DNA photoreacts by nucleobase ring cleavage to form labile isocyanates

Abstract

Differential infrared absorption spectroscopy was used to study the formation of isocyanates and further photo-products in the oligonucleotides dG10, dC10 and dT10 and in their mononucleosides by ultraviolet light at 266 nm. We find that α-cleavage takes place in oligonucleotides and mononucleosides both in films and in solution. The very intense and spectrally isolated isocyanate (N=C=O) asymmetric stretch vibration at 2277 cm(-1) is used as a spectroscopic marker for detection of the photo-product. The band disappears upon reaction with small amounts of water vapour as expected for isocyanates. Quantum yields for isocyanate formation by nucleobase ring cleavage in the α-position to the carbonyl group are ∼5 × 10(-5) in the mononucleosides and up to 5 × 10(-4) in the oligonucleotides. In the mixed oligonucleotides dG10/dC10 and dA10/dT10 the quantum yield of α-cleavage drops by a factor of 10 compared to the single oligonucleotides. Implications for DNA repair and photo-induced DNA-protein cross-linking via isocyanate reaction with NH2 groups of amino acids are discussed.

Ultraviolet Absorption Induces Hydrogen‐Atom Transfer in G⋅C Watson–Crick DNA Base Pairs in Solution

“…Thef ate of this product is unknown, and slow backreaction to the WC structure or formation of other products are possible. [14] The1 720 cm À1 product band shows al inear UV power dependence,a nd no build-up of other photoproducts was observed. Hence multi-photon-induced photochemistry can be excluded (see Sections S16 and S17 for an extended discussion).…”

Ultrafast intersystem crossing dynamics in uracil unravelled byab initiomolecular dynamics

“…In their work, it is also noted that this pathway would probably lead to photochemical products different from the equilibrium geometry. Interestingly Buschhaus et al 131 do observe ring opening after UV irradiation of nucleosides but the detected isocyanates (R-NQCQO) cannot arise directly from the N 3 -C 4 bond cleavage predicted by Nachtigallova et al 103 The third deactivation pathway involves a change of character to np* after initial trapping and relaxation through the S 2 /S 1 CoIn. The trapping in the np* minimum delays the ground state relaxation, affecting the time scales obtained.…”

Near-IR-Induced, UV-Induced, and Spontaneous Isomerizations in 5-Methylcytosine and 5-Fluorocytosine

“…These conclusions were in contrast with UV-induced hydrogen-atom-transfer and ring-opening processes experimentally found in isolated monomers of nucleic acid bases 4−8 as well as for these compounds in solution 9−11 and in the solid state. 12 Very recently, lifetimes have been determined for the excited 1 ππ state vibronic levels of the amino-oxo form of cytosine in the gas phase. 13,14 It has been shown that the lifetime of the 0− 0 level is as long as 44 ps and that up to 437 cm −1 of energy excess the lifetime does not decrease below 25 ps.…”