Covalent attachment of an alkylamine prevents the B to Z transition in poly(dG-dC)

Covalent complexes of n-butylamine and double-stranded poly(dG-dC) were prepared by coupling the amine to exocyclic amino groups of guanine bases with CH2O. Neither the absorption spectrum above 230 nm nor the s020,w of the complexes in low to moderate ionic strength solvents, freed of excess unreacted reagents, differs significantly from that of unreacted poly(dG-dC) or a control which had been exposed only to CH2O. In contrast, the CD spectra are profoundly altered. The minimum at 252 nm becomes more negative, and the rotational strength of the positive band above 260 nm is reduced as a linear function of the extent of amine attachment. At 0.22 mol of amine per mole of nucleotide, the transformation is similar to that observed by others in poly(dG-dC) when complexed to core histones in reconstituted core particles or in concentrated LiCl solvents at temperatures below the B----Z transition. Sedimentation studies reveal that these changes in the circular dichroism (CD) spectra reflect secondary structural effects rather than the formation of aggregates or psi type structures. Raman spectra reveal, however, that these secondary structural changes must occur within the B family as the amine complex retains B backbone geometry. The conformation produced by the attachment of the amine is probably a higher winding angle (overwound) B variant.(ABSTRACT TRUNCATED AT 250 WORDS)