M. S. de Vries

Guanine tautomerism revealed by UV–UV and IR–UV hole burning spectroscopy

The vibronic spectrum of laser desorbed and jet cooled guanine consists of bands from three different tautomers of guanine as revealed by UV–UV and IR–UV double resonance spectroscopy. 1-methylguanine, in which the Keto–Enol tautomerism is blocked, shows hole burning spectra from the 9H-and 7H-Keto form. A comparison of the vibronic pattern of the different tautomers demonstrates that the vibronic spectrum built on the redmost guanine band at 32u200a870 cm−1 (electronic origin 0) can be traced back to the 9H-Enol tautomer, while the spectra built on the origins at 0+404u200acm−1 and 0+1044u200acm−1 stem from the two Keto tautomers. The IR–UV double resonance spectra of the OH-and NH-stretch vibrations of the different tautomers support this assignment. The UV and IR spectra can be partly assigned by comparison with ab initio calculated vibrational frequencies and with the help of deuteration experiments.

REMPI spectroscopy of cytosine

We report resonant two-photon ionization spectra of laser desorbed, jet cooled, cytosine, 1-methyl cytosine, 5-methyl cytosine, and dimers of these. Unlike other pyrimidine bases, cytosine exhibits vibronic spectra with sharp features in two spectral regions, separated by about 5000 cm 1 . We interpret these as being due to two tautomeric forms, one keto and one enol. The dimers absorb at wavelengths that are intermediate between those of the two monomer forms. By UV–UV hole burning we determined the numbers of isomers contributing to each spectrum and by delayed two color ionization we determined triplet lifetimes. We observed hydrogen transfer between bases both in collisions between monomers and after photo-excitation in clusters. 2002 Published by Elsevier Science B.V.

Pairing of the nucleobases guanine and cytosine in the gas phase studied by IR–UV double-resonance spectroscopy and ab initio calculations

We present R2PI, IR–UV and UV–UV double resonance measurements of the guanine–cytosine (G–C) dimer formed in a supersonic jet. We show that there is only one isomer of G–C in the investigated wavelength range from 33200 to 34100 cm−1. We assigned the observed G–C isomer to a specific structure, based on comparisons of the IR spectra of the G and C monomers with the G–C dimer in the range of the OH and NH stretching vibrations and ab initio-calculated vibrational frequencies and dimer stabilities. The cluster exhibits an HNH⋯O/NH⋯N/CO⋯HNH bonding similar to the Watson–Crick G–C base pair bonding but with C as the enol tautomer. We did not observe any keto–keto or enol–enol G–C dimers in the investigated wavelength region.

IR-UV double-resonance spectroscopy of the nucleobase adenine

We present R2PI and IR–UV double resonance spectra of the nucleobase adenine seeded in a supersonic jet. We show that there is only one tautomer of adenine which absorbs in the wavelength range 36u2006050 to 36u2006700 cm−1. The IR spectra, measured in the range 3200 to 3700 cm−1, show bands at 3452, 3508 and 3569 cm−1, which we assign to the symmetric NH2u2006, N–H and antisymmetric NH2 stretching vibrations of a single tautomer of adenine. We compare the experimental IR–UV double resonance spectra with ab initio based normal mode calculations. The observed tautomer is most probably the 9H amino-form of adenine.

The nucleobase cytosine and the cytosine dimer investigated by double resonance laser spectroscopy and ab initio calculations

The vibronic spectrum of laser desorbed and jet cooled cytosine consists of bands from two major tautomers (keto and enol) as revealed by UV-UV and IR-UV double resonance spectroscopy and methyl blocking experiments. Only one isomer each was observed for the cytosine dimer and for the cytosine - 1-methylcytosine mixed dimer. These isomers form CO⋯HNH/NH⋯N hydrogen bonds. Cytosine - 5-methylcytosine exhibits three isomers: one again with CO⋯HNH/NH⋯N connectivity, the second with CO⋯HNH/NH⋯N interaction but one cytosine in the enol form and the third with symmetrical CO⋯NH/NH⋯OC bonds. These are also the most stable clusters according to molecular dynamics/quenching and ab initio quantum chemical calculations. The experimental IR spectra of these isomers agree well with the calculated normal mode vibrational spectra. The vibronic spectra of the clusters are blue shifted relative to the monomer spectra by more than 1000 cm−1 indicating a considerable reduction of dimer stability upon electronic excitation.

