Isomer-selected photoelectron spectroscopy of isolated DNA oligonucleotides: phosphate and nucleobase deprotonation at high negative charge states.

Fractionation according to ion mobility and mass-to-charge ratio has been used to select individual isomers of deprotonated DNA oligonucleotide multianions for subsequent isomer-resolved photoelectron spectroscopy (PES) in the gas phase. Isomer-resolved PE spectra have been recorded for tetranucleotides, pentanucleotides, and hexanucleotides. These were studied primarily in their highest accessible negative charge states (3-, 4-, and 5-, respectively), as provided by electrospraying from room temperature solutions. In particular, the PE spectra obtained for pentanucleotide tetraanions show evidence for two coexisting classes of gas-phase isomeric structures. We suggest that these two classes comprise: (i) species with excess electrons localized exclusively at deprotonated phosphate backbone sites and (ii) species with at least one deprotonated base (in addition to several deprotonated phosphates). By permuting the sequence of bases in various [A(5-x)T(x)](4-) and [GT(4)](4-) pentanucleotides, we have established that the second type of isomer is most likely to occur if the deprotonated base is located at the first or last position in the sequence. We have used a combination of molecular mechanics and semiempirical calculations together with a simple electrostatic model to explore the photodetachment mechanism underlying our photoelectron spectra. Comparison of predicted to measured photoelectron spectra suggests that a significant fraction of the detected electrons originates from the DNA bases (both deprotonated and neutral).

Probing electrostatic interactions and structural changes in highly charged protein polyanions by conformer-selective photoelectron spectroscopy.

We have recorded the first conformer-selective photoelectron spectra of a protein polyanion in the gas-phase. Bovine cytochrome c protein was studied in 8 different negative charge states ranging from 5- to 12-. Electron binding energies were extracted for all charge states and used as a direct probe of intramolecular Coulomb repulsion. Comparison of experimental results with simulations shows that the experimental outcome can be reproduced with a simple electrostatic model. Energetics are consistent with a structural transition from a folded to an unfolded conformational state of the protein as the number of charges increases. Furthermore, the additional ion-mobility data show that the onset of unfolding can be assigned to charge state 6- where three conformers can be distinguished.

Combining ion mobility spectrometry, mass spectrometry, and photoelectron spectroscopy in a high-transmission instrument.

We have developed a novel instrument that combines ion mobility spectrometry, mass spectro-metry, and photoelectron spectroscopy. The instrument couples an electrospray ion source, a high-transmission ion mobility cell based on ion funnels, a quadrupole mass filter, and a time-of-flight (magnetic bottle) photoelectron spectrometer operated with a pulsed detachment laser. We show that the instrument can resolve highly structured anion arrival time distributions and at the same time provide corresponding photoelectron spectra-using the DNA oligonucleotide ion [dC(6) - 5H](5-) as a test case. For this multianion we find at least four different, noninterconverting isomers (conformers) simultaneously present in the gas phase at room temperature. For each of these we record well-resolved and remarkably different photoelectron spectra at each of three different detachment laser wavelengths. Two-dimensional ion mobility/electron binding energy plots can be acquired with an automated data collection procedure. We expect that this kind of instrument will significantly improve the capabilities for structure determination of (bio)molecular anions in the gas phase.

Electronic relaxation dynamics in large anionic water clusters: (H2O)n(-) and (D2O)n(-) (n = 25-200).

Electronic relaxation dynamics subsequent to s --> p excitation of the excess electron in large anionic water clusters, (H(2)O)(n)(-) and (D(2)O)(n)(-) with 25 < or = n < or = 200, were investigated using time-resolved photoelectron imaging. Experimental improvements have enabled considerably larger clusters to be probed than in previous work, and the temporal resolution of the instrument has been improved. New trends are seen in the size-dependent p-state lifetimes for clusters with n > or = 70, suggesting a significant change in the electron-water interaction for clusters in this size range. Extrapolating the results for these larger clusters to the infinite-size limit yields internal conversion lifetimes tau(IC) of 60 and 160 fs for electrons dissolved in H(2)O and D(2)O, respectively. In addition, the time-evolving spectra show evidence for solvent relaxation in the excited electronic state prior to internal conversion and in the ground state subsequent to internal conversion. Relaxation in the excited state appears to occur on a time scale similar to that of internal conversion, while ground state solvent dynamics occur on a approximately 1 ps time scale, in reasonable agreement with previous measurements on water cluster anions and electrons solvated in liquid water.

Auger recombination and excited state relaxation dynamics in Hg(n)(-) (n=9-20) anion clusters.

Using femtosecond time-resolved photoelectron imaging, electron-hole pairs are created in size-selected Hg(n)(-) anion clusters (n=9-20), and the subsequent decay dynamics are measured. These clusters eject electrons via Auger decay on time scales of 100-600 fs. There is an abrupt increase in the Auger decay time for clusters larger than Hg(12)(-), coinciding with the onset of the transition from van der Waals to covalent bonding in mercury clusters. Our results also show evidence for subpicosecond excited state relaxation attributed to inelastic electron-electron and electron-hole scattering as well as hole-induced contraction of the cluster.

