Isabelle Compagnon

Secondary structures of short peptide chains in the gas phase: double resonance spectroscopy of protected dipeptides

The conformational structure of short peptide chains in the gas phase is studied by laser spectroscopy of a series of protected dipeptides, Ac-Xxx-Phe-NH(2), Xxx=Gly, Ala, and Val. The combination of laser desorption with supersonic expansion enables us to vaporize the peptide molecules and cool them internally; IR/UV double resonance spectroscopy in comparison to density functional theory calculations on Ac-Gly-Phe-NH(2) permits us to identify and characterize the conformers populated in the supersonic expansion. Two main conformations, corresponding to secondary structures of proteins, are found to compete in the present experiments. One is composed of a doubly gamma-fold corresponding to the 2(7) ribbon structure. Topologically, this motif is very close to a beta-strand backbone conformation. The second conformation observed is the beta-turn, responsible for the chain reversal in proteins. It is characterized by a relatively weak hydrogen bond linking remote NH and CO groups of the molecule and leading to a ten-membered ring. The present gas phase experiment illustrates the intrinsic folding properties of the peptide chain and the robustness of the beta-turn structure, even in the absence of a solvent. The beta-turn population is found to vary significantly with the residues within the sequence; the Ac-Val-Phe-NH(2) peptide, with its two bulky side chains, exhibits the largest beta-turn population. This suggests that the intrinsic stabilities of the 2(7) ribbon and the beta-turn are very similar and that weakly polar interactions occurring between side chains can be a decisive factor capable of controlling the secondary structure.

Mid-IR spectra of different conformers of phenylalanine in the gas phase

The experimental mid- and far-IR spectra of six conformers of phenylalanine in the gas phase are presented. The experimental spectra are compared to spectra calculated at the B3LYP and at the MP2 level. The differences between B3LYP and MP2 IR spectra are found to be small. The agreement between experiment and theory is generally found to be very good, however strong discrepancies exist when -NH2 out-of-plane vibrations are involved. The relative energies of the minima as well as of some transition states connecting the minima are explored at the CCSD(T) level. Most transition states are found to be less than 2000 cm(-1) above the lowest energy structure. A simple model to describe the observed conformer abundances based on quasi-equilibria near the barriers is presented and it appears to describe the experimental observation reasonably well. In addition, the vibrations of one of the conformers are investigated using the correlation-corrected vibrational self-consistent field method.

Beam deviation of large polar molecules in static electric fields: theory and experiment

Abstract A classical approach to calculate the energy and the orientation of symmetric top and linear molecules in high electric fields is proposed. This calculation is particularly well adapted to large molecules. It is used to simulate the deviation of a molecular beam in an inhomogeneous electric field. We give an example of experimental and calculated profiles of deviation for a transition metal-fullerene compound TiC60. This is the first direct measurement of the permanent dipole of a large rigid molecule in the gas phase.

Stiff, and sticky in the right places: Binding interactions in isolated mechanically interlocked molecules probed by mid-infrared spectroscopy

We report the results of high-resolution spectroscopic studies on isolated, jet-cooled [2]rotaxanes. Employing IR absorption spectroscopy, we show how these noncovalently bound, multicomponent molecular systems that so far have been out of reach from high-resolution techniques, can now be characterized at an unprecedented level. IR absorption spectra of prototypical hydrogen-bond assembled rotaxanes as well as their associated threads and macrocycles are shown to provide a direct view on the effects of interlocking the macrocycle and thread, and to offer a straightforward approach for the study of their structural and dynamical properties.

