Effect of Hg(II) on the spectroscopic properties of DNA bases: circular dichroism of deoxyadenosine and thymidine monomers and dimers.

The addition of Hg(ClO4)2(Hg(II)) to 2'-deoxyadenosine (dA), thymidine (dT), to their respective 5'-monophosphates (dAp,dTp) as well as to the dinucleoside phosphates 2'-deoxyadenylyl-(3'-->5')-2'-deoxyadenosine (d(ApA)), 2-deoxyadenylyl-(3'-->5')thymidine(d(ApT)), thymidylyl-(3'-->5')-2'-deoxyadenosine (d(TpA)), and thymidylyl-(3'-->5')thymidine (d(TpT))--all dissolved in 0.1 M NaClO4, 5 mM cacodylic acid buffer, pH 7--produces major alterations in the circular dichroism (CD) of the dimers but no or only small changes in the CD of the monomers. Of particular interest are the Hg(II)-induced changes in the CD of d(ApT) and d(TpA): they are strongly sequence-dependent and, within reason, progress in a "mirror"-like fashion when the concentration of Hg(II) is varied. In the absence of Hg(II), the CD of the dimers is conservative (d(TpT)), or near-conservative (d(ApA), d(ApT), d(TpA)), but becomes nonconservative upon the addition of Hg(II). The rotational strength R of the various Cotton effects of the dimers was evaluated as a function of Hg(II) concentration. Features of the CD spectra of mercurated d(ApA) and d(TpT) persist in the CD spectra of mercurated poly[d(A).(T)], but there is little obvious agreement of the CD spectra of mercurated d(ApT) and d(TpA) with the CD of mercurated poly[d(A-T).d(A-T)].