Valérie Brenner

Unraveling the Mechanisms of Nonradiative Deactivation in Model Peptides Following Photoexcitation of a Phenylalanine Residue

The mechanisms of nonradiative deactivation of a phenylalanine residue after near-UV photoexcitation have been investigated in an isolated peptide chain model (N-acetylphenylalaninylamide, NAPA) both experimentally and theoretically. Lifetime measurements at the origin of the first ππ* state of jet-cooled NAPA molecules have shown that (i) among the three most stable conformers of the molecule, the folded conformer NAPA B is ∼50-times shorter lived than the extended major conformer NAPA A and (ii) this lifetime is virtually insensitive to deuteration at the NH(2) and NH sites. Concurrent time-dependent density functional theory (TDDFT) based nonadiabatic dynamics simulations in the full dimensionality, carried out for the NAPA B conformer, provided direct insights on novel classes of ultrafast deactivation mechanisms, proceeding through several conical intersections and leading in fine to the ground state. These mechanisms are found to be triggered either (i) by a stretch of the N(Phe)H bond, which leads to an H-transfer to the ring, or (ii) by specific backbone amide distortions. The potential energy surfaces of the NAPA conformers along these critical pathways have been characterized more accurately using the coupled cluster doubles (CC2) method and shown to exhibit barriers that can be overcome with moderate excess energies. These results analyzed in the light of the experimental findings enabled us to assign the short lifetime of NAPA B conformer to a number of easily accessible exit channels from the initial ππ* surface, most importantly the one involving a transfer of electronic excitation to an nπ* surface, induced by distortions of the backbone peptide bond.

Spectroscopy, dynamics and structures of jet formed anthracene clusters

Abstract The spectroscopy and dynamics of the lowest excited states of the jet formed anthracene clusters (n

Energetics of a model NH–π interaction: the gas phase benzene–NH3 complex

The dissociation energy of the benzene–ammonia complex formed in a supersonic expansion has been determined (D0=1.84±0.12 kcal mol−1) from the features of its photoionisation curve as obtained by mass-resolved two-color resonant two-photon ionisation. The complex structure, stabilised by a π-type hydrogen bond between the benzene ring and the ammonia molecule located above, has also been obtained by a semi-empirical model. The neutral structure found is in good agreement with experiment and the best ab initio calculations in the literature. The ionic structures calculated enable us to interpret the slowly increasing photoionisation curve as a consequence of a large equilibrium geometry change between neutral and ion. The present study also shows that the benzene–ammonia complex is less bound than its homologue with water by ca. 0.6 kcal mol−1. However, the value found indicates that the NH–π interaction can be taken into account when modelling the structure of biological systems.

Laser induced fluorescence of jet-cooled non-conjugated bichromophores: bis-phenoxymethane and bis-2,6-dimethylphenoxymethane

Abstract The electronic spectroscopy of bichromophoric molecules linked by a OCH 2 O chain, such as bis-phenoxymethane (1), and bis-2,6-dimethylphenoxymethane (3), has been studied in a supersonic free jet by laser induced fluorescence. The experimental results have been compared to calculations resting on the perturbative CIPSI method, in which the di- and tri-excited configurations involving the π as well the v orbitals have been taken into account. The bis-phenoxymethane (1) molecule shows a single 0—0 transition which is blue-shifted relative to anisole by more than 400 cm −1 . This blue-shift has been theoretically related to the conformation of the bichromophore which displays an out-of-plane distorsion of the ether chain relative to anisole. The calculations clearly show that the observed blue-shift of the transition is related to this distorsion, and not to any electronic coupling between both cycles which is very weak. The single transition experimentally observed corresponds to the most stable structure of (1) which is in a gauche-gauche conformation relative to the CO bonds of the chain. In this structure the cycles are equivalent. The study of van der Waals complexes of (1) with usual solvents confirms this interpretation and shows that the equivalence of the cycles is removed by complexation. This contrasts with the bichromophore (3) whose 0—0 transition is blue-shifted by only 25 cm −1 relative to the 2,6-dimethylanisole subunit. Calculations have shown that in this case, the monomeric subunit as well as the bichromophore are in an out-of-plane conformation due to the steric hindrance introduced by the methyl groups. Moreover the excited state of (3) behaves as a weakly fluorescent exciplex whereas the emission resulting from the excitation of (1) is resonant. For the sake of comparison, the fluorescence excitation spectrum of the van der Waals dimer of anisole has also been studied and exhibits a red-shift with respect to bare anisole. The equilibrium geometry and the exciton coupling have also been calculated for the anisole van der Waals dimer, by means of the exchange perturbation theory.

Can we understand the different coordinations and structures of closed-shell metal cation-water clusters?

We present a model potential for studying Mq+(H2O)n=1,9 clusters where M stands for either Na+, Cs+, Ca2+, Ba2+, or La3+. The potential energy surfaces (PES) are explored by the Monte Carlo growth method. The results for the most significant equilibrium structures of the PES as well as for energetics are favorably compared to the best ab initio calculations found in the literature and to experimental results. Most of these complexes have a different coordination number in cluster compared to experimental results in solution or solid phase. An interpretation of the coordination number in clusters is given. In order to well describe the transition between the first hydration sphere and the second one we show that an autocoherent treatment of the electric field is necessary to correctly deal with polarization effects. We also explore the influence of the cation properties (charge, size, and polarizability) on both structures and coordination number in clusters, as well as the meaning of the second hydration sphere. Such an approach shows that the leading term in the interaction energy for a molecule in the second hydration sphere is an electrostatic attraction to the cation and not a hydrogen bond with the water molecules in the first hydration sphere.

