Time-resolved photoelectron and photoion fragmentation spectroscopy study of 9-methyladenine and its hydrates: a contribution to the understanding of the ultrafast radiationless decay of excited DNA bases.

The excited state dynamics of the purine base 9-methyladenine (9Me-Ade) has been investigated by time- and energy-resolved photoelectron imaging spectroscopy and mass-selected ion spectroscopy, in both vacuum and water-cluster environments. The specific probe processes used, namely a careful monitoring of time-resolved photoelectron energy distributions and of photoion fragmentation, together with the excellent temporal resolution achieved, enable us to derive additional information on the nature of the excited states (pp*, np*, ps*, triplet) involved in the electronic relaxation of adenine. The two-step pathway we propose to account for the double exponential decay observed agrees well with recent theoretical calculations. The near-UV photophysics of 9Me-Ade is dominated by the direct excitation of the pp* (1Lb) state (lifetime of 100 fs), followed by internal conversion to the np* state (lifetime in the ps range) via conical intersection. No evidence for the involvement of a ps* or a triplet state was found. 9Me- Ade–(H2O)n clusters have been studied, focusing on the fragmentation of these species after the probe process. A careful analysis of the fragments allowed us to provide evidence for a double exponential decay profile for the hydrates. The very weak second component observed, however, led us to conclude that the photophysics were very different compared with the isolated base, assigned to a competition between (i) a direct one-step decay of the initially excited state (pp* La and/or Lb, stabilised by hydration) to the ground state and (ii) a modified two-step decay scheme, qualitatively comparable to that occurring in the isolated molecule

SELECTIVE FOURIER TRANSFORM SPECTROSCOPY OF RADICALS

Author Institution: Laboratoire d'Infrarouge, Associ\'{e} au C.N.R.S., B\^{a}t. 350, Universit\'{e} Paris XI; Laboratoire de Physique Moleculaire et Atmospherique, C.N.R.S. et Universite Pierre et Marie CurieTwo types of selective detection of paramagnetic species, based on Zeeman effect, have been examined with both Fourier transform and diode laser spectrometers. A magnetic modulation requires both a constant and a periodic magnetic fields: however a variation of the magnetic field in a plasma induces, in addition to Zeeman modulation, a non selective modulation concentration. This effect is avoided with a polarization modulation which needs a polarizing system and a constant magnetic field only. We present here experimental results obtained using these technics

TAUTOMERISM OF THE DNA BASE GUANINE AND ITS METHYLATED DERIVATIVES AS STUDIED BY GAS-PHASE IR AND UV SPECTROSCOPY

Author Institution: Laboratoire Francis Perrin, URA CEA-CNRS 2453, Centre d'Etudes de SaclayCalculations suggest that only four tautomeric forms of the guanine DNA base should be significantly populated in the gas phase. Because of the fragility of the molecule only a few expermental studies have been dedicated to the gas-phase molecule so far and only three species have been identified. The present IR-UV depletion spectroscopic study, carried out in a supersonic expansion coupled to a desorption device, extends the recent data of literature by providing evidence for the existence of a fourth form of guanine in the gas phase. The comparison of the UV and IR signatures of the four forms together with those of the 7- and 9-methylated derivatives allows us to build up a new assignment in terms of enol/keto and 7/9NH tautomerisms. From this complete picture, it turns out that the UV spectroscopy of free guanine is mainly controlled by the 7/9NH tautomerism: both 7NH tautomers observed are red shifted compared to the 9NH ones, with the following origin transition order, from red to blue: 7NH enol $(32864 +/-5 cm^{-1}), 7NH keto (+405 cm^{-1}), 9NH keto (+1046 cm^{-1})$ and 9NH enol $(+1891 cm^{-1}); 7-, 9-$ or 1-methylations are found to cause only moderate red shifts (less than $400 cm^{-1})$. The opposite trend is observed for the IR spectroscopy, which appears to be essentially controlled by the enol/keto tautomerism. This study exemplifies the need for cross-checked experimental approaches, namely the IR/UV depletion spectroscopy or the study of relevant methylated species, in order to reach a global and consistent assignment, even in rather simple biological systems such as purine bases. Additional spectroscopic data, namely fluorescence spectra and lifetimes measurements, provide evidence for the fluorescent character of first near UV electronic state of all tautomers, in contrast to the short lifetimes reported in solution

Tautomerism of the DNA Base Guanine and Its Methylated Derivatives as Studied by Gas-Phase Infrared and Ultraviolet Spectroscopy

International audienceThe present IR-UV depletion spectroscopic study extends the recent data of literature by providing evidence for the existence of a fourth form of guanine in the gas phase. The comparison of the UV and IR signatures of the four forms together with those of the 7- and 9-methylated derivatives allows us to build up a new assignment in terms of enol/keto and 7/9NH tautomerisms. From this complete picture, it turns out that the UV spectroscopy of free guanine is mainly controlled by the 7/9NH tautomerism: both 7NH tautomers observed are red-shifted compared to the 9NH ones, with the following origin transition order, from red to blue: 7NH enol (32864 ± 5 cm-1), 7NH keto (+405 cm-1), 9NH keto (+1046 cm-1), and 9NH enol (+1891 cm-1); 7-, 9- or 1-methylations are found to cause only moderate red shifts (less than 400 cm-1). The opposite trend is observed for the IR spectroscopy, which appears to be essentially controlled by the enol/keto tautomerism. This study exemplifies the need for cross-checked experimental approaches, namely the IR/UV depletion spectroscopy or the study of relevant methylated species, to reach a global and consistent assignment, even in rather simple biological systems such as purine bases

