On the structure and photodissociation of cluster ions in the gas phase. (N2) (O2+) and (NO)2+

Results are presented for two experiments on N₂O₂⁺ cluster ions formed via the reactions O₂⁺ + N₂ + M → (N₂) (O₂⁺) + M (i), and NO⁺ + NO + M → (NO)₂⁺ + M (ii). In the first experiment the N₂O₂⁺ clusters are collisionally dissociated. The resulting collision-induced dissociation (CID) spectra show almost exclusively O₂⁺ and N₂⁺ products from N₂ O₂⁺ formed via the first reaction, and almost exclusively NO⁺ products from N₂O₂⁺ formed via the second reaction. In the second experiment, single-photon photodissociation of N₂O₂⁺ ions produced by both reactions (i) and (ii) was investigate using 514.5 and 634 nm radiation. The results indicate that the N₂O₂⁺ cluster from reaction (i) cannot be photodissociated while the N₂O₂⁺ cluster from reaction (ii) undergoes photodissociation at both wavelengths. These experiments indicate that two distinct N₂O₂⁺ cluster ions exist and that reactions (i) and (ii) selectively produce the two ions.     

The energy landscape of unsolvated peptides: helix formation and cold denaturation in Ac-A4G7A4 + H+

Ion mobility measurements and molecular dynamics simulations were performed for unsolvated A4G7A4 + H+ and Ac-A4G7A4 + H+ (Ac = acetyl, A = alanine, G = glycine) peptides. As expected, A4G7A4 + H+ adopts a globular conformation (a compact, random-looking, three-dimensional structure) over the entire temperature range examined (100-410 K). Ac-A4G7A4 + H+ on the other hand is designed to have a flat energy landscape with a marginally stable helical state. This peptide shows at least four different conformations at low temperatures (<230 K). The two conformations with the largest cross sections are attributed to - and partial -helices, while the one with the smallest cross section is globular. The other main conformation may be partially helical. Ac-A4G7A4 + H+ becomes predominantly globular at intermediate temperatures and then becomes more helical as the temperature is raised further. This unexpected behavior may be due to the helix having a higher vibrational entropy than the globular state, as predicted by some recent calculations (Ma, B.; Tsai, C.-J.; Nussinov, R. Biophys. J. 2000, 79, 2739-2753).     

The energy landscape of unsolvated peptides: the role of context in the stability of alanine/glycine helices

Ion mobility measurements have been used to examine the conformations present for unsolvated Ac-(AG)(7)A+H(+) and (AG)(7)A+H(+) peptides (Ac = acetyl, A = alanine, and G = glycine) over a broad temperature range (100-410 K). The results are compared to those recently reported for Ac-A(4)G(7)A(4)+H(+) and A(4)G(7)A(4)+H(+), which have the same compositions but different sequences. Ac-(AG)(7)A+H(+) shows less conformational diversity than Ac-A(4)G(7)A(4)+H(+); it is much less helical than Ac-A(4)G(7)A(4)+H(+) at the upper end of the temperature range studied, and at low temperatures, one of the two Ac-A(4)G(7)A(4)+H(+) features assigned to helical conformations is missing for Ac-(AG)(7)A+H(+). Molecular dynamics simulations suggest that the different conformational preferences are not due to differences in the stabilities of the helical states, but differences in the nonhelical states: it appears that Ac-(AG)(7)A+H(+) is more flexible and able to adopt lower energy globular conformations (compact random looking three-dimensional structures) than Ac-A(4)G(7)A(4)+H(+). The helix to globule transition that occurs for Ac-(AG)(7)A+H(+) at around 250-350 K is not a direct (two-state) process, but a creeping transition that takes place through at least one and probably several intermediates.     

Molecular dynamics simulations of the rehydration of folded and unfolded cytochrome C ions in the vapor phase.

