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 salt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>