photodissociation of adenine, gas phase adenine, multiphoton ionization spectroscopy
||August 6, 2018
||The Author(s) 2018. This article is published with open access at www.chitkara.edu.in/publications.
Laser interaction whit the gas phase nucleobase adenine is studied. A linear TOF mass spectrometer is utilized for measurements that require high mass resolution, high sensitivity, and sufficient ion yields of low mass fragment cations. The ion mass spectra are discussed at different laser energy intensities and two temperatures. In contrast to previous studies a number light ion is present in the mass spectra. The ion formation curves for 23 different ions are measured for the laser energy range from about 109 to 1010 W cm–2 and masses between 1 and 43 besides mass 57 which was present in the mass spectra and will be discuss. Data were taken heating the sample at 235 Co. The number of 355nm absorbed photons was calculated accordingly to Keldysh theory and similar results were found using adenine -Ar mixture. Our results are compared with those reported formed by protons, electrons or multiple charged ions interactions. Different ions were found indicating the possible effect of multiphoton absorption.
One of the interest on the study of biological molecules, clusters and their radicals, nitrogenous bases, glycopeptides and polysaccharides, could be focused on the effects exerted by radiation in the macromolecules as, DNA and RNA. Radiations can cause damage to biological material or the generation of genetic mutations mediated by free radicals, increasing the risk of cancer. This is one of the reasons of the present research, analyze the effect of the radiation on each of the DNA constitutes, thereby elucidating the mechanism of damage caused by radiation and preventing the multiplication of genetic damage. Aided in specific radiological therapies . Ultraviolet (UV) radiation is part of the spectrum of electromagnetic radiation emitted by the sun and includes the wavelength range from 100 to 400 nm: UVC (100–280 nm), UVB (280–315 nm), and UVA (315–400 nm). The oxygen and ozone in the atmosphere completely absorb the UVC radiation (< 280 nm) and absorb the majority (approximately 90%) of UVB. Thus, the solar UV radiation of relevance to human health and ecosystems consists of UVA and UVB wavelengths .
The study of biomolecules is essential to understand the development of life and why DNA replication processes show low fluorescence yields and very short excited life states. These structures have a high resistance ratio, but it is possible that they decompose and that they are very sensitive to thermal changes and that a low vapor pressure is present.  The evolution of the spectroscopic techniques, the optimization and the generation of technological devices together with the refinement of the laser sources, allows the study of the simple molecules and has increased the photochemical, photophysical studies and the effects of the UV light in biomolecules of simple structure. These characteristics were used to propose models of the kinetically unstable molecules, when analyzing the results of the mechanisms for the liberation of excess energy. Within these studies is the multiphoton ionization spectroscopy, this technique comprises a wide range of highly sensitive experiments, which study the interaction processes in the basal state of individual atoms or molecules with two or more photons; that it is not possible to study by means of conventional spectroscopy. When in a multiphoton interaction, the system absorbs two or more photons, the processes of absorption and emission are closely related, the simplest example of this type of experiment is when only absorption is involved [4, 5]. Adenine is one of the four nitrogen bases that belong to DNA. It is one of the building blocks of DNA the last years special interest has been concentrated on the study of the effects under UV radiation in nucleic acids also it has been considered the role that this molecule could have played in the origin and development of life on our planet [6-8]. Different aspects of astrochemical, medical or biological have being addressed. The objective of the present study are the effects of radiation on the adenine molecules, present on the processes of dissociation and ionization. Recently, it was shown that radiation can produce secondary ions/fragments with kinetic energies from thermal up to several hundred eV in a biological medium, and in the subsequent scattering events these energetic ions/fragments can also cause severe damage to DNA [9-10].
The Monte Carlo simulations for radiation damage studies on DNA and RNA , account for ionization. However, the probability of simultaneous ionization and dissociation and high energy deposition (known as dissociative ionization) has not been considered in these simulations, mainly due to lack of data until very recently. Most of the available experimental data by the interaction with electrons, protons laser in the range [12-14], synchrotron light of energy from 6-22 eV  or multiple charged ions [16-18]. In the present study, the photodissociation and photoionization of adenine in the multiple photon absorption regime were investigated at the wavelengths of 355 nm and intensities of radiation in the range 109 to 1010 W·cm–2. Radiation interacts with a molecular beam of Ar-adenine mixture, produced by the adiabatic expansion of vapors into a high vacuum chamber at 10-8 torr.
Adenine was heated at 235°C, the resulting ions were analyzed using a homeassembled Jordan R-TOF mass analyzer used in a linear mode. On the basis of the detected ions, the processes were identified as a Dissociation- Ionization. The mass spectra (MPI) were obtained, the resulting ions identified and compared with the previously reported. The identification of the ions in the mass spectra (MPI) is presented. The MPI show that the presence and intensity of the ions depends on the density power as well as the wave length and the fact of the use Ar as carrier gas. The order of the process was determined plotted the logarithm of the ion yield as a function of the logarithm of the laser intensity. Accordingly, with the Keldysh approximation  the order of the processes can be related whit number of photons required to form an ion. The MPI show that the presence and intensity of the ions depends on the density power. The results are compared with those reported by other laboratories under different experimental conditions [12-15]. Different ions from those previously identified were observed and its presence is discussed.
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In conclusion, we have presented here results based on the multiphoton ionization and dissociation of gas-phase adenine. This allowed us to analyze the low mass positive ions as well as detect ions not reported before, produced by the interaction of laser radiation. From the comparison with former experiments were a wide variety projectiles were used (from photons to multiple charged Ar) there is still to prove what of the many fragmentation paths contribute to the observed ions and to understand the response to such complicated molecules to radiation. We hope that the type of experiments reported here motivate further theoretical calculations and more refined experiments about the studies of these reactions and their influence on radiation biology.
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