Fragmentation of laser-desorbed 9-substituted adenines

Abstract In laser desorption of a series of cyclically 9-substituted adenines, followed by multiphoton ionization, we observed characteristic fragmentation. This is in marked contrast to laser desorption under identical conditions of cyclically 9-substituted guanines, for which we obtained predominantly the parent mass. By applying resonant 2 photon ionization spectroscopy (R2PI) to the laser-desorbed, jet-cooled fragments, we show that the fragmentation is induced by the desorption process and not by the excitation/ionization process. Fragmentation takes place between C′2 and C′3 and between C′1 and O6 in the sugar ring. By adding a propyl bridge between C′2 and C′3, the fragmentation pathway changes to a cleavage of the C′4ue5f8C′5 bond in the sugar ring. This allowed us to record R2PI spectra of a fragment species that can serve as a model for adenosine.

Characterizing the dark state in thymine and uracil by double resonant spectroscopy and quantum computation

We report on gas phase double resonant spectroscopy of both the ground state and the dark excited state in isolated uracil and thymine. We also report lifetimes of the dark state for different excitation wavelengths. In combination with ab initio calculations the results suggest that the dark state is of triplet ((3)ππ*) character.

The field-ionization of near-dissociation ion-pair states of I2

Using a resonant multiphoton excitation pathway, it is shown that electronic states down to 30 cm−1 below the first ion-pair dissociation threshold of I2 can be efficiently converted from initially prepared well-defined low-J states to long-lived (τd>4u200aμs), subthreshold zero ion kinetic energy (ZIKE) ion-pair states which are analogous to high, zero electron kinetic energy, Rydberg states. A pulsed electric field is used to dissociate the ZIKE states and produce free-ion pairs (i.e., I++I−). Direct excitation to very high vibrational levels of the ion-pair states is ruled out and polarization data are used to probe the spectroscopic character of the doorway states to free-ion formation. The ion-pair dissociation limit, determined from the extrapolation to zero field of the onset of the prompt I+/I− signal, agrees well with the literature value.

Sub-micron proximal probe thermal desorption and laser mass spectrometry on painting cross-sections

We demonstrate sub-micron, atomic force microscopy (AFM) proximal probe desorption of organic dyes, and subsequent detection via laser mass spectrometry. A nanothermal analysis (nano-TA) probe tip in contact with a surface is heated (10000 °C s−1) to induce thermal desorption, creating depression sizes ranging from 360–1500 nm in diameter and 20–100 nm in depth. Desorbed material is drawn through a heated capillary via vacuum, and deposits onto a graphite sample bar. Laser desorption, followed by supersonic jet-cooling and either resonant two-photon ionization (R2PI) or non-resonant ionization mass spectrometry is used to characterize the transferred material. Individual, microscopic layers of organic dyes within painting cross-sections were successfully analyzed using this new approach. Separating the AFM thermal desorption step from the detection step allows for the use of analytical techniques appropriate for individual samples of material, desorbed with high spatial resolution.

Nanometer-Scale Proximal Probe Thermal Desorption Coupled with Laser Mass Spectrometry

The analysis of cultural heritage materials presents a number of obstacles which impose limitations on the range of analytical techniques that can be utilized in their analysis: limited, and extremely small samples, complexity of sample structure, the importance of maintaining spatial integrity and, most importantly, the preciousness of the samples. These limitations present particular challenges for the identification of dyes and pigments within microscopic painting cross sections, in which complex mixtures and thin (often only a few microns) layers of organic material are common.