James S. Prell

Attosecond band-gap dynamics in silicon

Electron transfer from valence to conduction band states in semiconductors is the basis of modern electronics. Here, attosecond extreme ultraviolet (XUV) spectroscopy is used to resolve this process in silicon in real time. Electrons injected into the conduction band by few-cycle laser pulses alter the silicon XUV absorption spectrum in sharp steps synchronized with the laser electric field oscillations. The observed ~450-attosecond step rise time provides an upper limit for the carrier-induced band-gap reduction and the electron-electron scattering time in the conduction band. This electronic response is separated from the subsequent band-gap modifications due to lattice motion, which occurs on a time scale of 60 ± 10 femtoseconds, characteristic of the fastest optical phonon. Quantum dynamical simulations interpret the carrier injection step as light-field–induced electron tunneling. Excited electrons in semiconducting silicon are tracked on a time scale faster than the lattice vibrations. [Also see Perspective by Spielmann] Watching electrons dart through silicon The ultimate speed limit in electronic circuitry is set by the motion of the electrons themselves. Schultze et al. applied attosecond spectroscopy to glimpse this motion in a sample of silicon, the semiconducting building block of modern integrated circuits (see the Perspective by Spielmann). The technique distinguished the electron dynamics—which proceed faster than a quadrillionth of a second after laser excitation—from the comparatively slower lattice motion of the silicon atomic nuclei. Science, this issue p. 1348; see also p. 1293

Protein Conformation and Supercharging with DMSO from Aqueous Solution

The efficacy of dimethyl sulfoxide (DMSO) as a supercharging reagent for protein ions formed by electrospray ionization from aqueous solution and the mechanism for supercharging were investigated. Addition of small amounts of DMSO to aqueous solutions containing hen egg white lysozyme or equine myoglobin results in a lowering of charge, whereas a significant increase in charge occurs at higher concentrations. Results from both near-UV circular dichroism spectroscopy and solution-phase hydrogen/deuterium exchange mass spectrometry indicate that DMSO causes a compaction of the native structure of these proteins at low concentration, but significant unfolding occurs at ~63% and ~43% DMSO for lysozyme and myoglobin, respectively. The DMSO concentrations required to denature these two proteins in bulk solution are ~3–5 times higher than the concentrations required for the onset of supercharging, consistent with a significantly increased concentration of this high boiling point supercharging reagent in the ESI droplet as preferential evaporation of water occurs. DMSO is slightly more basic than m-nitrobenzyl alcohol and sulfolane, two other supercharging reagents, based on calculated proton affinity and gas-phase basicity values both at the B3LYP and MP2 levels of theory, and all three of these supercharging reagents are significantly more basic than water. These results provide additional evidence that the origin of supercharging from aqueous solution is the result of chemical and/or thermal denaturation that occurs in the ESI droplet as the concentration of these supercharging reagents increases, and that proton transfer reactivity does not play a significant role in the charge enhancement observed.

Shapes and vorticities of superfluid helium nanodroplets

X-raying superfluid helium droplets When physicists rotate the superfluid 4He, it develops a regular array of tiny whirlpools, called vortices. The same phenomenon should occur in helium droplets half a micrometer in size, but studying individual droplets is tricky. Gomez et al. used x-ray diffraction to deduce the shape of individual rotating droplets and image the resulting vortex patterns, which confirmed the superfluidity of the droplets. They found that superfluid droplets can host a surprising number of vortices and can rotate faster than normal droplets without disintegrating. Science, this issue p. 906 Vortex lattices inside individual helium droplets are imaged using x-ray diffraction. Helium nanodroplets are considered ideal model systems to explore quantum hydrodynamics in self-contained, isolated superfluids. However, exploring the dynamic properties of individual droplets is experimentally challenging. In this work, we used single-shot femtosecond x-ray coherent diffractive imaging to investigate the rotation of single, isolated superfluid helium-4 droplets containing ~108 to 1011 atoms. The formation of quantum vortex lattices inside the droplets is confirmed by observing characteristic Bragg patterns from xenon clusters trapped in the vortex cores. The vortex densities are up to five orders of magnitude larger than those observed in bulk liquid helium. The droplets exhibit large centrifugal deformations but retain axially symmetric shapes at angular velocities well beyond the stability range of viscous classical droplets.

