AN X-RAY STUDY OF INTERACTION OF DNA WITH SPERMINE

The tetramine spermine forms a complex with DNA, and thereby serves to protect it against denaturation and breakage. X-ray fibre photographs of the DNA-spermine complex were taken at various relative humidities. The most detailed pattern was obtained at 92% relative humidity. It shows the complex to have the same helical parameters as the naturally occurring B form of DNA, and presumably a closely similar conformation. However, at 76% and lower relative humidities, the complex conforms to the DNA C configuration. The intermolecular distance was found to vary with the degree of hydration of the specimen, but to have a definite upper limit. This and the range of intermolecular distance observed are consistent with spermine cross-links between DNA molecules. Studies of molecular models have shown that it is also possible for all four basic groups of a spermine molecule to make hydrogen bonds with phosphate groups on one molecule of DNA. The spermine molecules would lie across the small groove of the DNA and serve to bind its two strands together. Calculation of intensities for various models for the DNA-spermine complex indicates that no one model can account for the observed X-ray pattern. However, the assumption of spermine in the small groove of DNA as well as in intermolecular cross-links, or of an even greater variety of modes of attachment between DNA and spermine, does lead to satisfactory agreement between calculated and observed intensities. © 1969.