Iker León

Communication: Vibrational spectroscopy of Au4 from high resolution photoelectron imaging

High resolution photoelectron spectroscopy of Au4 (-) is reported using a new photoelectron imaging apparatus. A broad vibrational progression is resolved for the detachment transition from the ground electronic state of the Y-shaped Au4 (-) to that of the Y-shaped Au4 neutral (C2v, (1)A1) in the ν2 vibrational mode with a harmonic frequency of 171(7) cm(-1) and an anharmonicity of ∼0.5 cm(-1). In addition, two low frequency modes with weak Franck-Condon factors are observed: the v3 mode with a frequency of 97(7) cm(-1) and the v6 mode with a frequency of 17(7) cm(-1). An accurate electron affinity of 2.7098(6) eV is obtained for the Y-shaped Au4 neutral cluster. The current study shows that very low frequency vibrational modes can be resolved for size-selected clusters using high resolution photoelectron imaging, providing valuable additional experimental information for cluster structure determination.

Shaping Micelles: The Interplay Between Hydrogen Bonds and Dispersive Interactions†

A subtle interplay: In the formation of a 1.6 nm micelle containing up to six molecules of propofol, a hydrogen-bond network is shown to influence the structure of the micelle, whereas the nonpolar groups arrange in such a way that the remaining noncovalent interactions are maximized. Such globular structures present a characteristic signature in the IR spectrum that will allow their identification in more complex media.

A Spectroscopic and Computational Study of Propofol Dimers and Their Hydrated Clusters

Propofol (2,6-diisopropylphenol, PPF) homodimers and their complexes with one water molecule are analyzed by means of mass-resolved excitation spectroscopy. Using two-color resonance-enhanced multiphoton ionization (REMPI) the S(1) electronic spectra of these systems are obtained, avoiding fragmentation. Due to the large size of these species, the spectra present a large abundance of lines. Using UV/UV hole-burning spectroscopy, two isomers of PPF(2) are found and the existence of at least three isomers for propofol(2)(H(2)O)(1) (PPF(2)W(1)) is demonstrated. Comparison with the structures calculated at the M06-2X/6-311++G(d,p) and M06-2X/6-31+G(d) levels of theory shows that the main driving forces in PPF(2) are several C-H···π interactions accompanied by dipole-dipole interaction between the OH moieties. On the other hand, there is evidence for the formation of cyclic hydrogen-bond structures in the heterotrimers. A comparison of the results obtained herein with those of similar systems from previously published studies follows.

Unravelling Protein–DNA Interactions at Molecular Level: A DFT and NCI Study

Histone-DNA interactions were probed computationally at a molecular level, by characterizing the bimolecular clusters constituted by selected amino acid derivatives with polar (asparagine and glutamine), nonpolar (alanine, valine, and isoleucine), and charged (arginine) side chains and methylated pyrimidinic (1-methylcytosine and 1-methylthymine) and puric (9-methyladenine and 9-methylguanine) DNA bases. The computational approach combined different methodologies: a molecular mechanics (MMFFs forced field) conformational search and structural and vibrational density-functional calculations (M06-2X with double and triple-ζ Poples basis sets). To dissect the interactions, intermolecular forces were analyzed with the Non-Covalent Interactions (NCI) analysis. The results for the 24 different clusters studied show a noticeable correlation between the calculated binding energies and the propensities for protein-DNA base interactions found in the literature. Such correlation holded even for the interaction of the selected amino acid derivatives with Watson and Crick pairs. Therefore, the balance between hydrogen bonds and van der Waals interactions (specially stacking) in the control of the final shape of the investigated amino acid-DNA base pairs seems to be well reproduced in dispersion-corrected DFT molecular models, reinforcing the idea that the specificity between the amino acids and the DNA bases play an important role in the regulation of DNA.

Water Encapsulation by Nanomicelles

Reported is the hydration of nanomicelles in the gas-phase using spectroscopic methods and quantum chemical calculations. A fine-tuning of the experimental conditions allowed formation of a propofol trimer and tetramer with a water molecule and to determine the structure of the aggregates. Their electronic and IR spectra were obtained using mass-resolved laser spectroscopy, together with the number of conformational isomers for each stoichiometry. Interpretation of the spectra in the light of high-level calculations allowed determination of the clusters structure and demonstration that the trimer of propofol with a water molecule forms cyclic hydrogen-bond networks but, on the other hand, the tetramer is big enough to encapsulate the water molecule inside its hydrophilic core. Furthermore, these hydrated nanomicelles present an unusually high binding energy, thus reflecting their high stability and their capability to trap water inside.

A spectroscopic approach to the solvation of anesthetics in jets: propofol(H2O)n, n = 4-6.

