Complexation behavior of alpha-, beta-, and gamma-cyclodextrin in modulating and constructing polymer networks

A systematic study of the host-guest complexation by alpha-, beta-, and gamma-cyclodextrin (CD) in either the free state or as substituents of poly(acrylic acid) (PAA) with the hydrophobic n-octadecyl groups, C18, substituted onto PAA (HMPAA) and its effect on polymer aggregation and network formation is reported. Free alpha-CD, beta-CD, and gamma-CD mask hydrophobic associations between the C18 substituent of HMPAA in aqueous solution and form host-guest complexes with a 1:1 or CD:C18 substituent stoichiometry at 0.5 wt % polymer concentration. For alpha-CD this host-guest stoichiometry changes to 2:1 or 2alpha-CD:C18 at > or =1 wt % polymer concentrations but not for beta-CD and gamma-CD. Shear-thickening occurs when gamma-CD complexes C18 HMPAA substituents. Upon addition of sodium dodecyl sulfate, SDS (SDS:CD = 1:1), the hydrophobic associations between C18 diminished by alpha-CD masking were fully restored, were only partly restored in the case of beta-CD, and not restored for gamma-CD. When alpha- and beta-CD substituted PAA (alpha-CDPAA and beta-CDPAA) were mixed with HMPAA polymer, networks formed. As for free beta-CD, the beta-CD substituents of beta-CDPAA also formed 1:1 or beta-CD:C18 stoichiometry host-guest complexes with the C18 substituents of HMPAA. The alpha-CD substituents of alpha-CDPAA also formed 1:1 or alpha-CD:C18 stoichiometry host-guest complexes with some indication of the formation of 2:1 or 2alpha-CD:C18 stoichiometry host-guest complexes at polymer concentrations > or =1 wt %. The polymer networks formed by beta-CDPAA with HMPAA are less viscous than those formed by alpha-CDPAA, for which shear-thickening occurs at polymer concentrations > or =2 wt %. It is evident that the difference in CD annular size and its match with the C18 of HMPAA control the diversity of the interactions of alpha-CD, beta-CD, gamma-CD, alpha-CDPAA, and beta-CDPAA with HMPAA.     

Kinetic characterization of an extremely slow DNA binding equilibrium

We here exploit the recently reported thermodynamic preference for poly(dAdT)(2) over mixed-sequence calf thymus (ct) DNA of two binuclear ruthenium complexes, DeltaDelta-[mu-bidppz(bipy)4Ru2](4+) (B) and DeltaDelta-[mu-bidppz(phen)(4)Ru(2)](4+) (P), that bind to DNA by threading intercalation, to determine their intrinsic dissociation rates. After adding poly(dAdT)(2) as a sequestering agent to B or P bound to ct-DNA, the observed rate of change in luminescence upon binding to the polynucleotide reflects the rate of dissociation from the mixed sequence. The activation parameters for the threading and dissociation rate constants allow us for the first time to characterize the thermodynamics of the exceedingly slow threading intercalation equilibrium of B and P with ct-DNA. The equilibrium is found to be endothermic by 33 and 76 kJ/mol, respectively, and the largest part of the enthalpy difference between the complexes originates from the forward threading step. At physiological temperature (37 degrees C) B and P have dissociation half-lives of 18 and 38 h, respectively. This is to our knowledge the slowest dissociating noncovalently bound DNA-drug reported. SDS sequestration is the traditional method for determination of rate constants for cationic drugs dissociating from DNA. However, the rates may be severely overestimated for slowly dissociating molecules due to unwanted catalysis by the SDS monomers and micelles. Having determined the intrinsic dissociation rates with poly(dAdT)(2) as sequestering agent, we find that the catalytic effect of SDS on the dissociation rate may be up to a factor of 60, and that the catalysis is entropy driven. A simple kinetic model for the SDS concentration dependence of the apparent dissociation rate suggests an intermediate that involves both micelles and DNA-threaded complex.     

Characterization of PDMS-modified glass from cast-and-peel fabrication

In glass/poly(dimethylsiloxane) (PDMS) hybrid microfluidic chips, two different fabrication approaches are used: photolithographic or solid ink molds, or cast-and-peel methods. In the latter, a thin slab of PDMS is laid down and fluid channels are cut manually or by machine. The cast-and-peel approach has been used successfully for low-shear culture devices, among other applications. The main drawback, not reported to date, of cast-and-peel methods is that removal of PDMS (exposing the glass substrate) results in nanoscopic domains of PDMS still attached to the surface. This residual PDMS is not observable by eye, but affects the hydrophobicity of the device. Using contact angle measurement, atomic force and fluorescence microscopy, the changes in glass surfaces from the cast-and-peel technique were elucidated. This study demonstrates the enhanced protein (NeutrAvidin) adsorption on PDMS treated glass surfaces, and the potential influence of altered glass properties on microfluidic applications has been discussed as well.     

