Figure 4. The reduced P-Ap cross-link is stable and the unreduced P-Ap cross-link is relatively unstable.

The unreduced cross-link was generated by incubation of the Ap-containing duplex B in HEPES buffer (50 mM, pH 7) containing NaCl (100 mM) at 37 °C (72 h). The reduced cross-link was generated by incubation of the Ap-containing duplex B in sodium acetate buffer (750 mM, pH 5.2) and NaCNBH₃ (250 mM) at 37 °C (24 h). Following formation of the cross-links, the reaction mixtures were subjected to the following work-up conditions: pH adjusted to 3, 15 min; pH adjusted to 10, 15 min; 60 °C, 15 min; 90 °C, 15 min, 100 mM piperidine, 60° C, 15 min The amount of remaining cross-link following treatment to each condition (relative to that present at the end of the cross-link generation reaction) was measured by gel electrophoretic analysis. The yellow bars represent the relative yields of reduced cross-link and the red bars represent the relative yields of unreduced cross-link.