Intrinsic Folding Proclivities in Cyclic β-Peptide Building Blocks: Configuration and Heteroatom Effects Analyzed by Conformer-Selective Spectroscopy and Quantum Chemistry.

This work describes the use of conformer-selective laser spectroscopy following supersonic expansion to probe the local folding proclivities of four-membered ring cyclic β-amino acid building blocks. Emphasis is placed on stereochemical effects as well as on the structural changes induced by the replacement of a carbon atom of the cycle by a nitrogen atom. The amide A IR spectra are obtained and interpreted with the help of quantum chemistry structure calculations. Results provide evidence that the building block with a trans-substituted cyclobutane ring has a predilection to form strong C8 hydrogen bonds. Nitrogen-atom substitution in the ring induces the formation of the hydrazino turn, with a related but distinct hydrogen-bonding network: the structure is best viewed as a bifurcated C8/C5 bond with the N heteroatom lone electron pair playing a significant acceptor role, which supports recent observations on the hydrazino turn structure in solution. Surprisingly, this study shows that the cis-substituted cyclobutane ring derivative also gives rise predominantly to a C8 hydrogen bond, although weaker than in the two former cases, a feature that is not often encountered for this building block.

New model potentials for sulfur–copper(I) and sulfur–mercury(II) interactions in proteins: From ab initio to molecular dynamics

We have developed new force field and parameters for copper(I) and mercury(II) to be used in molecular dynamics simulations of metalloproteins. Parameters have been derived from fitting of ab initio interaction potentials calculated at the MP2 level of theory, and results compared to experimental data when available. Nonbonded parameters for the metals have been calculated from ab initio interaction potentials with TIP3P water. Due to high charge transfer between Cu(I) or Hg(II) and their ligands, the model is restricted to a linear coordination of the metal bonded to two sulfur atoms. The experimentally observed asymmetric distribution of metal ligand bond lengths (r) is accounted for by the addition of an anharmonic (r3) term in the potential. Finally, the new parameters and potential, introduced into the CHARMM force field, are tested in short molecular dynamics simulations of two metal thiolates fragments in water. (Brooks BR et al. J Comput Chem 1983, 4, 1987. 1 )

Secondary Structures in Phe-Containing Isolated Dipeptide Chains: Laser Spectroscopy vs Quantum Chemistry

The intrinsic conformational landscape of two phenylalanine-containing protein chain models (-Gly-Phe- and -Ala-Phe- sequences) has been investigated theoretically and experimentally in the gas phase. The near UV spectroscopy (first ππ* transition of the Phe ring) is obtained experimentally under jet conditions where the conformational features can be resolved. Single-conformation IR spectroscopy in the NH stretch region is then obtained by IR/UV double resonance in the ground state, leading to resolved vibrational spectra that are assigned in terms of conformation and H-bonding content from comparison with quantum chemistry calculations. For the main conformer, whose UV spectrum exhibits a significant Franck-Condon activity in low frequency modes involving peptide backbone motions relative to the Phe chromophore, excited state IR spectroscopy has also been recorded in a UV/IR/UV experiment. The NH stretch spectral changes observed in such a ππ* labeling experiment enable us to determine those NH bonds that are coupled to the phenyl ring; they are compared to CC2 excited state calculations to quantify the geometry change upon ππ* excitation. The complete and consistent series of data obtained enable us to propose an unambiguous assignment for the gallery of conformers observed and to demonstrate that, in these two sequences, three conceptually important local structural motifs of proteins (β-strands, 27 ribbons, and β-turns) are represented. The satisfactory agreement between the experimental conformational distribution and the predicted landscape anticipated from the DFT-D approach demonstrates the capabilities of a theoretical method that accounts for dispersive interactions. It also shows that the flaws, inherent to a resonant two-photon ionization detection scheme, often evoked for aromatic chromophores, do not seem to be significant in the case of Phe.

Infrared spectra of C2H4-HCl complex

We report the first rotationally resolved observation of the infrared spectrum of the molecular complex C2H4–HCl. The complex was produced by a supersonic expansion through a pulsed slit jet. By means of a high-resolution tunable diode laser spectrometer, we have recorded the spectrum of the HCl stretching vibration for the isotopes C2H4–H35Cl and C2H4–H37Cl. From the analysis of the spectra, we determined the rotational constants and vibrational frequencies of both isotopes. These experimental results have been interpreted in view of obtaining information on the intermolecular interaction. The experimental data have been complemented by standard coupled cluster singles and doubles model including connected triple excitations with a correlation consistent polarized valence triple zeta basis set ab initio followed by grid calculations, in order to study the anharmonicity, the coupling between intramolecular and intermolecular motions, and the basis set superposition error effects. The results obtained in t...

Non-radiative relaxation of UV photoexcited phenylalanine residues: probing the role of conical intersections by chemical substitution

A conformation-selective photophysics study in phenylalanine model peptides, combining pump-probe gas phase experiments and excited state calculations, highlights for the first time the quenching properties of a primary amide group (through its nπ* excited state) along with the effect of vibrational energy that facilitates access to the conical intersection area.