Low Energy Photoionization of Phosphorothioate DNA-Oligomers and Ensuing Hole Transfer

Abstract

Phosphorothioate (PS) modified oligonucleotides (S-DNA) naturally exist in bacteria and archaea genome and are widely used as an antisense strategy in gene therapy. However, the introduction of PS as a redox active site may trigger distinct UV photoreactions. Herein, by time-resolved spectroscopy, we observe that 266 nm excitation of S-DNA d(A ps ) 20 and d(A ps A) 10 leads to direct photoionization on the PS moiety to form hemi-bonded -P-S∴S-P-radicals, in addition to A base ionization to produce A +• /A(-H) • . Fluorescence spectroscopy and global analysis indicate that an unusual charge transfer state (CT) between the A and PS moiety might populate in competition with the common CT state among bases as key intermediate states responsible for S-DNA photoionization. Significantly, the photoionization bifurcating to PS and A moieties of S-DNA is discovered, suggesting that the PS moiety could capture the oxidized site and protect the remaining base against ionization lesion, shedding light on the understanding of its existence in living organisms.

Can methylated purine bases act as photoionization hotspots?

“…The experimental result indicates the occurrence of photoionization for 6 mA under the excitation of 266 nm laser, is much easier than that for A. By single exponential fitting of the kinetics at 700 nm (Figure 3B), a lifetime of 530 ns is obtained for the hydrated electron from 6 mA photoionization, which also falls in the range of sub microsecond of the lifetime for the hydrated electron reported in literatures 24,28,56 . This hydrated electron lifetime is different from that obtained for 6mG (240 ns), possibly due to differences in the second‐order reaction rate constants for the reactions of hydrated electrons with the methylated bases, and also the different concentrations of 6 mA (0.17 mM) and 6mG (0.23 mM).…”

“…of sub of the lifetime for the hydrated electron reported in literatures. 24,28,56 This hydrated electron lifetime is different from that obtained for 6mG (240 ns), possibly due to differences in the second-order reaction rate constants for the reactions of hydrated electrons with the methylated bases, and also the different concentrations of 6 mA (0.17 mM) and 6mG (0.23 mM). As the reaction proceeds with time until 2.13 μs, 320 nm band and a broad absorption feature peaking at ~560 nm are observed.…”

“…Nanosecond time-resolved absorption spectra were recorded using an Edinburgh LP980 flash photolysis spectrometer (Edinburgh Instruments Ltd.), combined with a Nd:YAG laser (Surelite II, Continuum Inc.) as described previously. 55,56 Solutions were excited with a 266 nm laser pulse (0.2 Hz, 20-60 mJ/pulse/cm 2 , FWHM ≈ 7 ns). The analyzing light was from a 150 W pulsed xenon lamp.…”