Experimental and theoretical investigation of the aromatic-aromatic interaction in isolated capped dipeptides

Selenium in Proteins: Conformational Changes Induced by Se Substitution on Methionine, as Studied in Isolated Model Peptides by Optical Spectroscopy and Quantum Chemistry

The side-chain of methionine residues is long enough to establish NH⋯S H-bonds with neighboring carbonyl groups of the backbone, giving rise to so-called intra-residue 6^δ and inter-residue 7^δ H-bonds. The aim of the present article is to document how the substitution of sulfur with a selenium atom affects the H-bonding of the Met system. This was investigated both experimentally and theoretically by conformation-resolved optical spectroscopy, following an isolated molecule approach. The present work emphasizes the similarities of the Met and Sem residues in terms of conformational structures, energetics, NH⋯Se/S H-bond strength and NH stretch spectral shifts, but also reveals subtle behavior differences between them. It provides evidence for the sensitivity of the H-bonding network with the folding type of the Sem/Met side-chains, where a simple flip of the terminal part of the side-chain can induce an extra 50 cm⁻¹ spectral shift of the NH stretch engaged in a 7^δ NH⋯S/Se bond.

Structural Properties of Phenylalanine-Based Dimers Revealed Using IR Action Spectroscopy

Peptide segments with phenylalanine residues are commonly found in proteins that are related to neurodegenerative diseases. However, the self-assembly of phenylalanine-based peptides can be also functional. Peptides containing phenylalanine residues with different side caps, composition, and chemical alteration can form different types of nanostructures that find many applications in technology and medicine. Various studies have been performed in order to explain the remarkable stability of the resulting nanostructures. Here, we study the early stages of self-assembly of two phenylalanine derived peptides in the gas phase using IR action spectroscopy. Our focus lies on the identification of the key intra- and intermolecular interactions that govern the formation of the dimers. The far-IR region allowed us to distinguish between structural families and to assign the 2-(2-amino-2-phenylacetamido)-2-phenylacetic acid (PhgPhg) dimer to a very symmetric structure with two intermolecular hydrogen bonds and its aromatic rings folded away from the backbone. By comparison with the phenylalanine-based peptide cyclic L-phenylalanyl-L-phenylalanine (cyclo-FF), we found that the linear FF dimer likely adopts a less ordered structure. However, when one more phenylalanine residue is added (FFF), a more structurally organized dimer is formed with several intermolecular hydrogen bonds.

A theoretical and experimental case study of the hydrogen bonding predilection of S-methylcysteine

S-containing amino acids can lead to two types of local NH···S interactions which bridge backbone NH sites to the side chain to form either intra- or inter-residue H-bonds. The present work reports on the conformational preferences of S-methyl-L-cysteine, Cys(Me), using a variety of investigating tools, ranging from quantum chemistry simulations, gas-phase UV and IR laser spectroscopy, and solution state IR and NMR spectroscopies, on model compounds comprising one or two Cys(Me) residues. We demonstrate that in gas phase and in low polarity solution, the C- and N-capped model compound for one Cys(Me) residue adopts a preferred C5-C6γ conformation which combines an intra-residue N-H···O=C backbone interaction (C5) and an inter-residue N-H···S interaction implicating the side-chain sulfur atom (C6γ). In contrast, the dominant conformation of the C- and N-capped model compound featuring two consecutive Cys(Me) residues is a regular type I β-turn. This structure is incompatible with concomitant C6γ interactions, which are no longer in evidence. Instead, C5γ interactions occur, that are fully consistent with the turn geometry and additionally stabilize the structure. Comparison with the thietane amino acid Attc, which exhibits a rigid cyclic side chain, pinpoints the significance of side chain flexibility for the specific conformational behavior of Cys(Me).

An intraresidue H-bonding motif in selenocysteine and cysteine, revealed by gas phase laser spectroscopy and quantum chemistry calculations

