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Calix[6]arene hexacarboxylic acid binds instantly and with low symmetry to Pb, Sr and Ba. Later a highly symmetric up-down alternating conformation emerges. The solution structures are identical to their p-tert-butylcalix[6]arene hexacarboxylic acid counterparts. With either receptor, an octahedral cage is formed around the metal. The transformation from low to high symmetry however proceeds at significantly faster rates for the de-t-butylated host. [ABSTRACT FROM AUTHOR]

The objective of this study is to transport and deliver resveratrol to intracellular sites using apolipoprotein E3 (apoE3). Reconstituted high-density lipoprotein (rHDL) bearing resveratrol (rHDL/res) was prepared using phospholipids and the low-density lipoprotein receptor (LDLr)-binding domain of apoE3. Biophysical characterization revealed that resveratrol was partitioned into the phospholipid bilayer of discoidal rHDL/res particles (~19 nm diameter). Co-immunoprecipitation studies indicated that the LDLr-binding ability of apoE3 was retained. Cellular uptake of resveratrol to intracellular sites was evaluated in glioblastoma A-172 cells by direct fluorescence using chemically synthesized NBD-labeled resveratrol (res/NBD) embedded in rHDL/res. Competition and inhibition studies indicate that the uptake is by receptor mediated endocytosis via the LDLr, with co-localization of apoE3 and res/NBD in late endosomes/lysosomes. We propose that rHDL provides an ideal hydrophobic milieu to sequester resveratrol and that rHDL containing apoE3 serves as an effective “nanovehicle” to transport and deliver resveratrol to targeted intracellular sites. [ABSTRACT FROM AUTHOR]

Lower rim carboxylic acid calix[n]arenes and upper rim phosphonic acid functionalized calix[4]arenes effect selective transport of distinct molecular payloads through a liquid membrane. The secret to this success lies in the attachment of a receptor-complementary handle. We find that the trimethylammonium ethylene group present in choline is a general handle for the transport of drug and drug-like species. Furthermore, neurotransmitters possessing ionizable amine termini are also transported. Some limitations to this strategy have been uncovered as payloads become increasingly lipophilic. These developments reveal new approaches to synthetic receptor-mediated selective small molecule transport in vesicular and cellular systems. [ABSTRACT FROM AUTHOR]

p-tert-Butylcalixarene hexacarboxylic acid initially binds with low symmetry, to later adopt a highly symmetric up-down alternating conformation in the presence of Pb, Sr or Ba. The conformational dynamics for the three ions are distinct, from 15 hours, to 20 days, to 38 days, respectively. [ABSTRACT FROM AUTHOR]

Inan effort to develop sustainable materials and methods for therecovery and recycling of precious metals, we recently developed twodifferent adsorbents by cross-linking pure cellulose with either epichlorohydrinor concentrated sulfuric acid and then modifying the surface with N-aminoguanidine functional groups. The adsorption behaviorof these sorption materials toward Au(III) from a multicomponent mixtureof Pt(IV), Pd(II), and some base-metal ions was studied in hydrochloricacid media. Both of the adsorbents exhibited outstanding selectivitytoward precious metals, with a selectivity order of Au(III) ≫Pd(II) > Pt(IV), over base metals in a wide range of acid concentrations.These materials contain a number of positive centers in HCl mediathat function as sorption active sites for chloroanionic species ofAu(III), Pd(II), and Pt(IV). The chloroanionic species of the correspondingprecious metals were thus adsorbed on these materials through anion-exchange-coupledelectrostatic interaction. The sulfuric acid cross-linked materialexhibited improved selectivity and greater adsorption capacity comparedto the epichlorohydrin cross-linked adsorbent. As equilibrium wasachieved within an hour with quantitative adsorption, the effectivenessof the sulfuric acid cross-linked material with regard to Au(III)was exemplified by the fact that 1 kg of the dry adsorbent materialhad a capacity to load 9.2 mol, that is, nearly 1.8 kg of Au(III).Moreover, the adsorbed Au(III) was subsequently reduced to the elementalform, yielding metallic gold particles, thereby demonstrating theconsiderable improvements in efficiency and effectiveness of the noveladsorbents for the recovery of gold in comparison to current commercialresins. [ABSTRACT FROM AUTHOR]