The infrared emission spectrum of SiN between 2.2 and 4.4 μ\mum

The emission spectrum of a plasma of silane and nitrogen, excited by a radio frequency discharge, has been recorded with a high resolution Fourier transform spectrometer. We report here the first observation by IR spectroscopy of the X $^2 \Sigma^+ (v=1, 2)$ and A $^2 \Pi_{\rm i} (v=2, 3, 4)$ vibrational levels of $^{28}$Si$^{14}$N, through the observation of 724 unperturbed transitions of the (1-0), (2-0), (2-1), (3-1), and (4-2) vibronic bands of the A $^2 \Pi_{\rm i}$ – X $^2 \Sigma^+$ system of this radical. The data were analyzed in a least squares fit procedure, with a standard deviation equal to $6\times10^{-4}$ cm$^{-1}$

THE ELECTRONIC EMISSION SPECTRUM OF CuCl

Author Institution: Department of Chemistry, University of Waterloo; Laboratoire de Photo Physique Moleculaire, Bat. 210, Universit\'{e} de Paris XIThe electronic emission spectrum of CuCl has been observed. The spectrum was observed when CuCl powder and Cu metal were placed in a copper hollow cathode lamp operated with He gas. On the basis of recent ab initio calculations, the labels for the electronic states were revised and the 0-0, 0-1 and 1-0 bands of the $a^{3}\Sigma^{+}_{1} - X^{1}\Sigma^{+}(A^{1}\Pi-X^{1}\Sigma^{+}),b^{3}\Pi-X^{1}\Sigma^{+}(B^{1}\Pi-X^{1} \Sigma^{+}), b^{3}\Pi_{o}-X^{1}\Sigma^{+} (C^{1}\Sigma^{+}-X^{1}\Sigma^{+}), A^{1}\Pi-X^{1}\Sigma^{+} (D^{1}\Pi-X^{1}\Sigma^{+})$ and $B^{1}\Sigma^{+}-X^{1}\Sigma^{+}(E^{1}\Sigma^{-}-X^{1}\Sigma^{+})$ transitions were measured. The old labels are in parentheses. Improved band constants were obtained for the excited and ground states

Tautomer contributions to the near UV spectrum of guanine: towards a refined picture for the spectroscopy of purine molecules

By using depopulation laser techniques, like IR-UV population labeling coupled to mass-selected R2PI detection, we confirm that four tautomers are responsible for the near UV spectroscopy (310–280 nm) of guanine: two enol and two keto forms, each pair having a 7NH and a 9NH form. Besides the UV spectroscopy of each tautomer, additional information on the excited state nature and dynamics is obtained from fluorescence studies. In particular, the quenching of fluorescence beyond 285 nm, the existence of a background absorption, as well as the existence of a strongly red-shifted component in the fluorescence emission provides evidence for a strong electronic mixing in the excited state together with an efficient non-radiative process. The details of these features are found to be tautomer-dependent. Comparison of the present results with literature data on other purine molecules, like adenine or 9-substituted guanines, enables us to propose a new insight on the spectroscopy and dynamics of the purine molecules. First, a large variability of the tautomer distribution in the gas phase is found within the purine family, in particular a molecular change, as simple as a 9-methylation on guanine, can reduce the tautomer distribution to a single species (enol form). Since the absorption spectrum is tautomer-dependent as well as substituent-dependent, it turns out that the tautomer population is one of the major parameters that control the overall shape of the UV spectrum. Second, the excited state model, often evoked in the literature, which involves electronic coupling between excited states of different natures, namely $\pi\pi^*$ and $n\pi^*$ states, might account for the present fluorescence measurements on guanine, providing an extensive excited state electronic mixing is assumed for these systems

Photoionization and dissociative photoionization of the allyl radical, C3H5

International audienceThe photoionization and dissociative photoionization of the allyl radical in the VUV range was investigated using synchrotron radiation. Allyl radicals were generated by flash pyrolysis from allyl iodide and 1,5-hexadiene. Mass spectra show the appearance Of C3H3+ as a fragment in the dissociative photoionization of allyl above 10 eV. Between 10.4 and 10.5 eV the C3H3+ signal reaches 50% of its maximum value. Cyclopropenyl cation, c-C3H3+, is the thermochemically most stable product, but accompanying ab initio computations indicate that formation of linear 1-C3H3+ becomes competitive at small excess energies. The dissociative photoionization of the precursor molecules is discussed as well. An ionization energy of 8.13 +/- 0.01 eV was deduced from threshold photoelectron spectra