Models of protein chains containing a seleno-cysteine (Sec) residue have been investigated by gas phase laser spectroscopy in order to document the effect of the H-bonding properties of the SeH group in the folding of the Sec side chain, by comparison with recent data on Ser- and Cys-containing sequences. Experimental data, complemented by quantum chemistry calculations and natural bonding orbital (NBO) analyses, are interpreted in terms of the formation of a so-called 5γ intra-residue motif, which bridges the acceptor chalcogen atom of the side chain to the NH bond of the same residue. This local structure, in which the O/S/Se atom is close to the plane of the N-terminal side amide, is constrained by local backbone-side chain hyperconjugation effects involving the S and Se atoms. Theoretical investigations of the Cys/Sec side chain show that (i) this 5γ motif is an intrinsic feature of these residues, (ii) the corresponding H-bond is strongly non-linear and intrinsically weak, (iii) but enhanced by γ- and β-turn secondary structures, which promote a more favorable 5γ H-bonding approach and distance. The resulting H-bonds are slightly stronger in selenocysteine than in cysteine, but nearly inexistent in serine, whose side chain in contrast behaves as a H-bonding donor. The modest spectral shifts of the Cys/Sec NH stretches measured experimentally reflect the moderate strength of the 5γ H-bonding, in agreement with the correlation obtained with a NBO-based H-bond strength indicator. The evolution along the Ser, Cys and Sec series emphasizes the compromise between the several factors that control the H-bonding in a hyperconjugation-constrained geometry, among them the chalcogen van der Waals and covalent radii. It also illustrates the 5γ H-bond enhancements with the Sec and Cys residues favoured by the constraints imposed by the γ- and β-turn structures of the peptide chain.

Conformational analysis by UV spectroscopy: the decisive contribution of environment-induced electronic Stark effects

UV chromophores are frequently used as probes of the molecular structure. In particular, they are sensitive to the electric field generated by the molecular environment, resulting in the observation of Stark effects on UV spectra. While these environment-induced electronic Stark effects (EI-ESE) are already used for conformational analysis in the condensed phase, this work explores the potential of such an approach when performed at much higher conformational resolution in the gas phase. By investigating model alkali benzylacetate and 4-phenylbutyrate ion pairs, where the electric field applied to the phenyl ring is chemically tuned by changing the nature of the alkali cation, this work demonstrates that precise conformational assignments can be proposed based on the correlation between the conformation-dependent calculated electric fields and the frequency of the electronic transitions observed in the experimental UV spectra. Remarkably, the sole analysis of Stark effects and fragmentation patterns in mass-selected UV spectra provided an accurate and complete conformational analysis, where spectral differences as small as a few cm-1 between electronic transitions were rationalized. This case study illustrates that the identification of EI-ESE together with their interpretation at the modest cost of a ground state electric field calculation qualify UV spectroscopy as a powerful tool for conformational analysis.

Neutral Peptides in the Gas Phase: Conformation and Aggregation Issues

Combined IR and UV laser spectroscopic techniques in molecular beams merged with theoretical approaches have proven to be an ideal tool to elucidate intrinsic structural properties on a molecular level. It offers the possibility to analyze structural changes, in a controlled molecular environment, when successively adding aggregation partners. By this, it further makes these techniques a valuable starting point for a bottom-up approach in understanding the forces shaping larger molecular systems. This bottom-up approach was successfully applied to neutral amino acids starting around the 1990s. Ever since, experimental and theoretical methods developed further, and investigations could be extended to larger peptide systems. Against this background, the review gives an introduction to secondary structures and experimental methods as well as a summary on theoretical approaches. Vibrational frequencies being characteristic probes of molecular structure and interactions are especially addressed. Archetypal biologically relevant secondary structures investigated by molecular beam spectroscopy are described, and the influences of specific peptide residues on conformational preferences as well as the competition between secondary structures are discussed. Important influences like microsolvation or aggregation behavior are presented. Beyond the linear α-peptides, the main results of structural analysis on cyclic systems as well as on β- and γ-peptides are summarized. Overall, this contribution addresses current aspects of molecular beam spectroscopy on peptides and related species and provides molecular level insights into manifold issues of chemical and biochemical relevance.

Conformation control through concurrent N-H⋯S and N-H⋯O[double bond, length as m-dash]C hydrogen bonding and hyperconjugation effects

In addition to the classical N-H⋯O[double bond, length as m-dash]C non-covalent interaction, less conventional types of hydrogen bonding, such as N-H⋯S, may play a key role in determining the molecular structure. In this work, using theoretical calculations in combination with spectroscopic analysis in both gas phase and solution phase, we demonstrate that both these H-bonding modes exist simultaneously in low-energy conformers of capped derivatives of Attc, a thietane α-amino acid. 6-Membered ring inter-residue N-H⋯S interactions (C6^γ), assisted by hyperconjugation between the thietane ring and the backbone, combine with 5-membered ring intra-residue backbone N-H⋯O[double bond, length as m-dash]C interactions (C5) to provide a C5-C6^γ feature that stabilizes a planar geometry in the monomer unit. Two contiguous C5-C6^γ features in the planar dimer implicate an unprecedented three-centre H-bond of the type C[double bond, length as m-dash]O⋯H(N)⋯SR₂, while the trimer adopts two C5-C6^γ features separated by a Ramachandran α-type backbone configuration. These low-energy conformers are fully characterized in the gas phase and support is presented for their existence in solution state.

