Spectroscopic studies on acetylaminofluorene-modified (dT-dG)n . (dC-dA)n suggest a left-handed conformation.

CD spectroscopy on the double-stranded strictly alternating dinucleotide polymer (dT-dG)n . (dC-dA)n partially modified by N-acetoxy-N-acetyl-2-aminofluorene suggests a left-handed conformation in concentrated NaCl solutions. Modification of the (dT-dG)n . (dC-dA)n polymer with acetylaminofluorene is required to promote formation of the left-handed helix since high salt concentrations and several other ionic conditions, which cause a similar transition for (dG-dC)n . (dG-dC)n, are ineffective. Furthermore, substitution of dC with 5-methyl dC in (dT-dG)n . (dC-dA)n does not facilitate formation of a left-handed helix, also in contrast to results found for (dG-dC)n . (dG-dC)n. A 62-base pair tract of almost perfectly alternating (dT-dG)n . (dC-dA)n from the 3'-side of the mouse kappa immunoglobulin gene modified with acetylaminofluorene undergoes the salt-induced transition to a left-handed helix when studied within a 140-base pair restriction fragment. High NaCl concentrations alone will not cause the transition for this 62-base pair tract in this fragment nor in the recombinant plasmid pRW777, which contains this fragment.