Propofol is a widely used nonvolatile anesthetic that exerts its action by docking to GABAA receptors. The docking process is a competition between solvation of the anesthetic by the extracellular medium and the stabilization inside the active site, and therefore a deep knowledge of the process requires of a good understanding of the solvation process. In this work we create propofol-water complexes containing up to six water molecules using supersonic expansions. We determine their structure by means of a number of mass-resolved laser-based excitation spectroscopic techniques, namely two-color REMPI, UV/UV, and IR/UV double resonance techniques, combined with computational chemistry. The results clearly show that water tends to self-aggregate, interacting with the hydrophilic side of propofol. Furthermore, a transition from planar to three-dimensional structures is observed in propofol(H2O)6. Comparison with structural data from similar systems such as phenol-water and pure water clusters follows.

A combined spectroscopic and theoretical study of propofol·(H2O)3

Propofol (2,6-di-isopropylphenol) is probably the most widely used general anesthetic. Previous studies focused on its complexes containing 1 and 2 water molecules. In this work, propofol clusters containing three water molecules were formed using supersonic expansions and probed by means of a number of mass-resolved laser spectroscopic techniques. The 2-color REMPI spectrum of propofol[middle dot](H(2)O)(3) contains contributions from at least two conformational isomers, as demonstrated by UV/UV hole burning. Using the infrared IR/UV double resonance technique, the IR spectrum of each isomer was obtained both in ground and first excited electronic states and interpreted in the light of density functional theory (DFT) calculations at M06-2X/6-311++G(d,p) and B3LYP/6-311++G(d,p) levels. The spectral analysis reveals that in both isomers the water molecules are forming cyclic hydrogen bond networks around propofols OH moiety. Furthermore, some evidences point to the existence of isomerization processes, due to a complicated conformational landscape and the existence of multiple paths with low energy barriers connecting the different conformers. Such processes are discussed with the aid of DFT calculations.

Resonant photoelectron spectroscopy of Au2− via a Feshbach state using high-resolution photoelectron imaging

Photodetachment cross sections are measured across the detachment threshold of Au2(-) between 1.90 and 2.02 eV using a tunable laser. In addition to obtaining a more accurate electron affinity for Au2 (1.9393 ± 0.0003 eV), we observe eight resonances above the detachment threshold, corresponding to excitations from the vibrational levels of the Au2(-) ground state (X(2)Σu(+)) to those of a metastable excited state of Au2(-) (or Feshbach resonances) at an excitation energy of 1.9717 ± 0.0003 eV and a vibrational frequency of 129.1 ± 1.5 cm(-1). High-resolution photoelectron spectra of Au2(-) are obtained using photoelectron imaging to follow the autodetachment processes by tuning the detachment laser to all the eight Feshbach resonances. We observe significant non-Franck-Condon behaviors in the resonant photoelectron spectra due to autodetachment from a given vibrational level of the Feshbach state to selective vibrational levels of the neutral final state. Using the spectroscopic data for the ground states of Au2(-) (X(2)Σu(+)) and Au2(X(1)Σg(+)), we estimate an equilibrium bond distance of 2.53 ± 0.02 Å for the Feshbach state of Au2(-) by simulating the Franck-Condon factors for the resonant excitation and autodetachment processes.

Mass-resolved infrared spectroscopy of complexes without chromophore by nonresonant femtosecond ionization detection.

Mass-resolved excitation spectroscopic techniques are usually limited to systems with a chromophore, that is, a functional group with electronic transitions in the Vis/UV, with lifetimes from hundreds of picoseconds to some microseconds. In this paper, we expand such techniques to any system, by using a combination of nanosecond IR pulses with nonresonant ionization with 800 nm femtosecond laser pulses. Furthermore, we demonstrate that the technique can achieve conformational specificity introducing an additional nanosecond IR laser. As a proof-of-principle, we apply the technique to the study of phenol(H(2)O)(1), propofol(H(2)O)(1) γ-butyrolactone(H(2)O)(n), n = 1-3, and (H(2)O)(2) complexes. While monohydrated phenol and propofol clusters permit a direct comparison with a well-studied system including an aromatic chromophore, γ-butyrolactone is a cyclic nonaromatic molecule, whose mass-resolved spectroscopy cannot be tackled by conventional techniques. Finally, we further demonstrate the potential of the technique by obtaining the first mass-resolved IR spectrum of the neutral water dimer, a nice example of a system whose ionization-based detection had not been possible to date.

Combined experimental and theoretical study of the benzocaine/Ar van der Waals system in supersonic expansions.

The electronic spectra of Benzocaine x Ar(n), n = 0-4 were obtained using two-color resonance enhanced multiphoton ionization; the 1:1 and 1:2 clusters were investigated by ultraviolet/ultraviolet hole burning, stimulated emission pumping, and other laser spectroscopies. A single isomer was found for the 1:1 cluster, while two isomers of the 1:2 cluster were found: one with the two Ar atoms on the same side of the chromophore, and the other with the two Ar atoms sitting on opposite sides of the chromophore. The observed shifts point to the existence of a single isomer for the 1:3 and 1:4 species. Dissociation energies for the neutral ground and first excited electronic state and the ion ground electronic state of the complexes have been determined by the fragmentation threshold method and by ab initio calculations conducted at the MP2 level with 6-31++g(2d, p), 6-311++g(2d, p) and AUG-cc-pVTZ basis sets. The results are compared with those obtained for other similar systems.