Laser-Induced Strand Break Formation of Polyuridylic Acid in the Presence and Absence of Tris(2,2’-Bipyridyl)Ruthenium Chloride and K₂S₂O₈

Abstract

The effect of light (steady state and laser pulse) on the system poly(U), Ru(bpy)² ₃ ⁺ (Ru(II)) and S₂O₈ ^2- has been studied and the following mechanism for the formation of single-strand breaks (ssb) is proposed: Electronically excited Ru(bpy)²⁺ ₃ ions, bound to poly(U), react with S₂O^2- ₈ by electron transfer resulting in Ru(bpy)₃ ³⁺ (Ru(III)) and SO₄ ^-. Both the ruthenium(III) complex and SO₄ react with the bases of poly(U) leading to base radicals which in turn leads to ssb formation by H abstraction from position 2’ of the sugar moiety. The reaction scheme is complicated by reactions of the base radicals with Ru(III) or with S₂O₈ ^2-,reactions which compete with ssb formation. The rate and yield of ssb formation was measured by transient conductivity.

The rate of the Ru(bpy)₃ ²⁺/S₂O₈ ^2- photosensitized ssb formation is not pH dependent (range 4-7), in contrast to that under OH radical-induced conditions. The explanation is that the ruthenium complex is so strongly bound to poly(U), due to its double positive charge, that protons do not compete in binding even at pH4. Therefore the rate is not accelerated by the presence of protons, as is observed in the case of the OH-induced ssb formation where only K⁺ ions serve as counterions. The relative yield and the rate of ssb formation depend on the nucleotide/sensitizer ratio (N/S). The rate constant for ssb formation is 4.5 s^-1 at N/S = 3 and 14 s^-1 at N/S = 10. The reason for the influence of the N/S ratio is not clear as yet. The formation and decay of Ru(III) has been measured spectroscopically. The decay rate constant at N/S = 10 is very similar to that for ssb formation in agreement with the postulated reaction scheme.