A series of supramolecular calixarenes efficiently transport distinct molecular species through a liquid membrane when attached to a receptor-complementary choline handle. Calix[6]arene hexacarboxylic acid was highly effective at transporting different target molecules against a pH gradient. Both carboxylic- and phosphonic-acid-functionalized calix[4]arenes effect transport without requiring a pH or ion gradient. NMR binding studies, two-phase solvent extraction, and three-phase transport experiments reveal the necessary and subtle parameters to effect the transport of molecules attached to a choline 'handle'. On the other hand, rescorin[4]arene cavitands, which have similar guest recognition profiles, did not transport guest molecules. These developments reveal new approaches towards attempting synthetic-receptor-mediated selective small-molecule transport in vesicular and cellular systems. [ABSTRACT FROM AUTHOR]

Studies on extraction equilibrium constants at different temperatures and thermodynamic parameters of solvent extraction of Pb2+ion with carboxylic acid derivatives of different ring size calixarenes and structure related monomeric compound have been carried out. The extraction equilibrium constants corresponding to calix[n]arene (n = 4, 5, 6) derivatives decrease in the order [5]arene > [6]arene > [4]arene. In all cases, the complexation process is primarily enthalpy driven. The favorable enthalpic contribution for extraction of Pb2+is in the order hexamer ≈ monomer > tetramer > pentamer. However, the unfavorable entropic loss follows the order: monomer > hexamer > tetramer > pentamer. Overall stability of the host-guest complex is the function of entropy-enthalpy compensation and the free energy of complexation is minimum for the pentamer, followed by tetramer ≈ hexamer and monomer. Although the carboxylic acid derivative of calix[4]arene is more preorganized than the calix[5]arene derivative, extraction of Pb2+ion with the tetramer passes through greater entropic loss than that with the pentamer and the degree of preorganization of calix[4]arene derivative is far from perfect for the complexation and extraction of Pb2+ion. As compared to tetrameric and hexameric counterparts, the structural features of the carboxylic acid derivative of calix[5]arene prior to complexation contribute much to interact with the Pb2+ion and form a thermodynamically stable complex. Supplementary materials are available for this article. Go to the publisher's online edition ofSolvent Extraction and Ion Exchangeto view the supplemental file. [ABSTRACT FROM AUTHOR]

A tannin-based novel adsorbent, named “BTU-PTgel”,was prepared by immobilizing bisthiourea (BTU) ligand on persimmontannin (PT) extract. The adsorption behaviors of precious metal ionsalong with other coexisting base metal ions onto BTU-PT gel were studiedby batch and continuous column methods. The gel exhibited remarkableselectivity for precious metal ions such as Au(III), Pd(II), and Pt(IV)over base metal ions such as Cu(II), Fe(III), Ni(II), and Zn(II) in1–5 mol dm–3hydrochloric acid. The adsorptionof precious metal ions on the present gel was found to obey the typicalmonolayer type of Langmuir model, and the maximum adsorption capacityof the adsorbent was evaluated as 5.18 mol kg–1forAu(III), 1.80 mol kg–1for Pd(II), and 0.67 molkg–1for Pt(IV). Combination of ion exchange, electrostaticinteraction, and coordination through the thiocarbonyl group is themechanism of adsorption of precious metals on BTU-PT gel. In the caseof Au(III) adsorption, the adsorbed species was simultaneously reducedto elemental gold by abundant polyphenolic groups of the tannin matrix.Elution by using acidothiourea solution in continuous column experimentrecovered the adsorbed precious metals almost quantitatively. Theresults of a potential reusability test of the gel for consecutiveadsorption and elution cycles by continuous column experiment indicatedthat the gel was stable and regenerated with undiminished metal uptakecapacity up to five cycles. The real time applicability of the adsorbentfor the recovery of precious metals from real industrial liquor wasevaluated from actual acidic leach liquor of printed circuit boardsof spent mobile phones. The gel selectively adsorbed precious metalspecies but exhibited negligible affinity toward base metals presentin the leach liquor. The BTU-PT gel is a potential adsorbent for selectiverecovery of precious metals from acidic leachate of spent mobile phonescontaining elevated concentrations of base metals. [ABSTRACT FROM AUTHOR]

