The mercury(II) and high‐salt‐induced conformational B↔Z transitions of poly[d(G‐m 5 C) · d(G‐m 5 C)] as studied by non‐polarized (ultraviolet) and circularly polarized (CD) ultraviolet spectroscopy

The B↔Z transition of poly[d(G‐m 5 C) · d(G‐m 5 C)] in buffered solution (0.002 M sodium cacodylate, pH 7) was studied by CD and ultraviolet spectroscopy as a function of the supporting electrolyte concentration (0.002–1.1 M NaClO 4 ) in the absence of Hg(ClO 4 ) 2 [Hg(II)], and as a function of the Hg(II) concentration at a given NaClO 4 level. NaClO 4 alone changes the conformation of the polymer from B↔Z at approximately 0.7 M NaClO 4 . The spectral changes caused by Hg(II) in the B‐form polymer (e.g. at 0.002 M ≤ [Na] ≤ 0.7) resemble those generated by salt alone during the B↔Z transition; the changes generated by Hg(II) in the Z‐form polymer (e.g. in 1.1 M [Na]) leave principally intact the Z‐form spectrum obtained at the higher levels of NaClO 4 (e.g. at [Na] > 0.7 M) in the absence of Hg(II). It is concluded that particularly the long‐wavelength positive‐CD band, located at 274 nm, is a correct indicator of duplex DNA right↔left‐screwness inversion. According to generally accepted criteria, the NaClO 4 ‐induced left‐handed form is Z DNA; Hg(II) generates a left‐handed form termed here Z Hg(II) . This form is close to (but not identical with) the salt‐induced Z‐form. All Hg(II)‐induced spectral changes are reversible upon removal of Hg(II) with a suitable complexing reagent (e.g. NaCN).