Molecular dynamics (MD) simulations have been performed to study the rehydration of compact and unfolded cytochrome c ions in the vapor phase. Experimental studies have shown that the compact conformations adsorb many more water molecules than unfolded ones when exposed to water vapor. MD simulations performed with up to 150 water molecules reproduce the key experimental observations, including a partial refolding caused by hydration. According to the calculations it is more energetically favorable to hydrate the compact conformation in the initial stages of hydration, because it is easier for a water molecule to interact simultaneously with several polar groups (due to their proximity). The protonated side chains are not favored hydration sites in the simulations because they have "self-solvation" shells which must be disrupted for the water to penetrate. For both conformations, the adsorbed water molecules are mainly located in surface crevices.     

Proton transfer-induced conformational changes and melting in designed peptides in the gas phase

The conformations of protonated RA15K, RA20K and RA15H (R = arginine, A = alanine, K = lysine, and H = histidine) have been examined in the gas phase as a function of temperature. These peptides were designed so that intramolecular proton transfer will trigger conformational changes between a helix (proton sequestered at the C-terminus) and globule (proton sequestered at the N-terminus). Kinetically controlled structural transitions occur below 400 K (from helix to globule for RA15H, and from globule to helix for RA15K and RA20K). As the temperature is raised, the compact globule found at room temperature expands, accesses more configurations, and becomes entropically favored. At around 500 K, the RA15K and RA20K helices undergo a melting transition. The transition is broad, as expected for a phase transition in a finite system, and becomes narrower as the peptide size increases. In the helical conformation, the two basic residues are well separated; as a result, the proton transfer necessary to drive the melting transition probably involves a mobile proton. For doubly protonated RA15K, a dumbbell-like conformation (resulting from repulsion between the two protonated basic residues) is found at high temperature.     

Stable copper-tin cluster compositions from high-temperature annealing

Copper-doped tin clusters can be thermally annealed to much more stable compositions with a substantially higher copper/tin ratio. The annealed clusters are only prominent over a narrow range of compositions: CuSn(10-15)+, Cu2Sn(12-18)+, Cu3Sn(15-21)+, Cu4Sn(18-(24)+, and Cu5Sn(21-(27)+. These compositions are close to those found for W(m)Si(n)+ clusters, raising the possibility that the Cu(m)Sn(n)+ clusters have core-shell geometries like those proposed for the W(m)Si(n)+ clusters. Increasing the number of copper atoms causes a change in the dissociation pattern from the fission processes that are characteristic of semiconductor clusters to the expulsion of individual atoms, which usually occurs for metal clusters. The change in the fragmentation pattern may result because the clusters rich in copper melt before they dissociate, while the pure tin clusters dissociate directly from a solidlike phase.     

Probing helix formation in unsolvated peptides

Ion mobility measurements have been used to examine helix formation in unsolvated glycine-based peptides containing three alanine residues. Nine sequence isomers of Ac-[12G3A]K+H(+) were studied (Ac = acetyl, G = glycine, A = alanine, and K = lysine). The amount of helix present for each peptide was examined using two metrics, and it is strongly dependent on the proximity and the location of the alanine residues. Peptides with three adjacent alanines have the highest helix abundances, and those with well-separated alanines have the lowest. The helix abundances for most of the peptides can be fit reasonably well using a modified Lifson-Roig theory. However, Lifson-Roig theory fails to account for several key features of the experimental results. The most likely explanation for the correlation between helix abundances and the number of adjacent alanines is that neighboring alanines promote helix nucleation.     

Observation of "Stick" and "Handle" intermediates along the fullerene road

The hypothesis that fullerenes grow in a carbon plasma by the addition of C2 units (the "fullerene road") has been widely acclaimed as the most plausible mechanism for formation of larger fullerenes including C60 and C70. Calculations suggest that the association of C2 with fullerenes proceeds through two classes of intermediates, "sticks" and "handles." Here we report the observation of these species using high-resolution ion-mobility measurements for C(n) cations generated by laser vaporization of graphite and laser desorption of C60. Sticks with up to eight-atom chains have also been found.     

Solid clusters above the bulk melting point

The fact that the melting points of nanoparticles are always lower than those of the corresponding bulk material is a paradigm supported by extensive experimental data for a large number of systems and by numerous calculations. Here we demonstrate that tin cluster ions with 10-30 atoms remain solid at approximately 50 K above the melting point of bulk tin. This behavior is possibly related to the fact that the structure of the clusters is completely different from that of the bulk element.