Effective recovery of gold from aqueous acidic chloride media was achieved with different kinds of polysaccharides with different structures by treating with concentrated sulfuric acid. The gels were found to be highly selective for Au(iii) over other precious and base metals at varying hydrochloric acid solution. The adsorption isotherms of Au(iii) followed the Langmuir type of adsorption and the maximum adsorption capacities for Au(iii) was evaluated as high as 7.57, 7.20, 5.64 and 4.80 mmol g−1 for the gels of cellulose, dextran, alginic acid and pectic acid, respectively. Analysis of kinetic data shows that the uptake of gold(iii) on celulose gel at the initial stage follows pseudo-first-order kinetics. Crystalline structure of the cellulose before and after the treatment with concentrated sulfuric acid was measured by XRD analysis. Furthermore, Au(iii) was found to be reduced to elemental form by all types of polysaccharides gels, which was confirmed by the formation of clearly visible elemental gold particles and by means of the XRD-spectra of the adsorbents after adsorption. Infrared spectrum studies provided supporting evidence for the reduction of Au(iii) to Au(0) with a suitable mechanism of Au(iii) adsorption followed by reduction using various polysaccharide gels. [ABSTRACT FROM AUTHOR]

Persimmon tannin was chemically modified to prepare a quaternary amine type of adsorption gel, named as quaternary amine modified persimmon tannin (QAPT) gel. The QAPT gel has been used to investigate the adsorption behaviors for Au(III), Pd(II), and Pt(IV) from HCl media. It was found that the gel exhibited good selectivity towards precious metals over a wide concentration range of HCl. However, it exhibited poor affinity towards base metals such as Cu(II), Fe(III), Ni(II), and Zn(II). The adsorption isotherms of the gel for precious metal ions were described by the Langmuir model. The maximum adsorption capacities for Au(III), Pd(II), and Pt(IV) were evaluated as 4.16, 0.84, and 0.52 mmol g−1, respectively. Although the anion exchange is the main mechanism for the adsorption of anionic species of Au(III), Pt(IV), and Pd(II), adsorption of Au(III) is followed by subsequent reduction, which results in the extraordinary high adsorption capacity for Au(III). Adsorption behavior of QATP gel for Au(III) was also compared to that of the persimmon tannin, the feed material. [ABSTRACT FROM PUBLISHER]

Abstract: A calix[5]arene-based solvent extraction reagent displaying very high selectivity for lead has been prepared, one molecule of which extracts two lead ions in a stepwise manner. The first Pb2+ ion is bound to five phenoxy oxygen atoms inside the calixarene cavity due to which the ligand undergoes conformational freezing in stable cone conformer. This causes positive allosteric effect for co-extraction of the second lead ion due to the aggregation of functional groups. [Copyright &y& Elsevier]

p-tert-Butylcalix[6]arene hexaacetic acid is in a symmetric cone conformation in CHCl3, but it becomes conformationally flexible in CHCl3/CH3CN (1 : 1). In this mixture the host has a strong binding affinity towards Pb(ii) and instantly forms a complex of low symmetry – shortly thereafter structural reorganization occurs resulting in a high symmetry complex of Pb(ii) in an octahedral cage of carboxylates. Sr(ii) and Ba(ii) display similar behavior over a longer period of time. [ABSTRACT FROM AUTHOR]