Doubly Charged Silver Clusters Stabilized by Tryptophan: Ag42+ as an Optical Marker for Monitoring Particle Growth

Metal clusters can serve as ultimate building blocks for nanometer-scale devices because of their unique optical and electronic properties, which are size and structure dependent. However, an important issue for optical and electronics applications is the influence of charges that can affect the formation and the electronic properties of such nanosystems. As the size of the cluster decreases, the charging that occurs during the photoexcitation or the charge transport in the device may even destroy the metal moiety as a result of strong Coulomb repulsion. Small multiply charged systems, in particular silver clusters, have been studied extensively in the gas phase. Although isolated metal clusters with double charge (e.g. Agn ) are not stable for n< 7, stabilization can be provided by the environment. Indeed, the attachment of protective ligands such as complex chemical or biomolecular 6] templates was used to stabilize multiply charged clusters. In particular, small multiply charged silver clusters were observed and stabilized in solution during the formation of colloidal Ag nanoparticles. The Ag4 2+ tetramer was even suggested as the main precursor for the particle formation and was investigated by optical spectroscopy. 9] Encapsulation of multiply charged small silver clusters within biomolecular templates is particularly attractive because it can provide both stabilization and functionalization of the metal cluster, thus forming new hybrid complexes with remarkable optical and photostability properties. In this case, the mechanism for stabilization usually involves the formation of salt-bridged structures. The recent progress in the synthesis of stabilized metal nanoclusters challenges the understanding of their electronic, spectroscopic, and chemical properties at the molecular level—these issues have not been addressed to date. Gas phase studies offer the opportunity to produce and study hybrid systems under well-defined conditions. Gas phase electronic spectroscopy has succeeded in giving structural information on isolated and microhydrated molecular ions. Combined with theoretical investigations, it provides a conceptual framework to unravel optical properties of multiply charged clusters and in particular the influence of stabilizing templates. In our combined theoretical and experimental study, we investigate an Ag4 metal cluster and its interaction with a tryptophan (Trp) molecule, which can be used as a reducing agent for the synthesis of silver nanoparticles. We demonstrate that this hybrid system exhibits an unambiguous optical fingerprint of the doubly charged silver cluster (Ag4 ) stabilized by a salt-bridge (SB) interaction with the organic moiety. Thus, we propose the use of its optical signature as a marker to monitor early stages of seeding processes for particle growth. Nanohybrids composed of a unit of the aromatic amino acid tryptophan and small silver clusters were produced in the gas phase by combining electrospray ionization and multiple stage mass spectrometry. The [(Trp H)+Ag4] complex containing Ag4 2+ was formed in a trapping cell by collisioninduced fragmentation of the precursor ion [(Trp)2 3H+4Ag]. A similar approach to producing complexes of silver clusters with amino acids has already been proposed by Khairallah and O Hair and Tabarin et al. . Theory predicts favorable formation of these complexes as a result of the interaction of the deprotonated carboxylic group and the cluster in the SB structure. Two types of saltbridged structures arise from exploration of structural properties (for details see Figure SM1 in the Supporting Information): 1) the lowest energy structure SB1 (Figure 1a) contains a doubly charged Ag4 cluster bound to the COO moiety and interacting with the indole ring. The distribution of the charge in this hybrid system is illustrated by the electrostatic potential map shown in Figure 2. This map shows strong localization of positive charges on the Ag4 subunit while the negative charge is distributed over the COO group. The natural bonding orbital (NBO) analysis shows that a net charge of + 1.76 e is localized on the Ag4 subunit, thus proving [*] A. Kulesza, Prof. V. Bonačić-Koutecký Institut f r Chemie, Humboldt-Universit t zu Berlin Brook-Taylor-Straße 2, 12489 Berlin (Germany) E-mail: vbk@chemie.hu-berlin.de

Internal Proton Transfer Leading to Stable Zwitterionic Structures in a Neutral Isolated Peptide

