Condensed form of (dG-dC)n X (dG-dC)n as an intermediate between the B- and Z-type conformations induced by sodium acetate.

Circular dichroism and laser Raman spectroscopy reveal that the synthetic DNA polymer (dG-dC)n X (dG-dC)n undergoes a cooperative transition induced by sodium acetate from a right-handed B-form to a left-handed Z-type conformation with a midpoint at 2.05 M. However, at concentrations only slightly higher than the end point of this transition (above approximately 2.2 M) and up to approximately 2.65 M, the Z-form is not stable in solution but aggregates to form highly condensed DNA. A manyfold increase of positive ellipticity in the range 340-250 nm is observed which is indicative of a psi (+)-type structure. At even higher concentrations (greater than or equal to 2.7 M), the Z-form is stable without condensation, and there is no change in the inverted CD spectrum. All structural transitions are reversible except that it is not possible to redissolve the highly condensed psi (+)-form by further increasing the salt concentration to greater than or equal to 2.7 M. The very high cooperativity of these transitions enables the DNA polymer to adopt three distinctly different structures (B-, Z-, and psi-forms) within a narrow range of sodium acetate concentration (approximately 200 mM). The Raman spectra of the condensed form and the Z-form in very concentrated sodium acetate show that the psi (+)-type state forms without substantial changes of the secondary conformation of the DNA. This indicates that the left-handed Z-helix of (dG-dC)n X (dG-dC)n can form psi-type aggregates with an ordered superstructure similar to those observed for natural right-handed DNA helices.