Gas-Phase Infrared Spectroscopy of Neutral Peptides: Insights from the Far-IR and THz Domain

Gas-phase, double resonance IR spectroscopy has proven to be an excellent approach to obtain structural information on peptides ranging from single amino acids to large peptides and peptide clusters. In this review, we discuss the state-of-the-art of infrared action spectroscopy of peptides in the far-IR and THz regime. An introduction to the field of far-IR spectroscopy is given, thereby highlighting the opportunities that are provided for gas-phase research on neutral peptides. Current experimental methods, including spectroscopic schemes, have been reviewed. Structural information from the experimental far-IR spectra can be obtained with the help of suitable theoretical approaches such as dynamical DFT techniques and the recently developed Graph Theory. The aim of this review is to underline how the synergy between far-IR spectroscopy and theory can provide an unprecedented picture of the structure of neutral biomolecules in the gas phase. The far-IR signatures of the discussed studies are summarized in a far-IR map, in order to gain insight into the origin of the far-IR localized and delocalized motions present in peptides and where they can be found in the electromagnetic spectrum.

Electronic Stark Effect in Isolated Ion Pairs

Stark spectral shifts of a molecular probe are commonly used to estimate the local electric field in condensed media. The very large fields reported, typically in the 0.1-10 GV m^-1 range, are, however, difficult to reproduce in a controlled manner, limiting the calibration of these molecular probes to ranges below 0.1 GV m^-1. In this context, we investigated gas-phase, isolated, molecular ion pairs, where a phenyl ring is immersed in the electric field produced by the nearby ionic groups. The intensity of the electric field is chemically tuned in the 1 GV m^-1 range by changing the nature of the cations, and the phenyl ring response is monitored by UV spectroscopy. A quadratic Stark effect is observed, demonstrating the possibility to characterize molecular probes in a solvent-free environment and in the very large field range they typically meet in condensed media such as biological environments.

Identification of ion pairs in solution by IR spectroscopy: crucial contributions of gas phase data and simulations

In a context where structure elucidation of ion pairs in solution remains a contemporary challenge, this work explores an original approach where accurate gas phase spectroscopic data are used to refine high level quantum chemistry calculations of ion pairs in solution, resulting in an unprecedented level of accuracy in vibrational frequency prediction. First, gas phase studies focus on a series of isolated contact ion pairs (M⁺, Ph-CH₂-COO^-, with M = Li, Na, K, Rb, Cs) for which conformer-selective IR spectra in the CO₂^- stretch region are recorded. These experiments reveal the interactions at play in isolated contact ion pairs, and provide vibrational frequencies enabling us to assess the accuracy of the theoretical approach used, i.e., mode-dependent scaled harmonic frequency calculations at the RI-B97-D3/dhf-TZVPP level. This level of calculation is then employed on large water clusters embedding either a free acetate ion or its contact or solvent-shared pairs with a sodium cation in order to simulate the individual vibrational spectra of these species in solution. This study shows that the stretching modes of carboxylate are sensitive to both solvent-shared and contact ion pair formation. FTIR spectra of solutions of increasing concentrations indeed reveal several spectral changes consistent with the presence of specific types of solvent-shared and contact ion pairs. By providing relevant guidelines for the interpretation of solution phase IR spectra, this work illustrates the potential of the approach for the elucidation of supramolecular structures in electrolyte solutions.

On the turn-inducing properties of asparagine: the structuring role of the amide side chain, from isolated model peptides to crystallized proteins

The anchoring properties of an asparagine (Asn) residue to its local backbone environment in turn model peptides is characterized using gas phase laser spectroscopy and compared to crystallized protein structures.