Interactions of mono- and divalent metal ions with aspartic and glutamic acid investigated with IR photodissociation spectroscopy and theory.

The interaction of metal ions with aspartic (Asp) and glutamic (Glu) acid and the role of gas-phase acidity on zwitterionic stability were investigated using infrared photodissociation spectroscopy in the spectral range 950-1900 cm (-1) and by hybrid density functional theory. Lithium ions interact with both carbonyl oxygen atoms and the amine nitrogen for both amino acids, whereas cesium interacts with both of the oxygen atoms of the C-terminus and the carbonyl oxygen of the side chain for Asp. For Glu, this structure is competitive, but a structure in which the cesium ion interacts with just the carbonyl oxygen atoms is favored and the calculated spectrum for this structure is more consistent with the experimentally measured spectrum. In complexes with either of these metal ions, both amino acids are non-zwitterionic. In contrast, Glu*Ca (2+) and Glu*Ba (2+) both adopt structures in which Glu is zwitterionic and the metal ion interacts with both oxygens of the C-terminal carboxylate and the carbonyl oxygen in the side chain. Assignment of the zwitterionic form of Glu is strengthened by comparisons to the spectrum of the protonated form, which indicate spectral features associated with a protonated amino nitrogen. Comparisons with results for glutamine, which adopts nearly the same structures with these metal ions, indicate that the lower Delta H acid of Asp and Glu relative to other amino acids does not result in greater relative stability of the zwitterionic form, a result that is directly attributed to effects of the metal ions which disrupt the strong interaction between the carboxylic acid groups in the isolated, deprotonated forms of these amino acids.

Structures of Protonated Dipeptides: The Role of Arginine in Stabilizing Salt Bridges

Structures of protonated dipeptides containing N-terminal Gly, Val, Pro, Lys, His, or Arg and C-terminal Arg are investigated with infrared multiple photon dissociation (IRMPD) spectroscopy between 900 and 1850 cm(-1) and theory. The IRMPD spectra clearly indicate that, when Gly, Val, Pro, Lys, or His are N-terminal to Arg, these protonated dipeptides adopt gas-phase structures with a single formal charge site (SCS), whereas ArgArg x H(+) has a salt-bridge (SB) structure in which the C-terminus is deprotonated and two basic sites are protonated. There are only subtle differences in the IRMPD spectra for dipeptides containing Gly, Val, Pro, and Lys. A sharp, intense peak at 1080 cm(-1) is observed for HisArg x H(+) that is attributed to the neutral histidine side chain, an assignment that is confirmed by comparison to the IRMPD spectrum of (HisArg x H(2))(2+). Lowest-energy B3LYP/6-31+G(d,p) structures and energies for the SCS and SB forms of these protonated dipeptides indicate that stability of the SB form relative to the SCS form generally increases with increasing gas-phase basicity of the N-terminal amino acid, but only ArgArg x H(+) is calculated to have a SB ground state at 298 K, in agreement with the results from IRMPD spectroscopy. This is the first direct experimental evidence for a salt-bridge structure in a gaseous protonated peptide, and ArgArg x H(+) is the smallest protonated peptide for which a SB structure has been reported. These results suggest that SB structures should be common for protonated peptides containing at least two arginine residues and may also occur for large protonated peptides or proteins with at least one arginine residue and other basic residues, such as lysine or histidine.

