Topography of nucleic acid helices in solutions. The interaction specificities of optically active amino acid derivatives

The synthesis and interactions of the d- and l-enantiomers of the amino acid amide derivatives [Formula: see text] (I) and lysyl dipeptides [Formula: see text] (II) with poly rI·poly rC, poly rA·poly rU and calf thymus DNA is reported. The following results were found. (1) The degree of stabilization of the helices as measured by the Tm (‘melting’ temperature) of the helix–coil transition was dependent on the nature of the amino acid. (2) For the poly rI·poly rC helix, the l-enantiomers of salts (I) and (II) stabilized more than the d-enantiomers. The same was true for calf thymus DNA in the presence of salts (II) and for poly rA·poly rU in the presence of salts (II) and the proline derivatives of salts (I). (3) As R increased in size and became more apolar, the amount of stabilization of the poly rI·poly rC helix in the presence of salts (I) decreased. On the other hand, the amount of stabilization increased with more polar substituents. An attempt was then made to determine whether the difference in stabilization of the double-stranded helices at the Tm in the presence of the l- and d-enantiomers of salts (I) is due to the interaction with the helix, the random coil or both. A new method was developed for determining the binding of small ions to polyions that involves a competition between an insoluble polystyrene ion-exchange resin and the soluble polyion for the counterion. Dissociation constants are obtained for the complexes of single- and double-stranded helices with the salts (I). The results are illuminating and indicate that with certain helices, i.e. poly rA·poly rU, the interactions of salts (I) with the single strands may not be ignored. It is concluded that the high optical specificity found in Nature, i.e. d-ribose in nucleic acids and l-amino acids in proteins, cannot be attributed solely to monomer–polymer interactions described by Gabbay (1968).