Stereochemistry-dependent hydrogen bonds stabilise stacked conformations in jet-cooled cyclic dipeptides: (LD) vs. (LL) cyclo tyrosine-tyrosine

Tyrosine-containing cyclic dipeptides based on a diketopiperazine (DKP) ring are studied under jet-cooled conditions using resonance-enhanced multi-photon ionisation (REMPI), conformer-selective IR-UV double resonance vibrational spectroscopy and quantum chemical calculations. The conformational landscape of the dipeptide containing natural L tyrosine (Tyr), namely c-LTyr-LTyr strongly differs from that of its diastereomer c-LTyr-DTyr. A similar family of conformers exists in both systems, with one aromatic ring folded on the dipeptide DKP ring and the other one extended. Weak NHπ and CHπ interactions are observed, which are slightly different in c-LTyr-LTyr and c-LTyr-DTyr. These structures are identical to those of LL and LD cyclo diphenylalanine, which only differ from c-Tyr-Tyr by the absence of hydroxyl on the benzene rings. While this is the only conformation observed for c-LTyr-DTyr, c-LTyr-LTyr exhibits an additional form stabilised by the interaction of the two hydroxyls, in which the two aromatic rings are in a stacked geometry. Stereochemical effects are still visible in the radical cation, for which one structure is observed for c-LTyr-DTyr, while the spectrum of the c-LTyr-LTyr radical cation is explained in terms of two co-existing structures.

Computational study on single molecular spectroscopy of tyrosin-glycine, tryptophane-glycine and glycine-tryptophane

Quantum chemistry calculations play a fundamental role in revealing the molecular structures observed in gas-phase spectroscopic measurements. The supersonic jet cooling widely used in single molecular spectroscopy experiment is a non-equilibrium process and often causes confusion on the theoretical and experimental comparison. A computational approach is proposed here to account for the effect of the non-equilibrium cooling on the experimental spectra and applied to the cases of tyrosin-glycine (YG), tryptophane-glycine (WG) and glycine-tryptophane (GW). The low energy conformers of YG, WG and GW are obtained through thorough conformational searches. The structural features and equilibrium distributions of conformations and the energy barriers for conformer conversions are then determined. Three classes of transition energy barriers, high, medium and low, are found for the conversions among conformers with distinctly different, similar and the same structural types, respectively. The final conformation populations are determined by assuming an initial temperature of about 450 K and allowing for only the conformation conversion with a low energy barrier to occur during the rapid cooling process. The results provide a natural explanation for the numbers of YG, WG and GW conformations observed experimentally. The theoretical conformation assignments are also in good agreement with the experimental IR data.

Unifying the microscopic picture of His-containing turns: from gas phase model peptides to crystallized proteins

The presence in crystallized proteins of a local anchoring between the side chain of a His residue, located in the central position of a γ- or β-turn, and its local main chain environment, is assessed by the comparison of protein structures with relevant isolated model peptides.

Local NH–π interactions involving aromatic residues of proteins: influence of backbone conformation and ππ* excitation on the π H-bond strength, as revealed from studies of isolated model peptides

Gas phase conformer-selective IR spectroscopy combined and relevant quantum chemistry methods document the NH–π interactions in Phe residues.

Structural investigations on a linear isolated depsipeptide: the importance of dispersion interactions

The first molecular beam investigations of an isolated linear depsipeptide are presented. By applying IR/UV spectroscopic methods and DFT calculations three structural arrangements are identified with the most stable structure being only stable by including dispersion interactions.

Chirality-dependent structuration of protonated or sodiated polyphenylalanines: IRMPD and ion mobility studies

Ion mobility experiments are combined with Infra-Red Multiple Photon Dissociation (IRMPD) spectroscopy and quantum chemical calculations for assessing the role of chirality in the structure of protonated and sodiated di- or tetra-peptides. Sodiated systems show a strong chirality dependence of the competition between Na(+)O and Na(+)π interactions. Chirality effects are more subtle in protonated systems and manifest themselves by differences in the secondary interactions such hydrogen bonds between neutral groups or those involving the aromatic rings.

Isolated neutral peptides

This chapter examines the structural characterisation of isolated neutral amino-acids and peptides. After a presentation of the experimental and theoretical state-of-the-art in the field, a review of the major structures and shaping interactions is presented. Special focus is made on conformationally-resolved studies which enable one to go beyond simple structural characterisation; probing flexibility and excited-state photophysics are given as examples of promising future directions.