Photodissociation dynamics of IrBr6(2-) dianions by time-resolved photoelectron spectroscopy.

We have used femtosecond time-resolved photoelectron spectroscopy to examine the photodissociation dynamics of doubly charged anions IrBr(6)(2-) after excitation at h nu(pump) = 1.6 eV and with a detachment photon energy of h nu(probe) = 4.8 eV. Excited state dynamics proceed by successive decay of the initially excited state, by way of an intermediate and back to the electronic ground state. This is associated with lifetimes of tau(1) = 2.1+/-0.3 ps and tau(2) = 21+/-2 ps, respectively. After nonadiabatic relaxation, the internal energy of the dianion is sufficiently large to induce fragmentation and delayed emission of Br(-) over the repulsive Coulomb barrier with a 79+/-21 ps time constant. As both fragments are negatively charged, Coulomb repulsion at early times (and correspondingly small separations) is reflected in the transient photoelectron spectra. Analysis of both shifts and intensities of the time-dependent bromide detachment features allows determination of the shape of the dissociation barrier. A lower limit of the outer height was retrieved from the kinetic energy release of KER > or = 1.6 eV. Modeling of the dissociation rate with statistical rate theory results in an inner barrier height of E(RCB) = 0.95 eV.

Dynamics of electron solvation in molecular clusters.

Solvated electrons, and hydrated electrons in particular, are important species in condensed phase chemistry, physics, and biology. Many studies have examined the formation mechanism, reactivity, spectroscopy, and dynamics of electrons in aqueous solution and other solvents, leading to a fundamental understanding of the electron-solvent interaction. However, key aspects of solvated electrons remain controversial, and the interaction between hydrated electrons and water is of central interest. For example, although researchers generally accept that hydrated electrons, eaq-, occupy solvent cavities, another picture suggests that the electron resides in a diffuse orbital localized on a H3O radical. In addition, researchers have proposed two physically distinct models for the relaxation mechanism when the electron is excited. These models, formulated to interpret condensed phase experiments, have markedly different timescales for the internal conversion from the excited p state to the ground s state.Studies of negatively charged clusters, such as (H2O)n- and I-(H2O)n, offer a complementary perspective for studying aqueous electron solvation. In this Account, we use time-resolved photoelectron spectroscopy (TRPES), a femtosecond pump-probe technique in which mass-selected anions are electronically excited and then photodetached at a series of delay times, to focus on time-resolved dynamics in these and similar species. In (H2O)n-,TRPES gives evidence for ultrafast internal conversion in clusters up to n=100. Extrapolation of these results yields a p-state lifetime of 50 fs in the bulk limit. This is in good agreement with the nonadiabatic solvation model, one of the models proposed for relaxation of eaq-. Similarly, experiments on (MeOH)n- up to n=450 give an extrapolated p-state lifetime of 150fs. TRPES investigations of I-(H2O)n and I-(CH3CN)n probe a different aspect of electron solvation dynamics. In these experiments,an ultraviolet pump pulse excites the cluster analog of the charge-transfer-to-solvent (CTTS) band, ejecting an electron from the iodide into the solvent network. The probe pulse then monitors the solvent response to this excess electron,specifically its stabilization via solvent rearrangement. In I-(H2O)n, the iodide sits outside the solvent network, as does the excess electron initially formed by CTTS excitation. However, the iodide in I-(CH3CN)n is internally solvated, and both experimental and theoretical evidence indicate that electrons in (CH3CN)n- are internally solvated. Hence, these experiments reflect the complex dynamics that ensue when the electron is photo detached within a highly confined solvent cavity.

Photoinduced electron transfer and solvation in iodide-doped acetonitrile clusters.

We have used ultrafast time-resolved photoelectron imaging to measure charge transfer dynamics in iodide-doped acetonitrile clusters I(-)(CH(3)CN)(n) with n = 5-10. Strong modulations of vertical detachment energies were observed following charge transfer from the halide, allowing interpretation of the ongoing dynamics. We observe a sharp drop in the vertical detachment energy (VDE) within 300-400 fs, followed by a biexponential increase that is complete by approximately 10 ps. Comparison to theory suggests that the iodide is internally solvated and that photodetachment results in formation of a diffuse electron cloud in a confined cavity. We interpret the initial drop in VDE as a combination of expansion of the cavity and localization of the excess electron on one or two solvent molecules. The subsequent increase in VDE is attributed to a combination of the I atom leaving the cavity and rearrangement of the acetonitrile molecules to solvate the electron. The n = 5-8 clusters then show a drop in VDE of around 50 meV on a much longer time scale. The long-time VDEs are consistent with those of (CH(3)CN)(n)(-) clusters with internally solvated electrons. Although the excited-state created by the pump pulse decays by emission of a slow electron, no such decay is seen by 200 ps.

Photoelectron spectroscopy of gramicidin polyanions: competition between delayed and direct emission.

