Experimental and theoretical investigations on photoabsorption and photoionization of trimethylphosphate in the vacuum-ultraviolet energy range

Abstract

In this work, we report a joint experimental-theoretical investigation on interaction of vacuum-ultraviolet radiation with trimethylphosphate (TMP) molecule (C(3)H(9)O(4)P) in gas phase. This species together with tetrahydrofuran (THF) are model compounds of deoxyribose nucleic acids (DNA)/ribose nucleic acids (RNA) backbone. Absolute photoabsorption cross sections (σ(a)) and ionization yields (η) are measured using the double-ion-chamber technique in the 11.0-21.45 eV energy range. Photoionization (σ(i)) and neutral-decay (σ(n)) cross sections in absolute scale are also derived. Moreover, theoretical photoabsorption cross sections are calculated using the time-dependent density functional theory from the excitation threshold up to 16 eV. Good agreement between the present calculated and experimental photoabsorption cross sections in the 11.0-14.5 eV range is encouraging. Also, the present measured data of σ(a) and σ(i) for TMP are about 1.3 and 1.5 times of those of THF, respectively. Thus, the experimental evidences that the majority of strand breaks being located at sugar rings in the irradiated DNA/RNA backbone moiety may be induced by a possible migration of the hole, initially created at phosphate group, to the linked sugar groups. Finally, absolute partial photoionization cross sections are derived from the experimental time-of-flight mass spectra.

Ion induced fragmentation cross-sections of DNA constituents

“…As a consequence of the induced charge rearrangement, formaldehyde (CH 2 O) is dissociating and C 2 H 7 O 3 P + , the largest mass peak at 110 amu, is formed. Mass 109 amu is formed when the σ P OCH orbital is ionized and the methoxy radical (OCH 3 ) dissociated without previous isomerization [43]. Further dissociation pathways are illustrated by Bafus [45] and Holtzclaw [47] The cation yields as a function of proton energy in Figure 8 is almost constant over the displayed range between 1.55 MeV and 16 MeV.…”

“…The next lighter fragments, around 95 amu and around 79 amu, contribute less then 10% and 20%, respectively. Yields for cations around 65 amu, 47 amu and 31 amu are very low and not included in the data for 21.35 eV photon impact [43] and for electron impact by massbank.jp [26]. Mass 15 amu is found in all data sets.…”

“…Photoelectron spectra show an initial electron emission from a σ CH bond which is followed by a hydrogen migration from the methyl-group to the nonmethylated oxygen [43]. Such keto-enol isomerization is common for phosphorus esters [47].…”

“…The data is compared to literature [26,27,43]. The relative fragmentation cross sections are shown in Figure 8 for proton energies between 1.55 MeV and 16 MeV.…”

“…Apparently, the sugar backbone is more fragile than the nucleic acid base, which aligns very well to our findings for the chemical analogs THF and pyrimidine. 16 MeV are compared with the electron impact mass spectra of massbank.jp [26] and NIST [27], and the photo-fragmentation mass spectrum of Homem (21.35 eV) [43]. …”