Multiple phases in the system MgF2---Nb2O5 an electron microscope study of intergrowths, defects, and disordered crystals

Mixtures of MgF₂·xNb₂O₅ with x = 7 and 14 have, on annealing at high temperatures yielded a wide variety of block structures in which MgF₂ has replaced MeO₂ (Me = Ti⁴⁺, Nb⁴⁺). With F⁻ stabilizing the formation of large blocks, numerous defect structures have been characterized by direct lattice resolution electron microscopy. Features thus observed include planar faults, intergrowths, segregated domain structures, and disordered crystals. Chemical implications of these features are discussed.     

The synthesis of prostaglandin E1 and related substances

The authors describe in detail the conversion of 4 isometric exo-substituted bicyclo (3.1.0) hexanones 1a, 1b, 2a and 2b (R=CH3) to 4 prostaglandins of the E series, including crystalline dl-prostaglandin E1 (PGE1). Oxidative solvolysis of the keto esters 1a and 2a and of the desired keto acids 1a and 2a (R=H) according to the method of Just and Simonovitch resulted in a good yield of the mixture of vic-glycols of unrearranged carbon skeleton. The presence of dl-PGE1, or dl-8-iso PGE1 or their methyl esters was not detected in either case. Analysis of the unalkylated ketones 5a and 5b under the same and other conditions yielded unrearranged vic-glycols 6 as essentially the only product. A reaction sequence produced dl-8-isoprostaglandin E1, the 15-epimers of these prostaglandins, and dl-PGA1 and dl-PGB1 methyl esters.     

Enantioselective luminescence quenching of DNA light-switch [Ru(phen)2dppz]2+ by electron transfer to structural homologue [Ru(phendione)2dppz]2+

The quenching of the luminescence of [Ru(phen)(2)dppz](2+) by structural homologue [Ru(phendione)(2)dppz](2+), when both complexes are bound to DNA, has been studied for all four combinations of Delta and Lambda enantiomers. Flow linear dichroism spectroscopy (LD) indicates similar binding geometries for all the four compounds, with the dppz ligand fully intercalated between the DNA base pairs. A difference in the LD spectrum observed for the lowest-energy MLCT transition suggests that a transition, potentially related to the final localization of the excited electron to the dppz ligand in [Ru(phen)(2)dppz](2+), is overlaid by an orthogonally polarized transition in [Ru(phendione)(2)dppz](2+). This would be consistent with a low-lying LUMO of the phendione moiety of [Ru(phendione)(2)dppz](2+) that can accept the excited electron from [Ru(phen)(2)dppz](2+), thereby quenching the emission of the latter. The lifetime of excited Delta-[Ru(phen)(2)dppz](2+) is decreased moderately, from 664 to 427 ns, when bound simultaneously with the phendione complex to DNA. The 108 ns lifetime of opposite enantiomer, Lambda-[Ru(phen)(2)dppz](2+), is only shortened to 94 ns. These results are consistent with an average rate constant for electron transfer of approximately 1.10(6) s(-1) between the phenanthroline- and phendione-ruthenium complexes. At binding ratios close to saturation of DNA, the total emission of the two enantiomers is lowered equally much, but for the Lambda enantiomer, this is not paralleled by a decrease in luminescence lifetime. A binding isotherm simulation based on a generalized McGhee-von Hippel approach shows that the Delta enantiomer binds approximately 3 times stronger to DNA both for [Ru(phendione)(2)dppz](2+) and [Ru(phen)(2)dppz](2+). This explains the similar decrease in total emission, without the parallel decrease in lifetime for the Lambda enantiomer. The simulation also does not indicate any significant binding cooperativity, in contrast to the case when Delta-[Rh(phi)(2)bipy](3+) is used as quencher. The very slow electron transfer from [Ru(phen)(2)dppz](2+) to [Ru(phendione)(2)dppz](2+), compared to the case when [Rh(phi)(2)phen](3+) is the acceptor, can be explained by a much smaller driving free-energy difference.