IR and UV Spectroscopy of Gas-Phase Monohydrated Protonated Guanine.

We use UV and infrared photodissociation spectroscopy to study monohydrated protonated guanine in a dual cryogenic ion trap spectrometer. The monohydrated complexes are formed through helium-mediated collisions between bare electrosprayed protonated guanine and low-pressure water vapor in a clustering trap maintained at 180 K, before being transferred to a quadrupole ion trap at 10 K. The spectrum of the monohydrated complex exhibits sharp vibronic transitions at the band origin and becomes broader and higher in intensity further in blue, which is very similar to protonated guanine but with a notable blue shift of ∼1850 cm (∼0.

Selective Tautomer Production and Cryogenic Ion Spectroscopy of Radical Cations: The Uracil and Thymine Cases.

The vibrational and electronic spectroscopy of the radical cations of two nucleobases (NB) (uracil and thymine) was studied by cryogenic ion photodissociation spectroscopy. The radical cations have been generated from the photodissociation of NB-Ag complexes. A charge transfer process from the NB to Ag governs the deactivation mechanism, leading to the formation of the radical cation without further tautomerization.

Cryogenic IR and UV spectroscopy of isomer-selected cytosine radical cation.

Oxidation of the nucleobases is of great concern for the stability of DNA strands and is considered as a source of mutagenesis and cancer. However, precise spectroscopy data, in particular in their electronic excited states are scarce if not missing. We here report an original way to produce isomer-selected radical cations of DNA bases, exemplified in the case of cytosine, through the photodissociation of cold cytosine-silver (C-Ag) complex.

Conformer-selective Photodynamics of TrpH -H O.

The photodynamics of protonated tryptophan and its mono hydrated complex TrpH -H O has been revisited. A combination of steady-state IR and UV cryogenic ion spectroscopies with picosecond pump-probe photodissociation experiments sheds new lights on the deactivation processes of TrpH and conformer-selected TrpH -H O complex, supported by quantum chemistry calculations at the DFT and coupled-cluster levels for the ground and excited states, respectively. TrpH excited at the band origin exhibits a transient of less than 100 ps, assigned to the lifetime of the excited state proton transfer (ESPT) structure.

Excited-state proton transfer in protonated adrenaline revealed by cryogenic UV photodissociation spectroscopy.

We report a comprehensive study of the structures and deactivation processes of protonated adrenaline through cryogenic UV photodissociation spectroscopy. Single UV and double-resonance UV-UV hole burning spectroscopies have been performed and compared to coupled-cluster SCS-CC2 calculations performed on the ground and first electronic states. Three conformers were assigned, two lowest energy gauche conformers along with a higher energy conformer with an extended structure which is indeed the global minimum in solution.

Water binding to Fe hemes studied in a cooled ion trap: characterization of a strong 'weak' ligand.

The interaction of a water molecule with ferric heme-iron protoporphyrin ([PP Fe]) has been investigated in the gas phase in an ion trap and studied theoretically by density functional theory. It is found that the interaction of water with ferric heme leads to a stable [PP-Fe-HO] complex in the intermediate spin state (S = 3/2), in the same state as its unligated [PP-Fe] homologue, without spin crossing during water attachment. Using the Van't Hoff equation, the reaction enthalpy for the formation of a Fe-OH bond has been determined for [PP-Fe-HO] and [PP-Fe-(HO)].

UV Photofragmentation of Cold Cytosine-M Complexes (M: Na, K, Ag).

The UV photofragmentation spectra of cold cytosine-M complexes (M: Na, K, Ag) were recorded and analyzed through comparison with geometry optimizations and frequency calculations of the ground and excited states at the SCS-CC2/Def2-SVPD level of theory. While in all complexes, the ground state minimum geometry is planar ( symmetry), the ππ* state minimum geometry has the NH group slightly twisted and an out-of-plane metal cation. This was confirmed by comparing the simulated ππ* Franck-Condon spectra with the vibrationally resolved photofragmentation spectra of CytNa and CytK.

The dramatic effect of N-methylimidazole on trans axial ligand binding to ferric heme: experiment and theory.

The binding energy of CO, O2 and NO to isolated ferric heme, [FeIIIP]+, was studied in the presence and absence of a σ donor (N-methylimidazole and histidine) as the trans axial ligand. This study combines the experimental determination of binding enthalpies by equilibrium measurements in a low temperature ion trap using the van't Hoff equation and high level DFT calculations. It was found that the presence of N-methylimidazole as the axial ligand on the [FeIIIP]+ porphyrin dramatically weakens the [FeIIIP-ligand]+ bond with an up to sevenfold decrease in binding energy owing to the σ donation by N-methylimidazole to the FeIII(3d) orbitals.

