2 research outputs found
Absorption Spectra of Model Single Chains of Conducting Polyaniline
The theoretical study addresses the type and nature of
the transitions
in the absorption spectra of octamers, dodecamers, and hexadecamers
of the emeraldine saltî—¸the conducting form of polyaniline.
Each of the fully protonated oligomers is modeled in its lowest singlet
(bipolaronic form) and highest possible multiplicity (polaronic form).
Two configurations of the chloride counterions with respect to the
oligomer chains are considered. All structures are optimized with
BLYP/6-31G*/PCM, while the spectra are evaluated with CIS/6-31G*/PCM.
The optical behavior of the bipolaronic and polaronic forms of the
investigated systems is discussed and compared to relevant experimental
data. The optical transitions at about 400 and 800 nm characteristic
for the emeraldine salt are registered for all model structures. Weighed
against experimental and earlier theoretical findings the results
prove that CIS gives qualitatively correct electronic spectra of these
conjugated species. While the two configurations have almost identical
spectra in the highest multiplicity, the singlets’ absorption
conduct turns out to be sensitive to the counterions position. In
all cases the most intensive absorption is the longest wavelength
one in the near-IR region, but the number and oscillator strengths
of the polaronic and bipolaronic bands are noticeably dissimilar.
The bands of the low-spin oligomers are grouped, while those of the
high-spin species cover the entire visible region. Each extension
of the chain with one elementary unit contributes systematically a
set of new bands to the spectrum. The possibility for a solvatochromic
effect is estimated
Molecular Dynamics Simulation of the Aggregation Patterns in Aqueous Solutions of Bile Salts at Physiological Conditions
Classical molecular dynamics simulations
are employed to monitor
the aggregation behavior of six bile salts (nonconjugated and glycine-
and taurine-conjugated sodium cholate and sodium deoxycholate) with
concentration of 10 mM in aqueous solution in the presence of 120
mM NaCl. There are 150 ns trajectories generated to characterize the
systems. The largest stable aggregates are analyzed to determine their
shape, size, and stabilizing forces. It is found that the aggregation
is a hierarchical process and that its kinetics depends both on the
number of hydroxyl groups in the steroid part of the molecules and
on the type of conjugation. The micelles of all salts are similar
in shape–deformed spheres or ellipsoids, which are stabilized
by hydrophobic forces, acting between the steroid rings. The differences
in the aggregation kinetics of the various conjugates are rationalized
by the affinity for hydrogen bond formation for the glycine-modified
salts or by the longer time needed to achieve optimum packing for
the tauro derivatives. Evidence is provided for the hypothesis from
the literature that the entirely hydrophobic core of all aggregates
and the enhanced dynamics of the molecules therein should be among
the prerequisites for their pronounced solubilization capacity for
hydrophobic substances <i>in vivo</i>