Maria Demireva

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.

Directly Relating Reduction Energies of Gaseous Eu(H2O)n3+, n = 55–140, to Aqueous Solution: The Absolute SHE Potential and Real Proton Solvation Energy

In solution, half-cell potentials are measured relative to other half-cells resulting in a ladder of thermodynamic values that is anchored to the standard hydrogen electrode (SHE), which is assigned an arbitrary value of exactly 0 V. A new method for measuring the absolute SHE potential is introduced in which reduction energies of Eu(H(2)O)(n)(3+), from n = 55 to 140, are extrapolated as a function of the geometric dependence of the cluster reduction energy to infinite size. These measurements make it possible to directly relate absolute reduction energies of these gaseous nanodrops containing Eu(3+) to the absolute reduction enthalpy of this ion in bulk solution. From this value, an absolute SHE potential of +4.11 V and a real proton solvation energy of -269.0 kcal/mol are obtained. The infrared photodissociation spectrum of Eu(H(2)O)(119-124)(3+) indicates that the structure of the surface of the nanodrops is similar to that at the bulk air-water interface and that the hydrogen bonding of interior water molecules is similar to that in aqueous solution. These results suggest that the environment of Eu(3+) in these nanodrops and the surface potential of the nandrops are comparable to those of the condensed phase. This method for obtaining absolute potentials of redox couples has the advantage that no explicit solvation model is required, which eliminates uncertainties associated with these models, making this method potentially more accurate than previous methods.

Average Sequential Water Molecule Binding Enthalpies of M(H2O)19-1242+ (M = Co, Fe, Mn, and Cu) Measured with Ultraviolet Photodissociation at 193 and 248 nm

The average sequential water molecule binding enthalpies to large water clusters (between 19 and 124 water molecules) containing divalent ions were obtained by measuring the average number of water molecules lost upon absorption of an UV photon (193 or 248 nm) and using a statistical model to account for the energy released into translations, rotations, and vibrations of the products. These values agree well with the trend established by more conventional methods for obtaining sequential binding enthalpies to much smaller hydrated divalent ions. The average binding enthalpies decrease to a value of ~10.4 kcal/mol for n > ~40 and are insensitive to the ion identity at large cluster size. This value is close to that of the bulk heat of vaporization of water (10.6 kcal/mol) and indicates that the structure of water in these clusters may more closely resemble that of bulk liquid water than ice, owing either to a freezing point depression or rapid evaporative cooling and kinetic trapping of the initial liquid droplet. A discrete implementation of the Thomson equation using parameters for liquid water at 0 °C generally fits the trend in these data but provides values that are ~0.5 kcal/mol too low.

Electron hydration and ion-electron pairs in water clusters containing trivalent metal ions.

The hydrated electron is one of the most fundamental nucleophiles in aqueous solution, yet it is a transient species in liquid water, making it challenging to study. The solvation thermodynamics of the electron are important for determining the band structure and properties of water and aqueous solutions. However, a wide range of values for the electron solvation enthalpy (-1.0 to -1.8 eV) has been obtained from previous methods, primarily because of the large uncertainty as to the value for the absolute proton solvation enthalpy. In the gas phase, electron interactions with water can be investigated in stable water clusters that contain an excess electron, or an electron and a solvent-separated monovalent or divalent metal ion. Here, we report the generation of stable water clusters that contain an excess electron and a solvent-separated trivalent metal ion that are formed upon electron capture by hydrated trivalent lanthanide clusters. From the number of water molecules lost upon electron capture, adiabatic recombination energies are obtained for La(H(2)O)(n)(3+) (n = 42-160). The trend in recombination energies as a function of hydration extent is consistent with a structural transition from a surface-located excess electron at smaller sizes (n <or= approximately 56) to a more fully solvated electron at larger sizes (n >or= approximately 60). The recombination enthalpies for n > 60 are extrapolated as a function of the geometrical dependence on cluster size to infinite size to obtain the bulk hydration enthalpy of the electron (-1.3 eV). This extrapolation method has the advantages that it does not require estimates of the absolute proton or hydrogen hydration enthalpies.

Water-Induced Folding of 1,7-Diammoniumheptane

Effects of hydration on the gaseous structures of diprotonated 1,7-diaminoheptane and protonated heptylamine are investigated by infrared photodissociation (IRPD) spectroscopy and computational chemistry. IRPD spectra in the hydrogen bond stretching region (2800-3900 cm(-1)) indicate that 1,7-diammoniumheptane is linear and that hydration occurs predominantly by alternate solvation of the two protonated amine groups for clusters with up to 10 water molecules. The relative intensities of bonded versus free hydroxyl (OH) stretches are greater in the spectra of 1,7-diammoniumheptane with more than 12 water molecules attached than the corresponding reference spectra of heptylammonium. This indicates that in the larger clusters, 1,7-diammoniumheptane adopts a more folded conformation in which the two protonated amine groups are solvated by a single water nanodrop. These results are supported by molecular dynamics simulations which show more hydrogen bonds in representative folded structures of hydrated 1,7-diammoniumheptane versus those with linear structures. These results indicate that the increase in Coulomb energy as a result of bringing the two positive charges closer together in the folded structures is compensated for by the additional hydrogen bonds that are possible when a single nanodrop solvates both protonated amine groups.