Photodissociation Spectroscopy of Cold Protonated Synephrine: Surprising Differences between IR-UV Hole-Burning and IR Photodissociation Spectroscopy of the O-H and N-H Modes.

We report the UV and IR photofragmentation spectroscopies of protonated synephrine in a cryogenically cooled Paul trap. Single (UV or IR) and double (UV-UV and IR-UV) resonance spectroscopies have been performed and compared to quantum chemistry calculations, allowing the assignment of the lowest-energy conformer with two rotamers depending on the orientation of the phenol hydroxyl (OH) group. The IR-UV hole burning spectrum exhibits the four expected vibrational modes in the 3 μm region, i.

Tautomerism and electronic spectroscopy of protonated 1- and 2-aminonaphthalene.

Experimental and theoretical investigations of the excited states of protonated 1- and 2-aminonaphthalene are presented. The electronic spectra are obtained by laser induced photofragmentation of the ions captured in a cold ion trap. Using ab initio calculations, the electronic spectra can be assigned to different tautomers which have the proton on the amino group or on the naphthalene moiety.

Excited State Dynamics of Cold Protonated Cytosine Tautomers: Characterization of Charge Transfer, Intersystem Crossing, and Internal Conversion Processes.

Charge transfer reactions are ubiquitous in chemical reactivity and often viewed as ultrafast processes. For DNA, femtochemistry has undeniably revealed the primary stage of the deactivation dynamics of the locally excited state following electronic excitation. We here demonstrate that the full time scale excited state dynamics can be followed up to milliseconds through an original pump-probe photodissociation scheme applied to cryogenic ion spectroscopy.

Dioxygen Binding to Protonated Heme in the Gas Phase, an Intermediate Between Ferric and Ferrous Heme.

With a view to characterizing the influence of the electronic structure of the Fe atom on the nature of its bond with dioxygen (O ) in heme compounds, a study of the UV/Vis action spectra and binding energies of heme-O molecules has been undertaken in the gas phase. The binding reaction of protonated ferrous heme [Fe -hemeH] with O has been studied in the gas phase by determining the equilibrium of complexed [Fe -hemeH(O )] with uncomplexed protonated heme in an ion trap at controlled temperatures. The binding energy of O to the Fe atom of protonated ferrous heme was obtained from a van't Hoff plot.

Vibronic spectra of protonated hydroxypyridines: contributions of prefulvenic and planar structures.

Various hydroxypyridine derivatives are endogenous or synthetic photosensitizers which could contribute to solar radiation damage. The study of their excited states could lead to a better understanding of their action mechanisms. We present here the ultraviolet (UV) spectra of the protonated 2-, 3- and 4-hydroxypyridine.

Experimental and ab initio characterization of HCN vibronic structure. II. High-resolution VUV PFI-ZEKE spectroscopy.

Vacuum-ultraviolet pulsed-field-ionization zero-kinetic-energy photoelectron spectra of XΠ2←XΣ+1 and BΠ2←XΣ+1 transitions of the HCN and HCN isotopologues of cyanoacetylene have been recorded. The resolution of the photoelectron spectra allowed us to resolve the vibrational structures and the spin-orbit splittings in the cation. Accurate values of the adiabatic ionization potentials of the two isotopologues (E/hc(HCN)=93 909(2) cm and E/hc(HCN)=93 912(2) cm), the vibrational frequencies of the ν, ν, and ν vibrational modes, and the spin-orbit coupling constant (A = -44(2) cm) of the XΠ2 cationic ground state have been derived from the measurements.

Hydrogen bonds vs. π-stacking interactions in the p-aminophenolp-cresol dimer: an experimental and theoretical study.

The gas phase structure and excited state lifetime of the p-aminophenolp-cresol heterodimer have been investigated by REMPI and LIF spectroscopy with nanosecond laser pulses and pump-probe experiments with picosecond laser pulses as a model system to study the competition between π-π and H-bonding interactions in aromatic dimers. The excitation is a broad and unstructured band. The excited state of the heterodimer is long lived (2.

Multiscale excited state lifetimes of protonated dimethyl aminopyridines.

The excited state dynamics of protonated ortho (2-) and para (4-) dimethyl aminopyridine molecules (DMAPH(+)) has been studied through pump-probe photofragmentation spectroscopy and excited state coupled-cluster CC2 calculations. Multiscale temporal dynamics has been recorded over 9 orders of magnitude from subpicosecond to millisecond. The initially locally excited ππ* state rapidly decays within about 100 fs into a charge transfer state following 90° twist motion of the dimethyl amino group.

Photofragmentation mechanisms in protonated chiral cinchona alkaloids.

The photo-stability of protonated cinchona alkaloids is studied in the gas phase by a multi-technique approach. A multi-coincidence technique is used to demonstrate that the dissociation is a direct process. Two dissociation channels are observed.