A conformation-selective IR-UV study of the dipeptides Ac-Phe-Ser-NH2 and Ac-Phe-Cys-NH2: probing the SH···O and OH···O hydrogen bond interactions

The conformational preferences of peptides are mainly controlled by the stabilizing effect of intramolecular interactions. In peptides with polar side chains, not only the backbone but also the side chain interactions determine the resulting conformations. In this paper, the conformational preferences of the capped dipeptides Ac-Phe-Ser-NH2 (FS) and Ac-Phe-Cys-NH2 (FC) are resolved under laser-desorbed jet cooling conditions using IR-UV ion dip spectroscopy and density functional theory (DFT) quantum chemistry calculations. As serine (Ser) and cysteine (Cys) only differ in an OH (Ser) or SH (Cys) moiety; this subtle alteration allows us to study the effect of the difference in hydrogen bonding for an OH and SH group in detail, and its effect on the secondary structure. IR absorption spectra are recorded in the NH stretching region (3200-3600 cm(-1)). In combination with quantum chemical calculations the spectra provide a direct view of intramolecular interactions. Here, we show that both FS as FC share a singly γ-folded backbone conformation as the most stable conformer. The hydrogen bond strength of OH···O (FS) is stronger than that of SH···O (FC), resulting in a more compact gamma turn structure. A second conformer is found for FC, showing a β turn interaction.

Unraveling non-covalent interactions within flexible biomolecules: from electron density topology to gas phase spectroscopy

The NCI (Non-Covalent Interactions) method, a recently-developed theoretical strategy to visualize weak non-covalent interactions from the topological analysis of the electron density and of its reduced gradient, is applied in the present paper to document intra- and inter-molecular interactions in flexible molecules and systems of biological interest in combination with IR spectroscopy. We first describe the conditions of application of the NCI method to the specific case of intramolecular interactions. Then we apply it to a series of stable conformations of isolated molecules as an interpretative technique to decipher the different physical interactions at play in these systems. Examples are chosen among neutral molecular systems exhibiting a large diversity of interactions, for which an extensive spectroscopic characterization under gas-phase isolation conditions has been obtained using state-of-the-art conformer-specific experimental techniques. The interactions presently documented range from weak intra-molecular H-bonds in simple amino-alcohols, to more complex patterns, with interactions of various strengths in model peptides, as well as in chiral bimolecular systems, where invaluable hints for the understanding of chiral recognition are revealed. We also provide a detailed technical appendix, which discusses the choices of cut-offs as well as the applicability of the NCI analysis to specific constrained systems, where local effects require attention. Finally, the NCI technique provides IR spectroscopists with an elegant visualization of the interactions that potentially impact their vibrational probes, namely the OH and NH stretching motions. This contribution illustrates the power and the conditions of use of the NCI technique, with the aim of providing an easy tool for all chemists, experimentalists and theoreticians, for the visualization and characterization of the interactions shaping complex molecular systems.

Intra-residue interactions in proteins: interplay between serine or cysteine side chains and backbone conformations, revealed by laser spectroscopy of isolated model peptides

Intra-residue interactions play an important role in proteins by influencing local folding of the backbone. Taking advantage of the capability of gas phase experiments to provide relevant information on the intrinsic H-bonding pattern of isolated peptide chains, the intra-residue interactions of serine and cysteine residues, i.e., OH/SH···OC(i) C6 and NH(i···)O/S C5 interactions in Ser/Cys residues, are probed by laser spectroscopy of isolated peptides. The strength of these local side chain-main chain interactions, elegantly documented from their IR spectral features for well-defined conformations of the main chain, demonstrates that a subtle competition exists between the two types of intra-residue bond: the C6 H-bond is the major interaction with Ser, in contrast to Cys where C5 interaction takes over. The restricted number of conformers observed in the gas phase experiment with Ser compared to Cys (where both extended and folded forms are observed) also suggests a significant mediation role of these intra-residue interactions on the competition between the several main chain folding patterns.

Strength of NH···S Hydrogen Bonds in Methionine Residues Revealed by Gas-Phase IR/UV Spectroscopy

Despite of being ubiquitous in proteins, NHbackbone···S hydrogen bonds linking the sulfur atom of methionine or cysteine to backbone NH groups remain poorly documented. Here, we report vibrationally resolved IR NH stretch spectra of two methionine-containing dipeptides (Ac-Phe-Met-NH2 and Ac-Met-Phe-NH2). The conformations observed for both molecules, assigned with the help of DFT-D quantum chemistry, provide spectroscopic evidence for the formation of NHbackbone···S H-bonds, surprisingly strong enough to challenge the classical intrabackbone NH···O═C H-bonds. The methionine side chain is found to fold locally, forming a H-bond with the neighboring amide groups (NH(i) or NH(i+1)). Comparison with protein data bank structural information shows that such a local folding is also common in proteins where it concerns 24% of the methionine residues that have a sulfur atom linked to a backbone NH group. This convergence between the strength of these NH···S H-bonds and protein structural data illustrates their contribution to the stability of protein chains.

