12 research outputs found
Valence structures of aromatic bioactive compounds: a combined theoretical and experimental study.
Valence electronic structures of three recently isolated aryl bioactive compounds, namely 2-phenylethanol (2PE), p-hydroxyphenylethanol (HPE) and 4-hydroxybenzaldehyde (HBA), are studied using a combined theoretical and experimental method. Density functional theory-based calculations indicate that the side chains cause electron charge redistribution and therefore influence the aromaticity of the benzene derivatives. The simulated IR spectra further reveal features induced by the side chains. Solvent effects on the IR spectra are simulated using the polarizable continuum model, which exhibits enhancement of the O-H stretch vibrations with significant red-shift of 464 cm(-1) in 2PE. A significant spectral peak splitting of 94 cm(-1) between O(4)-H and O(8)-H of HPE is revealed in an aqueous environment. Experimental measurements for valence binding energy spectra for 2PE, HPE and HBA are presented and analyzed using outer-valence Green function calculations. The experimental (predicted) first ionization energies are measured as 9.19 (8.79), 8.47 (8.27) and 8.97 (8.82) eV for 2PE, HPE and HBA, respectively. The frontier orbitals (highest occupied molecular orbitals, HOMOs, and lowest unoccupied molecular orbitals, LUMOs) have similar atomic orbital characters although the HOMO-LUMO energy gaps are quite different
Adsorption of cytosine and aza derivatives of cytidine on Au single crystal surfaces
The adsorption of cytosine on the Au(111) and Au(110) surfaces has been
studied using both aqueous deposition and evaporation in vacuum to prepare the
samples. Soft X-ray photoelectron spectroscopy (XPS) and near edge X-ray
absorption fine structure spectroscopy (NEXAFS) were used to determine the
electronic structure and orientation of the adsorbates. In addition, three
derivatives of cytosine, 6-azacytosine, 6-azacytidine and 5- azacytidine, were
studied. Monolayer films of the latter three samples were adsorbed on Au(111)
from aqueous solution, and the nature of bonding was determined. Spectra have
been interpreted in the light of published calculations of free cytosine
molecules and new ab initio calculations of the other compounds. Surface core
level shifts of Au 4f imply that all of these compounds are chemisorbed.
Cytosine adsorbs as a single tautomer, but in two chemical states with
different surface-molecule bonding. For deposition in vacuum, a flat-lying
molecular state bonded through the N(3) atom of the pyrimidine ring dominates,
but a second state is also present. For deposition from solution, the second
state dominates, with the molecular plane no longer parallel to the surface.
This state also bonds through the N(3) atom, but in addition interacts with the
surface via the amino group. Two tautomers of 6-azacytosine were observed, and
they and 6-azacytidine adsorb with similar geometries, chemically bonding via
the azacytosine ring. The ribose ring does not appear to perturb the adsorption
of azacytidine compared with azacytosine. The azacytosine ring is nearly but
not perfectly parallel to the surface, like 5-azacytidine, which adsorbs as an
imino tautomer. ...Comment: 40 pages, 3 tables and 8 figure
Bonding of Histidine to Cerium Oxide
Adsorption of histidine on cerium
oxide model surfaces was investigated
by synchrotron radiation photoemission, resonant photoemission, and
near edge X-ray absorption fine structure spectroscopies. Histidine
was evaporated in a vacuum onto ordered stoichiometric CeO<sub>2</sub>(111) and partially reduced CeO<sub>1.9</sub> thin films grown on
Cu(111). Histidine binds to CeO<sub>2</sub> in anionic form via the
carboxylate group and all three nitrogen atoms, with the imidazole
ring parallel to the surface. The amino nitrogen atom of the imidazole
ring (IM) is deprotonated, and both IM nitrogen atoms form strong
bonds via π orbitals, while the α-amino nitrogen interacts
with the oxide via its hydrogen atoms. In the case of CeO<sub>1.9</sub>, the deprotonation of the amino nitrogen of the imidazole ring is
less pronounced and N K-edge spectra do not show a clear orientation
of the ring with respect to the surface. A minor reduction of the
cerium surface on adsorption of histidine was observed and explained
by charge exchange as a result of hybridization of the π orbitals
of the IM ring with the f and d orbitals of ceria. Knowledge of histidine
adsorption on the cerium oxide surface can be used for design of mediator-less
biosensors where the histidine-containing proteins can be strongly
bound to the oxide surface via the imidazole side chain of this residue
Adsorption of Cytosine and AZA Derivatives of Cytidine on Au Single Crystal Surfaces
The adsorption of cytosine on the
Au(111) and Au(110) surfaces
has been studied using both aqueous deposition and evaporation in
vacuum to prepare the samples. Soft X-ray photoelectron spectroscopy
(XPS) and near edge X-ray absorption fine structure spectroscopy (NEXAFS)
were used to determine the electronic structure and orientation of
the adsorbates. In addition, three derivatives of cytosine, 6-azacytosine,
6-azacytidine, and 5-azacytidine, were studied. Monolayer films of
the latter three samples were adsorbed on Au(111) from aqueous solution,
and the nature of bonding was determined. Spectra have been interpreted
in the light of published calculations of free cytosine molecules
and new ab initio calculations of the other compounds. Surface core
level shifts of Au 4f imply that all of these compounds are chemisorbed.
Cytosine adsorbs as a single tautomer but in two chemical states with
different surface-molecule bonding. For deposition in vacuum, a flat-lying
molecular state bonded through the N<sub>(3)</sub> atom of the pyrimidine
ring dominates, but a second state is also present. For deposition
from solution, the second state dominates, with the molecular plane
no longer parallel to the surface. This state also bonds through the
N<sub>(3)</sub> atom, but in addition interacts with the surface via
the amino group. Two tautomers of 6-azacytosine were observed, and
they and 6-azacytidine adsorb with similar geometries, chemically
bonding via the azacytosine ring. The ribose ring does not appear
to perturb the adsorption of azacytidine compared with azacytosine.
The azacytosine ring is nearly but not perfectly parallel to the surface,
like 5-azacytidine, which adsorbs as an imino tautomer. This work
highlights the complications, which can occur when medicinally significant
compounds are adsorbed on gold, for example, in drug delivery systems,
but also the amount of chemical information and detailed understanding
that can be extracted from such complex systems