Role of Sequence in Salt-Bridge Formation for Alkali Metal Cationized GlyArg and ArgGly Investigated with IRMPD Spectroscopy and Theory

The roles of hydrogen bonding, electrostatic interactions, sequence, gas-phase basicity, and molecular geometry in determining the structures of protonated and alkali metal-cationized glycyl-L-arginine (GlyArg) and L-arginylglycine (ArgGly) were investigated using infrared multiple photon dissociation spectroscopy in the spectral range 900-1800 cm(-1) and theory. The IRMPD spectra clearly indicate that GlyArg x M(+), M = Li, Na, and Cs, form similar salt-bridge (SB) structures that do not depend significantly on metal ion size. In striking contrast, ArgGly x Li(+) exists in a charge-solvated (CS) form, whereas ArgGly x M(+), M = K and Cs, form SB structures. SB and CS structures are similarly populated for ArgGly x Na(+). Computed energies of low-energy structures are consistent with these results deduced from the experimental spectra. By comparison to Arg x M(+), GlyArg x M(+) and ArgGly x M(+) have a greater and lesser propensity, respectively, to form SB structures. The greater propensity for GlyArg to adopt SB structures in complexes with smaller metal cations than for ArgGly is due to the ability of alkali metal-cationized GlyArg to adopt a nearly linear arrangement of formal charge sites, a structure unfavorable for ArgGly complexes due to geometric constraints induced by its different amino acid sequence. The amide carbonyl oxygen solvates charge in both the SB and CS form of both dipeptides. ArgGly is calculated to be slightly more basic than GlyArg, indicating that differences in intrinsic basicity do not play a role in the relative SB stabilization of these ions. Loss of a neutral water molecule from complexes in which SB structures are most stable indicates that CS structures are intermediates in the dissociation pathway, but these intermediates do not contribute to the measured IRMPD spectra.

Structural and electric field effects of ions in aqueous nanodrops.

Ensemble infrared photodissociation (IRPD) spectra in the hydrogen stretch region (∼2950-3800 cm(-1)) are reported for M(H(2)O)(35-37), with M = I(-), Cl(-), HCO(3)(-), OH(-), tetrabutyl-, tetrapropyl-, and tetramethylammonium, Cs(+), Na(+), Li(+), H(+), Ba(2+), Ca(2+), Co(2+), Mg(2+), La(3+), and Tm(3+), at 133 K. A single, broad feature is observed in the bonded-OH region of the spectra that indicates that the water network in these clusters is bulk-like and likely resembles liquid water more strongly than ice. The free-OH region for all of these clusters is dominated by peaks corresponding to water molecules that accept two and donate one hydrogen bond (AAD water molecules), indicating that AAD water molecules are more abundant at the surface of these ions than AD water molecules. A-only water molecules are present in significant abundance only for the trivalent metal cations. The frequency of the AAD free-OH stretch band shifts nearly linearly with the charge state of the ion, consistent with a Stark shift attributable to the ions electric field. From these data, a frequency range of 3704.9-3709.7 cm(-1) is extrapolated for the free-OH of AAD water molecules at the (uncharged) bulk liquid water surface, consistent with sum-frequency generation spectroscopy experiments. Differences in both the bonded- and the free-OH regions of the spectra for these ions are attributable to ion-induced patterning of the water network that extends to the surface of the clusters, which includes water molecules in the third and fourth solvation shells; that is, these ions pattern water molecules at long distance to various extents. These spectra are simulated using two different electrostatic models previously used to calculate OH-stretch spectra of bulk water and aqueous solutions and parametrized for bonded-OH frequencies. These models qualitatively reproduce a number of features in the experimental spectra, although it is evident that more sophisticated treatment of water molecule and ion polarizability and vibrational coupling is necessary for more quantitative comparisons.

Coordination of trivalent metal cations to peptides: results from IRMPD spectroscopy and theory.

