Photoelectron spectra of cold (10 K) size selected water cluster anions (H(2)O)(n) (-) and (D(2)O)(n) (-) have been measured in the size range n=20-120. A new isomer with a higher binding energy than the so-called isomer I has been identified, which appears in the size range n=25-30 and for (H(2)O)(n) (-) becomes dominant at n=46. Magic numbers observed in the mass spectra of the cluster anions provide evidence that this new isomer class consists of clusters with an internal electron.

The use of Forster resonance energy transfer (FRET) as a probe of the structure of biological molecules through fluorescence measurements in solution is well-attested. The transposition of this technique to the gas phase is appealing since it opens the perspective of combining the structural accuracy of FRET with the specificity and selectivity of mass spectrometry (MS). Here, we report FRET results on gasphase polyalanine ions obtained by measuring FRET efficiency through specific photofragmentation rather than fluorescence. The structural sensitivity of the method was tested using commercially available chromophores (QSY 7 and carboxyrhodamine 575) grafted on a series of small, alanine-based peptides of differing sizes. The photofragmentation of these systems was investigated through action spectroscopy, and their conformations were probed using ion mobility spectrometry (IMS) and Monte Carlo minimization (MCM) simulations. We show that specific excitation of the donor chromophore results in the observation of fragments that are specific to the electronic excitation of the acceptor chromophore. This shows that energy transfer took place between the two chromophores and hence that the action-FRET technique can be used as a new and sensitive probe of the structure of gas-phase biomolecules, which opens perspectives as a new tool in structural biology.F orster resonance energy transfer (FRET) is a widely used probe of molecular structure in solution. 1−4 It requires a photon source to electronically excite the so-called "donor chromophore" and a light-harvesting setup to detect either the "donor" or "acceptor" chromophore fluorescence. The occurrence of FRET is then usually evidenced through a decrease in the fluorescence of the donor chromophore (quenching), with the concurrent onset of the fluorescence of the acceptor chromophore or by changes in fluorescence decay times. The interpretation of FRET results relies on the known distance dependence of the effect and on the possibility to graft specific chromophores at relatively well-defined sites on a molecule. FRET is then used to characterize the distance between the chromophores and hence separation between the grafting sites, although extracting exact distances is difficult due to the uncertainty of the exact orientation of the transition dipole moments of the chromophores. This allows the use of FRET to probe intra-or intermolecular distances, especially the change in distance, depending on whether the chromophores are attached to the same or to different molecules.The versatility of FRET makes it a powerful tool to assess the conformation and/or association of molecules. It has been shown that the overall structure of complex molecular edifices can be preserved in the gas phase using soft ionization techniques. 5,6 Therefore, the development of techniques capable of probing FRET in the gas phase is of high interest and could be integrated into a global approach for structure determination of proteins and protein complexes. 7−9 There are few tec...

Ion mobility spectrometry coupled with mass spectrometry was used to experimentally determine the three-dimensional structure of multiply charged sodium cationized polylactides (PLA). In particular, the experiments were conducted to evaluate the influence of the charge state and the size on the gas-phase conformation of cationized PLA. The measured collision cross sections were then compared to calculated values obtained by computational chemistry methods. The most striking feature was the experimental and theoretical observation of a breaking point in the quasilinear relationship between the average collision cross sections and the number of monomer units for the triply charged cations. This breaking point was theoretically demonstrated, for the doubly and triply charged cations, to be associated with a significant folding of the polymer chains around the cationizing agents. The occurrence of such breaking points could be exploited to correlate the charge state of the most intense ion series observed upon electrospray ionization with the number-average molecular mass of a polymer.