INTERACTIONS OF HORMONAL STEROIDS WITH NUCLEIC ACIDS .2. STRUCTURAL AND THERMODYNAMIC ASPECTS OF BINDING

Structural attributes governing associations of estradiol, progesterone, and testosterone with polyguanylic acid and denatured deoxyribonucleic acid in aqueous buffer have been determined using equilibrium dialysis as an assay method. Structural substitution of the steroids indicates that in each case two functional groups are necessary for binding to guanine residues in polyguanylic acid: the 3- and 20-keto groups of progesterone, the 3- and 17-hydroxyl groups of estradiol, and the 3-keto and 17-hydroxyl groups of testosterone. These observations, the fact that these functional groups can act as proton donors or acceptors, and the minimal effect of slightly protic solvents on the binding of the parent steroids to polynucleotides suggest that two hydrogen bonds are formed in each case between functional groups of steroid and purine. Values for free energy of binding of 7-10 kcal/mole of steriod bound, derived from binding constants corresponding to the maximum slopes of binding isotherms, are consistent with the formation of two hydrogen bonds. While the relevant functional groups of guanine residues are not defined in all cases, studies with polyinosinic acid and hydroxy methylation of the 2-amino group indicate that this group is essential to the binding of progesterone and testosterone but not of estradiol. Alteration of binding properties by factors affecting polynucleotide conformation and the finding that the number of binding sites corresponding to maximum binding constants is less than 1/10,000 nucleotide residues demonstrate the critical role of polymer geometry in the provision of optimal binding sites. Construction of space-filling molecular models indicates that these hormonal steriods may interact with short, singlestranded nucleotide sequences. © 1969, American Chemical Society. All rights reserved.