACID - SMALL MOLECULE INTERACTIONS. VII. FURTHER CHARACTERIZATION OF DEOXYRIBONUCLEIC ACID - STEROIDAL DIAMINE COMPLEXES. H.R. Mahler, G. Green, R. Goutarel and Q. Khuong-Huu. Biochemistry, 7, 1568-1582 (1968).

This paper, along with others in this series of papers, describes studies done to characterize the nature of a series of steroidal diamine complexes with DNA. These studies provided early evidence for the existence of partial intercalation by these steroidal diamines into the low energy beta-DNA form.

Dr. Mahler and his associates have shown that:

Irehdiamine and other steroidal diamines bind tightly to DNA under low salt conditions, as evidenced by a hyperchromic effect at 260 millimicrons. The magnitude of this hyperchromic effect is in the same order as that seen with DNA melting, suggesting significant base pair unstacking.

Irehdiamine and other steroidal diamines form stoichiometic complexes that contain one steroidal diamine for every two base pairs (i.e., one steroidal diamine/ four phosphates). Very similar observations are observed with simple intercalators, suggesting neighbor-exclusion binding. Another complex has been detected at higher steroidal diamine - DNA ratios. This complex is less stable to thermal melting, and, although structurally uncharacterized, could be a superhelical variant of the preceding form.

The melting temperature of the first steroidal diamine - DNA complex is about 20 degrees higher than native DNA, suggesting that complex formation may interfere with the nucleation and propagation of DNA melting.

Irehdiamine and other steroidal diamines show little effect on DNA viscosity. As a function of steroidal diamine concentration, the intrinsic viscosity first falls (about 10%), and then rises to the initial intrinsic viscosity for native DNA. Very different observations are observed with simple intercalators (a large rise in viscosity accompanies binding), suggesting that complex formation neither stiffens or lengthens DNA.

Spectroscopic (ORD) measurements suggest DNA to have a very different conformation in these complexes, when compared with either A- or B- DNA.

And, finally, irehdiamine reverses proflavine induced mutations in T2 bacteriophage, suggesting that irehdiamine binds to DNA in a manner similar to intercalation.

VARIATION OF SUPERCOILS IN CLOSED CIRCULAR DNA BY BINDING OF ANTIBIOTICS AND DRUGS: EVIDENCE FOR MOLECULAR MODELS INVOLVING INTERCALATION. M.J. Waring. J. Mol. Biol. 54, 247-279 (1970).

This study (of the unwinding of negatively superhelical DNA by intercalating agents) is the most complete of its kind. Irehdiamine is observed to unwind DNA much like other planar intercalators, unwinding DNA roughly half as much as ethidium (see Figure below). This suggests that irehdiamine binds to DNA by partial intercalation into the beta-structural element. Our model predicts an angular unwinding of about 12 degrees, in good agreement with the data.