Article pubs.acs.org/JPCA
Secondary Structures in Phe-Containing Isolated Dipeptide Chains: Laser Spectroscopy vs Quantum Chemistry Yohan Loquais,†,‡ Eric Gloaguen,‡,† Sana Habka,†,‡ Vanesa Vaquero-Vara,†,‡ Valérie Brenner,†,‡ Benjamin Tardivel,†,‡ and Michel Mons†,‡,* †
CEA, IRAMIS, Laboratoire Interactions, Dynamique et Lasers, CEA Saclay, Bât 522, 91191 Gif-sur-Yvette, France CNRS, INP, Laboratoire Francis Perrin, URA 2453, CEA Saclay, Bât 522, 91191 Gif-sur-Yvette, France
‡
ABSTRACT: 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.
1. INTRODUCTION Flexible biomolecules, such as peptides, exhibit a complex conformational landscape with many local minima populated at room temperature, making difficult the interpretation of their spectroscopy in solution or in the gas phase.1−7 In the past decade, the coupling of supersonic expansions with thermal vaporization or laser desorption/ablation8−10 devices have provided powerful laboratory procedures to overcome this experimental difficulty. In particular, double-resonance UV/UV and IR/UV spectroscopic techniques11 have proved to be elegant and efficient to resolve conformational features in IR spectra of various molecules of biological interest, in particular peptide chains.3,12−20 The goal of the early works was to explore the diversity of structures observed, leading to a flourishing gallery of secondary structures detected under isolated conditions.7,17,21−29 The modest signal-to-noise ratio achieved in the early days explained that studies were focused on the main experimental signals, preventing an in-depth analysis of the experimental conformational landscape, usually achieved for coexpanded molecules.2 This field of research has nowadays somewhat reached its maturity with the availability of efficient theoretical methods at least for the interpretation of the ground state spectroscopy.16,18,19,30−33 Once a secured assignment is provided, useful information can be learned from © XXXX American Chemical Society
a close comparison between experimental and theoretical landscapes. Confrontation of experimental abundances with the lowest energy predicted minima enables spectroscopists to test the capacity of theoretical methods to properly account for the energetics of these molecules. Conversely, once efficient theoretical methods have been validated, the same confrontation can unravel manifestations of possible flaws in the experiment, such as a specific low detectivity of some conformations in nanosecond resonant two-photon ionization (R2PI) experiments, due, for example, to an ultrafast photophysics in the excited state,34,35 or to an unfavorable Franck−Condon factor toward ionization, due to a too large geometry change between neutral and ionic states.36 Though a series of neutral peptides or peptoid molecules have been investigated in this spirit,16,18,19,21,22,29,30,32 careful studies combining state-of-the-art theoretical descriptions and experimental investigations, extended up to minor conformations, remain to be done. The present paper aims at providing such Special Issue: Jean-Michel Mestdagh Festschrift Received: September 19, 2014 Revised: October 20, 2014
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dx.doi.org/10.1021/jp509494c | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
collisions in the supersonic expansion (in regions much colder than the room temperature T0; it is, for instance, useful to notice that at the nozzle the translational temperature of the carrier gas has already dropped to three-quarter T0 ∼225 K50). One usually assumes that the conformational distribution is described by a Bolzmann temperature, corresponding to the quasi equilibrium present until kinetic trapping occurs in the expanding gas. The corresponding temperatures are expected to range between that of the desorption plume (∼600 K51) and that of the expansion, where the molecules are desorbed (
