Secondary structure of Ac-Alan_n-LysH+^+ polyalanine peptides (nn=5,10,15) in vacuo: Helical or not?

The polyalanine-based peptide series Ac-Ala_n-LysH+ (n=5-20) is a prime example that a secondary structure motif which is well-known from the solution phase (here: helices) can be formed in vacuo. We here revisit this conclusion for n=5,10,15, using density-functional theory (van der Waals corrected generalized gradient approximation), and gas-phase infrared vibrational spectroscopy. For the longer molecules (n=10,15) \alpha-helical models provide good qualitative agreement (theory vs. experiment) already in the harmonic approximation. For n=5, the lowest energy conformer is not a simple helix, but competes closely with \alpha-helical motifs at 300K. Close agreement between infrared spectra from experiment and ab initio molecular dynamics (including anharmonic effects) supports our findings.Comment: 4 pages, 4 figures, Submitted to JPC Letter

Action spectroscopy of gas-phase carboxylate anions by multiple photon IR electron detachment/attachment

We report on a form of gas-phase anion action spectroscopy based on infrared multiple photon electron detachment and subsequent capture of the free electrons by a neutral electron scavenger in a Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometer. This method allows one to obtain background-free spectra of strongly bound anions, for which no dissociation channels are observed. The first gas-phase spectra of acetate and propionate are presented using SF6 as electron scavenger and a free electron laser as source of intense and tunable infrared radiation. To validate the method, we compare infrared spectra obtained through multiple photon electron detachment/attachment and multiple photon dissociation for the benzoate anion. In addition, different electron acceptors are used, comparing both associative and dissociative electron capture. The relative energies of dissociation (by CO2 loss) and electron detachment are investigated for all three anions by DFT and CCSD(T) methods. DFT calculations are also employed to predict vibrational frequencies, which provide a good fit to the infrared spectra observed. The frequencies of the symmetric and antisymmetric carboxylate stretching modes for the aliphatic carboxylates are compared to those previously observed in condensed-phase IR spectra and to those reported for gas-phase benzoate, showing a strong influence of the solution environment and a slight substituent effect on the antisymmetric stretch.Comment: Revised version, Submitted to J Phys Chem

IR spectroscopy of gas-phase C-60(-)

We present the IR spectrum of the gas-phase C-60(-) anion. The C-60(-) anions are mass-selected and trapped in an ion cyclotron mass spectrometer cell. The spectrum is obtained by recording the yield of electrons following infrared multiple photon absorption and subsequent electron detachment of the anions. Two bands are observed that correlate to two of the four IR-allowed transitions in neutral C-60. The results show that the higher frequency band is strongly shifted from its position in neutral C-60 and is a sensitive marker for the charge state. The band positions and intensities are compared to results obtained by theory as well as to known bands in solid samples and good agreement is found

Gas-phase IR spectra of intact alpha-helical coiled coil protein complexes

Electrospray ionization (ESI) is the softest ionization method that is currently available and it is widely accepted, that ESI generated ions of proteins and protein assemblies at certain conditions retain characteristic aspects of their solution-state conformation. ESI mass spectrometry (MS) therefore evolved as a useful tool to obtain information on composition, stoichiometry, and dynamics of non-covalently associated protein complexes. While tertiary structure information of proteins can be obtained from ion mobility spectrometry (IMS), only a few techniques yield direct information on the secondary structure of gas-phase peptides and proteins. We present here the mid-IR spectroscopic secondary structural analysis of three de novo designed alpha-helical coiled coil model peptides and their non-covalently associated complexes in the gas-phase. The conformational stability of such coiled coil peptides in solution is primarily driven by aggregation. Isolated monomers usually remain unfolded. Two of the investigated peptides were designed to assemble into stable alpha-helical complexes in acidic solution, while the third one remains monomeric and unfolded at these conditions. Monomer ions of all three peptides show comparable photodissociation IR spectra and therefore suggest an unfolded conformation in the gas phase. in contrast, considerable C=O stretch (amide-1) and N-H bend (amide-II) band shifts have been observed for the dieters which is consistent with an elevated H-bond content. These findings provide evidence that at least a fraction of the condensed phase a.-helical structure is retained in the gas-phase coiled coil complexes. (C) 2009 Elsevier B.V. All rights reserved

Exploring the conformational preferences of 20-residue peptides in isolation: Ac-Ala(19)-Lys + H+ vs. Ac-Lys-Ala(19) + H+ and the current reach of DFT

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Structure determination of anionic metal clusters via infrared resonance enhanced multiple photon electron detachment spectroscopy

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