Differential effect of Hg(II) on [d(A)n.d(T)n] and [d(A-T)n.d(A-T)n] sequences: circular dichroism (CD) measurements and endonuclease digestion studies using poly[d(A).d(T)] and poly[d(A-T).d(A-T)] as substrates.

The long-wavelength positive CD bands of poly[d(A).d(T)] and poly[d(A-T).d(A-T)] become inverted upon the addition of Hg(ClO4)2. Poly[d(A).d(T)] requires higher levels of mercury to undergo inversion than poly[d(A-T).d(A-T)]. Mercurated poly[d(A).d(T)] is digested more rapidly than the control by DNase I or staphylococcal nuclease at low levels of Hg(ClO4)2. Let r identical to [Hg(ClO4)2]added/[DNA-P]. A 4- to 5-fold rate increase occurs with DNase I at r = 0.25; a 2-fold increase with staphylococcal nuclease at r = 0.2. By contrast, digestion of poly[d(A-T).d(A-T)] decreases immediately with increasing r. The noted rate increases appear to be due to a modification of poly[d(A).d(T)] helix structure prior to the chiroptical conversion. The modification is interpreted as a widening of the minor groove, permitting, thus, a better binding of DNase I to its substrate. The overall changes in CD as well as enzymatic digestion rates are taken to signal mercury-induced alterations in helix screwness from right-to-left. They are totally reversible subsequent to the removal of mercury.