Twisted Intramolecular Charge Transfer in Protonated Amino Pyridine.

The excited state properties of protonated ortho (2-), meta (3-), and para (4-) aminopyridine molecules have been investigated through UV photofragmentation spectroscopy and excited state coupled-cluster CC2 calculations. Cryogenic ion spectroscopy allows recording well-resolved vibronic spectroscopy that can be reproduced through Franck-Condon simulations of the ππ* local minimum of the excited state potential energy surface. The excited state lifetimes have also been measured through a pump-probe excitation scheme and compared to the estimated radiative lifetimes.

Non-radiative processes in protonated diazines, pyrimidine bases and an aromatic azine.

The excited state lifetimes of DNA bases are often very short due to very efficient non-radiative processes assigned to the ππ*-nπ* coupling. A set of protonated aromatic diazine molecules (pyridazine, pyrimidine and pyrazine C4H5N2(+)) and protonated pyrimidine DNA bases (cytosine, uracil and thymine), as well as the protonated pyridine (C5H6N(+)), have been investigated. For all these molecules except one tautomer of protonated uracil (enol-keto), electronic spectroscopy exhibits vibrational line broadening.

Trapped Hydronium Radical Produced by Ultraviolet Excitation of Substituted Aromatic Molecule.

The gas phase structure and excited state dynamics of o-aminophenol-H2O complex have been investigated using REMPI, IR-UV hole-burning spectroscopy, and pump-probe experiments with picoseconds laser pulses. The IR-UV spectroscopy indicates that the isomer responsible for the excitation spectrum corresponds to an orientation of the OH bond away from the NH2 group. The water molecule acts as H-bond acceptor of the OH group of the chromophore.

Conformer- and Mode-Specific Excited State Lifetimes of Cold Protonated Tyrosine Ions.

The excited state lifetimes of conformer- and mode-selected cold protonated tyrosine have been measured for the first time through a picosecond pump-probe photodissociation scheme. Whereas the photofragmentation mechanism of protonated tyrosine ions strongly depends upon the interaction of the carboxylic acid group with the phenol ring, their excited state lifetimes are quite similar and decrease as the excess energy increases from 1.5 ns at the band origin to 900 ps and less than 500 ps for the 10(1) and 10(2) bands, respectively.

Bonding of heme Fe(III) with dioxygen: observation and characterization of an incipient bond.

While ferrous heme (Fe(II)) within hemoproteins binds dioxygen efficiently, it has not yet been possible to observe the analog complex with ferric heme (Fe(III)). We present the first observation and characterization of the latter complex in a cooled ion trap. The bond formation enthalpy of ferric heme-O2 has been derived from the Van't Hoff equation by means of temperature dependent measurements.

A comprehensive study of cold protonated tyramine: UV photodissociation experiments and ab initio calculations.

We present a comprehensive experimental study of protonated tyramine ions in a cold 3D quadrupole ion trap coupled to a time-of-flight mass spectrometer. Multiple UV photodissociation techniques have been developed, including single and double resonance spectroscopy along with time-resolved excited state lifetime measurements through a picosecond pump-probe scheme. An original UV-UV hole burning method is presented which can be used without modification of the quadrupole ion trap.

Excited State Dynamics of Protonated Phenylalanine and Tyrosine: Photo-Induced Reactions Following Electronic Excitation.

The electronic spectroscopy and the electronic excited state properties of cold protonated phenylalanine and protonated tyrosine have been revisited on a large spectral domain and interpreted by comparison with ab initio calculations. The protonated species are stored in a cryogenically cooled Paul trap, maintained at ∼10 K, and the parent and all the photofragment ions are mass-analyzed in a time-of-flight mass spectrometer, which allows detecting the ionic species with an improved mass resolution compared to what is routinely achieved with a quadrupole mass spectrometer. These new results emphasize the competition around the band origin between two proton transfer reactions from the ammonium group toward either the aromatic chromophore or the carboxylic acid group.

Electronic and vibrational spectra of protonated benzaldehyde-water clusters, [BZ-(H2O)n≤5]H+: evidence for ground-state proton transfer to solvent for n ≥ 3.

Vibrational and electronic photodissociation spectra of mass-selected protonated benzaldehyde-(water)n clusters, [BZ-(H2O)n]H(+) with n ≤ 5, are analyzed by quantum chemical calculations to determine the protonation site in the ground electronic state (S0) and ππ(*) excited state (S1) as a function of microhydration. IR spectra of [BZ-(H2O)n]H(+) with n ≤ 2 are consistent with BZH(+)-(H2O)n type structures, in which the excess proton is localized on benzaldehyde. IR spectra of clusters with n ≥ 3 are assigned to structures, in which the excess proton is located on the (H2O)n solvent moiety, BZ-(H2O)nH(+).