Ions in Size-Selected Aqueous Nanodrops: Sequential Water Molecule Binding Energies and Effects of Water on Ion Fluorescence

The effects of water on ion fluorescence were investigated, and average sequential water molecule binding energies to hydrated ions, M(z)(H(2)O)(n), at large cluster size were measured using ion nanocalorimetry. Upon 248-nm excitation, nanodrops with ~25 or more water molecules that contain either rhodamine 590(+), rhodamine 640(+), or Ce(3+) emit a photon with average energies of approximately 548, 590, and 348 nm, respectively. These values are very close to the emission maxima of the corresponding ions in solution, indicating that the photophysical properties of these ions in the nanodrops approach those of the fully hydrated ions at relatively small cluster size. As occurs in solution, these ions in nanodrops with 8 or more water molecules fluoresce with a quantum yield of ~1. Ce(3+) containing nanodrops that also contain OH(-) fluoresce, whereas those with NO(3)(-) do not. This indirect fluorescence detection method has the advantages of high sensitivity, and both the size of the nanodrops as well as their constituents can be carefully controlled. For ions that do not fluoresce in solution, such as protonated tryptophan, full internal conversion of the absorbed 248-nm photon occurs, and the average sequential water molecule binding energies to the hydrated ions can be accurately obtained at large cluster sizes. The average sequential water molecule binding energies for TrpH(+)(H(2)O)(n) and a doubly protonated tripeptide, [KYK + 2H](2+)(H(2)O)(n), approach asymptotic values of ~9.3 (n ≥ 11) and ~10.0 kcal/mol (n ≥ 25), respectively, consistent with a liquidlike structure of water in these nanodrops.

Weighing photon energies with mass spectrometry: effects of water on ion fluorescence.

We report a new, highly sensitive method for indirectly measuring fluorescence from ions with a discrete number of water molecules attached. Absorption of a 248 nm photon by hydrated protonated proflavine, PH(+)(H(2)O)(n) (n = 13-50), results in two resolved product ion distributions that correspond to full internal conversion of the photon energy (loss of approximately 11 water molecules) and to partial internal conversion of the photon energy and emission of a lower energy photon (loss of approximately 6 water molecules). In addition to fluorescence, a long-lived triplet state with a half-life of approximately 0.5 s (for n = 50) is formed. The energy of the emitted photon can be obtained from the number of water molecules lost from the precursor to form each distribution. The photon energies generally red shift from approximately 450 to 580 nm with increasing cluster size (the onset of the PH(+)(aq) fluorescence spectrum is 600 nm and the maximum is 518 nm) consistent with preferential stabilization of the first excited singlet state versus the ground state. The fluorescence quantum yield of PH(+)(H(2)O)(n) for n > or = 30 is 0.36 +/- 0.02, the same as that in bulk solution, and increases dramatically with decreasing cluster sizes, due to less efficient conversion of electronic-to-vibrational energy. The high sensitivity of this method should make it possible to perform Forster resonance energy transfer experiments with gas-phase biomolecules in a microsolvated environment to investigate how a controlled number of water molecules facilitates dynamical motions in proteins or other molecules of interest.

Measuring internal energy deposition in collisional activation using hydrated ion nanocalorimetry to obtain peptide dissociation energies and entropies

The internal energy deposited in both on- and off-resonance collisional activation in Fourier transform ion cyclotron resonance mass spectrometry is measured with ion nanocalorimetry and is used to obtain information about the dissociation energy and entropy of a protonated peptide. Activation of Na+(H2O)30 results in sequential loss of water molecules, and the internal energy of the activated ion can be obtained from the abundances of the product ions. Information about internal energy deposition in on-resonance collisional activation of protonated peptides is inferred from dissociation data obtained under identical conditions for hydrated ions that have similar m/z and degrees-of-freedom. From experimental internal energy deposition curves and Rice-Ramsperger-Kassel-Marcus (RRKM) theory, dissociation data as a function of collision energy for protonated leucine enkephalin, which has a comparable m/z and degrees-of-freedom as Na+(H2O)30, are modeled. The threshold dissociation energies and entropies are correlated for data acquired at a single time point, resulting in a relatively wide range of threshold dissociation energies (1.1 to 1.7 eV) that can fit these data. However, this range of values could be significantly reduced by fitting data acquired at different dissociation times. By measuring the internal energy of an activated ion, the number of fitting parameters necessary to obtain information about the dissociation parameters by modeling these data is reduced and could result in improved accuracy for such methods.