A temperature-dependent Z to B to single-strand transition in d(CGCG).

A thermally induced left- to right-handed helical interconversion for the self-complementary tetradeoxynucleotide d(CGCG) has been observed in the presence of NaCl at concentrations from 1.5 to 2.25M. Analysis of the Raman spectrum of d(CGCG) in aqueous solution as a function of temperature indicates that at low temperature a left-handed Z-helical form is the predominant species. An increase in temperature results in a decrease in the population of the left-handed form and an increase in the population of the right-handed form. Further elevation in temperature results in extensive disruption of base stacking and a loss of secondary structure. This unstacking presumably represents dissociation to a single-stranded structure. The data suggest that a temperature-dependent Z to B to single-strand transition occurs under the conditions employed. A direct Z to single-strand conversion was not identified. In contrast to previous examples, the thermally induced left- to right-handed helical conversion for d(CGCG) can occur in the absence of chemically modified nucleic acid bases, alcohol solutions, or divalent ions.