Binding of holons and spinons in the one-dimensional anisotropic t-J model

We study the binding of a holon and a spinon in the one-dimensional anisotropic t-J model using a Bethe-Salpeter equation approach, exact diagonalization, and density matrix renormalization group methods on chains of up to 128 sites. We find that holon-spinon binding changes dramatically as a function of anisotropy parameter alpha=J( perpendicular)/J(z): it evolves from an exactly deducible impuritylike result in the Ising limit to an exponentially shallow bound state near the isotropic case. A remarkable agreement between the theory and numerical results suggests that such a change is controlled by the corresponding evolution of the spinon energy spectrum.     

One-pot parallel solution-phase synthesis of 1-substituted 4-(2-aminoethyl)-1H-pyrazol-5-ols

Two variations of enaminone-based parallel solution-phase synthesis of 1-substituted 4-(2-aminoethyl)-1 H-pyrazol-5-ols 8 and their NH-tautomers 8' were developed. The synthetic strategy comprises a two step preparation of the N-protected alpha-enamino lactams 3a and 3b from 2-pyrrolidinone (1), "ring switching" transformation of 3a, b with monosubstituted hydrazines 4a-u, and acidolytic removal of the N-protecting group. In order to ensure a clean and fast conversion, reactions of Cbz-enaminone 3a with hydrazines 4a-k were carried out under microwave irradiation to afford the "ring-switched" intermediates 7a-k. Deprotection of 7a-k with HBr-AcOH at 50 degrees C gave a library of 11 analytically pure 4-(2-aminoethyl)-1 H-pyrazol-5-ols (di)hydrobromides 8/ 8'a-k in 16-75% yields over two steps. The other reagent, Boc-enaminone 3b, was more reactive and ring switching transformations with hydrazines 4b, d, k proceeded smoothly and cleanly under conventional heating. Finally, a parallel one-pot transformation of the Boc-enaminone 3b with hydrazines 4a-u followed by subsequent deprotection of the intermediates 9a-u with HCl-EtOAc furnished a library of 21 analytically pure 4-(2-aminoethyl)-1 H-pyrazol-5-ols (di)hydrochlorides 8/ 8'a-u in 40-100% yields.     

Bridging like-charged macroions through long divalent rodlike ions

Like-charged macroions in aqueous electrolyte solution can attract each other because of the presence of inter- and/or intramolecular correlations. Poisson-Boltzmann theory is able to predict attractive interactions if the spatially extended structure (which reflects the presence of intramolecular correlations) of the mobile ions in the electrolyte is accounted for. We demonstrate this for the case of divalent, mobile ions where each ion consists of two individual charges separated by a fixed distance. Variational theory applied to this symmetric 2:2 electrolyte of rodlike ions leads to an integro-differential equation, valid for arbitrary rod length. Numerical solutions reveal the existence of a critical rod length above which electrostatic attraction starts to emerge. This electrostatic attraction is distinct from nonelectrostatic depletion forces. Analysis of the orientational distribution functions suggests a bridging mechanism of the rodlike ions to hold the two macroions together. For sufficiently large rod length, we also observe "overcharging", that is, an over-compensation of the macroion charges by the diffuse layer of mobile rodlike ions. Our results emphasize the importance of the often rodlike internal structure that condensing agents such as polyamines, peptides, or polymer segments exhibit. The results were compared with Monte Carlo simulations.     

Energetics in correlation with structural features: the case of micellization

Understanding micellization processes at the molecular level has direct relevance for biological self-assembly, folding, and association processes. As such, it requires complete characterization of the micellization thermodynamics, including its correlation with the corresponding structural features. In this context, micellization of a series of model non-ionic surfactants (poly(ethylene glycol) monooctyl ethers, C(8)E(gamma)) was studied by isothermal titration calorimetry (ITC) and differential scanning calorimetry (DSC). The corresponding structural properties of C(8)E(gamma) micelles were investigated by small-angle X-ray scattering (SAXS). The C(8)E(gamma) micellization, characterized independently from ITC, DSC, and structural data, reveals that deltaH(M)(o) > 0, deltaS(M)(o) > 0, and deltaC(P)(M)(o) < 0, while the dissection of its energetics shows that it is primarily governed by the transfer of 20-30 C(8) alkyl chains from aqueous solution into the nonpolar core (r approximately 1.3 nm) of the spherical micelle. Moreover, thermodynamic parameters of micellization, estimated from the structural features related to the changes in solvent-accessible surface areas upon micellization, are in a good agreement with the corresponding parameters obtained from the analysis of ITC and DSC data. We have shown that the contributions to deltaS(M)(o) other than from hydration (deltaS(M)(other)(o)), estimated from experimental data, appear to be small (deltaS(M)(other)(o) < 0.1 deltaS(M)(other)(o)) and agree well with the theoretical estimates expressed as a sum of the corresponding translational, conformational, and size contributions. These deltaS(M)(other)(o) contributions are much less unfavorable than those estimated for a rigid-body association, which indicates the dynamic nature of the C(8)E(gamma) micellar aggregates. the dynamic nature of the C8EY micellar aggregates.