Understanding the influence of guest-host interactions on the conformation of short peptides in a hydrophobic cavity: a computational study

We performed a computational investigation to understand the conformational preferences of four short peptides in a self-assembled cage based on the experimental work by Y. Hatakeyama et al. (Angew. Chem. Int. Ed.2009, 48, 8695). For this purpose, we combined molecular dynamics simulations, Monte Carlo simulations, and quantum mechanical calculations to obtain energies and structures for several low-lying conformers of four peptides and the corresponding peptide-cage inclusion complexes. Our calculations at both B3LYP and MP2 levels show that for each peptide, the corresponding conformation within the host (as revealed by the crystal structure) does not represent the lowest-energy conformation of this peptide in vacuum. By comparing some low-lying conformers in vacuum and in the cavity (for the same peptide), we found that the cage has a significant influence on the conformational propensities of peptides. First, one carbonyl oxygen of each peptide tends to bind to one Zn(II) atom of the cage, forming a Zn-O bond. The formation of this bond leads to significant charge transfer from the cage to the peptide. Second, this Zn-O bond causes the peptide to go through some local conformational changes. For larger peptides, such as penta- and hexapeptides, our calculations also show that some of their conformers must undergo significant structural changes, due to the confinement of the host. This computational study reveals the noticeable influence of the guest-host interaction on the conformational preferences of short peptides.

Gas-phase and Ar-matrix SQM scaling factors for various DFT functionals with basis sets including polarization and diffuse functions

Scaling factors for Pulay's scaled quantum mechanical (SQM) scheme have been determined for four different widely used DFT functionals (PBE, B3LYP, B3PW91, and M06-2X) and for two basis sets (6-31++G** and aug-cc-pVTZ) by fitting computed results to 347 fundamental experimental vibrational frequencies of 33 molecules. Measurements in the gas phase and in solid argon matrices were used independently in the fitting procedure in order to provide a simple method of estimating matrix shifts. The accuracy of the new scaling factors is demonstrated on test molecules including hydrogen-bonded systems and molecules containing chlorine and sulfur atoms.

Isolated monohydrates of a model peptide chain: effect of a first water molecule on the secondary structure of a capped phenylalanine

The formation of monohydrates of capped phenylalanine model peptides, CH(3)-CO-Phe-NH(2) and CH(3)-CO-Phe-NH-CH(3), in a supersonic expansion has been investigated using laser spectroscopy and quantum chemistry methods. Conformational distributions of the monohydrates have been revealed by IR/UV double-resonance spectroscopy and their structures assigned by comparison with DFT-D calculations. A careful analysis of the final hydrate distribution together with a detailed theoretical investigation of the potential energy surface of the monohydrates demonstrates that solvation occurs from the conformational distribution of the isolated peptide monomers. The distribution of the monohydrates appears to be strongly dependent on both the initial monomer conformation (extended or folded backbone) and the solvation site initially occupied by the water molecule. The solvation processes taking place during the cooling can be categorized as follows: (a) solvation without significant structural changes of the peptide, (b) solvation inducing significant distortions of the backbone but retaining the secondary structure, and (c) solvation triggering backbone isomerizations, leading to a modification of the peptide secondary structure. It is observed that solvation by a single water molecule can fold a β-strand into a γ-turn structure (type c) or induce a significant opening of a γ-turn characterized by an elongated C(7) hydrogen bond (type b). These structural changes can be considered as a first step toward the polyproline II condensed-phase structure, illustrating the role played by the very first water molecule in the solvation process.

Chirality-induced conformational preferences in peptide-metal ion binding revealed by IR spectroscopy

Chirality reversal of a residue in a peptide can change its mode of binding to a metal ion, as shown here experimentally by gas-phase IR spectroscopy of peptide-metal ion complexes. The binding conformations of Li(+), Na(+), and H(+) with the LL and DL stereoisomers of PhePhe were compared through IR ion spectroscopy using the FELIX free-electron laser. For the DL isomer, both Li(+) and Na(+) exclusively coordinate to the amide O atom, the carboxyl O atom, and one of the aromatic rings (the OOR conformation), while for the LL isomer, a mixture of the OOR and NOR conformations was found. The stereochemically induced change in conformation is shown to reflect the strength of an NH···π interaction remote from the metal ion site. Protonated PhePhe shows no stereochemically induced variation in binding geometry.