Analysing DNA complexes by circular and linear dichroism.

The application of linear and circular dichroism (LD and CD) in nucleic acid research is illustrated by recent results aimed at answering specific structural problems in the interaction of DNA with molecules of biological importance. We first consider the circumstances under which ligands, such as DAPI (4',6-diamidino-2-phenylindole), change their preferred binding mode in the minor groove to major groove binding or intercalation. As an extension of this problem we refer to the switch between groove binding and intercalation of structurally similar ligands such as ellipticines and trigonal ruthenium complexes. We also explore the use of LD and CD in the determination of the structure of the complex formed between the polynucleotide poly(dA) and the novel 'peptide nucleic acid', consisting of nucleic acid bases joined by a polyamide homomorphous with the deoxyribose-phosphate backbone of DNA. Finally, the structure and interaction of the recombination enzyme RecA with DNA is discussed, in particular the influence of the presence of intercalators, groove binders or covalent DNA adducts.