Salt bridge chemistry has recently been realized as a determining
factor in the structures and reaction
dynamics of biological molecules in the gas phase. In this paper,
we further investigate salt bridge chemistry
in studies of the low-energy collision-induced dissociation (CID) of
sodiated peptides. MALDI and electrospray
ionization are used to generate singly and multiply charged sodiated
peptides which are analyzed by using an
external ion source Fourier transform ion cyclotron resonance mass
spectrometer. Of particular interest is the
observation that sodiated peptides exhibit highly selective cleavage at
aspartic acid residues. Sodiated peptides
that lack acidic residues, however, undergo sequential cleavages from
the C-terminus on low-energy CID.
We propose a mechanism for cleavage at aspartic acid residues that
involves a salt bridge intermediate in
which the sodium ion stabilizes the ion pair formed by proton transfer
from aspartic acid to the adjacent amide
nitrogen. This proposal is supported by ab initio calculations to
quantify the reaction energetics. In several
instances the less selective low-energy fragmentation processes of the
protonated peptides have also been
investigated for comparison.
Straight-stranded anatase TiO2 nanotubes were produced by anodic oxidation on a pure titanium substrate
in an aqueous solution containing a 0.45 wt % NaF electrolyte (pH 4.3 fixed). The average length of the TiO2
nanotubes was approximately 3 μm, which had an effect on the level of dye adsorption in the dye-sensitized
solar cells. The anodic TiO2 nanotubes were applied as a working electrode in a solid-state dye-sensitized
solar cell. An approximately 1 nm ZnO shell was coated on the TiO2 nanotube to improve the open-circuit
voltage (V
oc) and conversion efficiency of the solar cell, and to retard any back reaction. Although the V
oc
and short-circuit current (J
sc) of the cell were improved, there was a low fill factor as a result of the formation
of a thick TiO2 barrier layer in the anodic TiO2/Ti substrate. A parameter on the degradation of fill factor
(37%) is related to the formation of a thick TiO2 barrier layer in the anodic TiO2/Ti substrate interface. A
hydrogen peroxide treatment was performed in an attempt to narrow the TiO2 barrier layer. This treatment
was found to influence not only fill factor (37−49%) but also the conversion efficiency (0.704−0.906%) of
the cell by eliminating the remnant after anodic reaction and barrier narrowing through an etching effect.
This result was confirmed by X-ray photoelectron spectroscopy (XPS) and photocurrent-voltage measurements.
The longer electron lifetime on the ZnO coated TiO2 film was measured by the open-circuit voltage decay.
The improvement in the electron lifetime from the thin ZnO coating affects the number of electrons collected
on the Ti substrate and the retardation of charge recombination. Therefore, the ZnO coating on the TiO2
nanotube film improves the efficiency of dye-sensitized TiO2 solar cells from the extended V
oc from ZnO
coating confirmed by the Mott−Schottky plots and the increased J
sc through the inhibition of charge
recombination confirmed by IPCE measurements.
Selective binding of crown ethers to model protonated peptides is utilized to study the site selectivity and mechanisms of gas-phase hydrogen/deuterium exchange reactions with ND 3 in an external ion source FT-ICR mass spectrometer. Mechanisms for H/D exchange reactions in the gas phase can be classified into two different types: Type I involving direct participation of the labile protons at the charge site and Type II in which the charge site at most plays only an ancillary role in the process (e.g., salt bridge formation). Localization of the labile proton at the charge site by crown ether attachment inhibits Type I processes, as evidenced by a dramatic reduction in the rates of H/D exchange. For example, crown ether attachment to protonated ethylenediamine and 1,4-diaminobutane inhibits H/D exchange reactions, while the free protonated species undergo rapid exchange of all five labile hydrogens. Type II processes are still observed with the crown ether adducts. Both the amide and the carboxyl hydrogens of peptides exchange via a Type II process for which a salt bridge mechanism has been proposed. In the salt bridge mechanism, the charge site may play an important role by stabilizing a charge separated ion pair. Immobilization of the labile proton by crown ether attachment does not eliminate this stabilization. Charge localization by crown ether attachment also affects the dissociation processes of protonated peptides, inhibiting charge directed mechanisms where endothermic proton transfer from the most basic group to a less basic site is a prerequisite for fragmentation. Collisional activation of the crown ether complex with protonated GGDPG and GGI results in no backbone cleavage in the peptide, while the free protonated peptides lead to cleavage at the C-terminus side of aspartic acid and the second glycine, respectively.
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