We present the first photoelectron (PE) spectra of polypeptide polyanions. Combining PE spectroscopy and mass spectrometry provides a direct measurement of the stability of the polyanions with respect to electron detachment and of the repulsive energy between excess charges. The second electron affinity of gramicidin was found to amount to 2.35 +/- 0.15 eV, and the value of the repulsive Coulomb barrier was estimated to be 0.5 +/- 0.15 eV. The spectra are interpreted as resulting from a competition between delayed and direct emission.

Excited state dynamics of metastable phthalocyanine-tetrasulfonate tetra-anions probed by pump/probe photoelectron spectroscopy.

Femtosecond time-resolved pump-probe photoelectron spectroscopy was used to study elementary relaxation processes occurring in isolated phthalocyanine-tetrasulfonate tetra-anions ([MPc(SO3)4]4-, M=Cu,Ni, and "free-base" [H2Pc(SO3)4]4-) following Q band excitation by one-photon absorption at 775 nm. Whereas the Cu and Ni systems decay rapidly by means of internal conversion without electron loss, the free-base phthalocyanine primarily undergoes excited state tunneling electron emission. This reflects less efficient coupling to lower lying states within the corresponding spin manifold. Results are interpreted in terms of (time-dependent) density functional theory calculations of ground and electronically excited states and kinetically modeled to yield the associated rates.

Femtosecond pump/probe photoelectron spectroscopy of isolated C60 negative ions.

We have measured pump/probe photoelectron spectra of mass-selected, near room temperature C60- in the gas phase. The lifetime of the vibrationally excited B- (2Eg) state at a calculated energy of 1.26 eV was found to be tau = 2.2+/-0.2 ps. The dominant decay process corresponds to intramolecular radiationless transitions into ground state C60-. This is in contrast to C60 for which pumping at the absorption onset (1.95 eV) leads to predominantly intersystem crossing.

Quantifying electron transfer during hyperthermal scattering of C60+ from Au(111) and n-alkylthiol self-assembled monolayers.

A tandem time-of-flight mass spectrometer with an intermediate surface was used to quantify electron transfer during glancing incidence scattering of hyperthermal C(60) (+) (E(coll)=250-500 eV, theta(in)=75 degrees ) from (i) self-assembled monolayers of n-alkylthiols on gold (of various chain lengths), (ii) partly fluorinated alkylthiols on gold, as well as (iii) clean gold surfaces. Self-assembled monolayers (SAMs) behave as insulating layers with their thicknesses determining the electron tunneling probability during collision. Correspondingly, a roughly exponential dependence of the neutralization probability on the chain length n was found. A pronounced dependence of the neutral yield on the primary beam kinetic energy indicates that dynamic SAM deformation and associated projectile penetration depth also play a role in determining electron transfer efficiency. Results are consistent with the molecular deformability of SAMs as determined with other experimental methods.

Photoelectron spectroscopy of fullerene dianions C76(2-), C78(2-), and C84(2-).

We report laser photoelectron spectra of the doubly negatively charged fullerenes C(76) (2-), C(78) (2-), and C(84) (2-) at 2.33, 3.49, and 4.66 eV photon energy. From these spectra, second electron affinities and vertical detachment energies, as well as estimates for the repulsive Coulomb barriers are obtained. These results are discussed in the context of electrostatic models. They reveal that fullerenes are similar to conducting spheres, with electronic properties scaling with their size. The experimental spectra are compared with the accessible excited states of the respective singly charged product ions calculated in the framework of time dependent density functional theory.

Oxytocin-receptor binding: why divalent metals are essential.

Biologists have observed that the presence of divalent metal is essential for the binding of the hormone oxytocin (OT) to its cellular receptor. However, this interaction is not understood on the molecular level. Because conformation is a key factor controlling ligand binding in biomolecule systems, we have used ion mobility experiments and molecular modeling to probe the conformation of the oxytocin-zinc complex. Results show that Zn2+ occupies an octahedral site in the interior of the OT peptide that frees the N-terminus and creates a structured hydrophobic binding site on the peptide exterior; both factors are conducive to binding oxytocin to its receptor.

Photoelectron spectroscopy of isolated multiply negatively charged oligonucleotides.

Ultraviolet photoelectron spectroscopy in an ion beam was used to investigate the electronic properties of isolated DNA oligonucleotides [dA(5)-4H](4-) and [dT(5)-4H](4-), carrying four excess negative charges. We find the fourth adiabatic electron affinity to be slightly negative for [dA(5)-4H](4-), while it is positive for [dT(5)-4H](4-). This implies a significant influence of the base composition on energetics, which is in turn relevant for analytic applications and also for charge transport properties.

Photoelectron spectroscopy of C(84) dianions.

We report the laser photoelectron spectra of doubly negatively charged C84 (D2 and D(2d)) using 532 nm and 355 nm radiation. From these spectra, values for the second electron affinity and vertical detachment energy, as well as upper and lower limits for the repulsive Coulomb barrier, are obtained. These values are discussed in the context of classical electrostatic models. The experimental spectra are compared with the accessible excited states of the C-84 product ion calculated in the framework of time dependent density functional theory.