Decades of gas-phase spectroscopy of small biomolecules have enabled some of the intrinsic physical and chemical properties of the building blocks of life to be unraveled. Bridging the gap towards an understanding of the biological function of these biomolecules has become an essential issue. For the processes that take place in the active sites of functional proteins at the molecular level to be understood, two critical aspects must be taken into account: 1) interactions with the biological environment (protons, electrons, metal ions, water molecules) and 2) the specific organization of a few significant amino acid residues nested in the welldefined local environment shaped by the entire protein. In this context, it is important to pursue a bottom-up approach, whereby elements of the environment can be introduced stepby-step in a controlled fashion until the biological function emerges. A crucial discrepancy between the gas-phase structure of isolated amino acids and peptides and their biologically relevant counterparts is the transition from the canonical to the zwitterionic form. Whereas neutral, isolated amino acids and peptides have always been found in their canonical form, studies on ionic complexes have shown that zwitterionic forms may be stabilized by the addition of a proton, an electron, a metal cation, or a metal dication or by microsolvation. In the case of overall-neutral complexes, the canonical-to-zwitterionic transition was observed upon the stepwise addition of solvent molecules. We report herein the first observation of an “autozwitterion” formed by intramolecular proton transfer between nearby residues in a neutral, isolated peptide. We specifically designed the pentapeptide Ac-Glu-Ala-Phe-Ala-Arg-NHMe (EAFAR; Scheme 1) with an appropriate structure for

Photoabsorption and photofragmentation of isolated cationic silver cluster-tryptophan hybrid systems

We present a theoretical study of the size and structure selective absorption properties of cationic silver cluster-tryptophan Trp-Ag(n)(+) (n = 2-5,9) hybrid systems supported by photofragmentation experiments. Our time-dependent density functional theory calculations provide insight into the nature of excitations in interacting nanoparticle-biomolecule subunits and allow to determine characteristic spectral features as fingerprints of two different classes of structures: charge solvated and zwitterionic. Moreover, different types of charge transfer transitions have been identified. Charge transfer from pi system of tryptophan to silver cluster occurs for charge solvated structures while charge transfer from silver to the NH(3) (+) group takes place for zwitterionic structures. This has been confirmed by experimentally measured photofragmentation channels and molecular dynamics simulations. Our findings provide fundamental insight into the structure- and size-dependent mechanism responsible for the enhanced absorption and emission in nanoparticle-biomolecular hybrid systems.

Spectroscopy of isolated, mass-selected tryptophan-Ag3 complexes: a model for photoabsorption enhancement in nanoparticle-biomolecule hybrid systems.

We wish to show that gas-phase studies of small metal cluster-biomolecule complexes provide fundamental insights into the mechanism leading to enhanced optical absorption in nanoparticle-biomolecular hybrid systems. Here we present, for the first time, a joint experimental and theoretical study of photoabsorption and photofragmentation of the silver trimer-tryptophan cation complex ([Trp.Ag3]+). We demonstrate that binding the metal cluster to a biomolecule leads to a remarkably high optical absorption as compared to the bare tryptophan or the [Trp.Ag]+ complex. As calculations show this arises due to coupling between the excitations in the metallic subunit with a charge transfer excitation to the tryptophan molecule.

Enhanced electric polarizability in metal C60 compounds: Formation of a sodium droplet on C60

We measured the electric polarizability of NaNC60 (N=1–34) molecules. The experimental values can be interpreted by the existence of a permanent electric dipole for every size. This cannot be explained by a metal shell around the C60, but this is in agreement with a sodium cluster bound to the C60.

Binding motifs of silver in prion octarepeat model peptides: a joint ion mobility, IR and UV spectroscopies, and theoretical approach

Transition metal-ion complexation is essential to the function and structural stability of many proteins. We studied silver complexation with the octarepeat motif ProHisGlyGlyGlyTrpGlyGln of the prion protein, which shows competitive sites for metal chelation including amide, indole and imidazole groups. This octapeptide is known as a favourable transition metal binding site in prion protein. We used ion mobility spectrometry (IMS), infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory calculations (DFT) to identify the binding motifs of a silver cation on HisGlyGlyGlyTrp peptide as well as on peptide subsequences. Ultra-violet photodissociation (UVPD) and collision induced dissociation mass spectrometry together with the time-dependent density functional method was then exploited to study the influence of binding sites on optical properties and on the ground and excited states reactivity of the peptide. We show that the metal cation is bound to the π-system of the indole group and a nitrogen atom of the imidazole group and that charge transfers from the indole group to the silver cation occur in excited electronic states.