Structures of trivalent lanthanide metal cations La(3+), Ho(3+), and Eu(3+) with deprotonated Ala(n) (n = 2-5) or Leu-enk (Tyr-Gly-Gly-Phe-Leu) are investigated with infrared multiple photon dissociation (IRMPD) spectroscopy between 900 and 1850 cm(-1) and theory. In all of these complexes, a salt bridge is formed in which the metal cation coordinates to the carboxylate group of the peptide, resulting in a limited conformational space and many sharp IRMPD spectral bands. The IRMPD spectra clearly indicate that all carbonyl groups solvate the metal cation in each of the Ala(n) complexes. Due to strong vibrational coupling between the carbonyl groups, a sharp, high-energy amide I band due to in-phase stretching of all of the amide carbonyl groups bound to the metal cation is observed that is separated by approximately 50 cm(-1) from a strong, lower-energy amide I band. This extent of carbonyl coupling, which is sometimes observed in condensed-phase peptide and protein IR spectroscopy, has not been reported in IRMPD spectroscopy studies of other cationized peptide complexes. Intense bands due to carbonyl groups not associated with the metal cation are observed for Leu-enk complexes, indicating that a side chain group, such as the Tyr or Phe aromatic ring, prevents complete carbonyl coordination of the metal cation. Substitution of smaller lanthanide cations for La(3+) in these peptide complexes results only in minor structural changes consistent with the change in metal cation size. These are the first IRMPD spectra reported for lanthanide metal cationized peptides, and comparison to previously reported protonated and alkali metal or alkaline earth metal cationized peptide complexes reveals many trends consistent with the higher charge state of the lanthanide cations.

IRPD spectroscopy and ensemble measurements: Effects of different data acquisition and analysis methods

Three different commonly used infrared photodissociation (IRPD) spectroscopy acquisition and analysis methods are described, and results from these methods are compared using the same dataset for an extensively hydrated metal cation, La3+(H2O)36. Using the first-order laser-induced photodissociation rate constant as an IRPD intensity has several advantages over photodissociation yield and depletion/appearance methods in that intensities can be more directly compared with calculated infrared absorption spectra, and the intensities can be readily corrected for changes in laser power or irradiation times used for optimum data acquisition at each frequency. Extending IRPD spectroscopy to large clusters can be complicated when blackbody infrared radiative dissociation competes strongly with laser-induced photodissociation. A new method to obtain IRPD spectra of single precursor ions or ensembles of precursor ions that is nearly equivalent to the photodissociation rate constant method for single precursor ions is demonstrated. The ensemble IRPD spectra represent the “average” structure of clusters of a given size range, and this method has the advantage that spectra with improved signal-to-noise ratios can be obtained with no increase in data acquisition time. Results using this new method for a precursor ensemble consisting of La3+(H2O)35–37 are compared with results for La3+(H2O)36.

Hydration of Alkaline Earth Metal Dications: Effects of Metal Ion Size Determined Using Infrared Action Spectroscopy

Infrared laser action spectroscopy is used to characterize divalent Mg, Ca, and Ba ions solvated by discrete numbers of water molecules in the gas phase. The spectra of the hexahydrated ions are very similar and indicate that all six water molecules directly solvate the metal ion. The spectra of the heptahydrated ions indicate the presence of populations of structures that have a water molecule in the outer shell for all ions and an average coordination number (CN) for Ba that is higher than that for Ca or Mg. Comparisons between CN values obtained from M06 density functional and local MP2 theory indicate that the B3LYP density functional favors smaller CN values. The spectra of clusters containing at least 12 water molecules indicate that the relative abundance of single-acceptor water molecules for a given cluster size decreases with increasing metal ion size, indicating that tighter water binding to smaller metal ions disrupts the hydrogen bond network and results in fewer net hydrogen bonds. The spectra of the largest clusters (n = 32) are very similar, suggesting that intrinsic water properties are more important than ion-water interactions by that size, but subtle effects of Mg on surface water molecules are observed even for such large cluster sizes.