Improved syntheses of bis(beta-cyclodextrin) derivatives, new carriers for gadolinium complexes

In the last decade a number of reports have been published on the synthesis and characterization of bridged cyclodextrin dimers (bis-CDs) connected with linkers of different lengths and structures. These dimers, having two hydrophobic cavities in close proximity, display much higher binding affinities and molecular selectivities than parent CDs, forming stable supramolecular adducts. We describe new synthetic protocols for the preparation of bis(beta-CDs) bearing 2-2', 3-3' and 6-6' bridges. Some of the critical steps were carried out either under high-intensity ultrasound (US) or microwave (MW) irradiation. Bis(beta-CDs) containing 6-6' ureido- and thioureido-bridges were prepared in high yields by a MW-promoted aza-Wittig reaction using polymer-bound triphenylphosphine, while those containing 2,2' and 3,3' bridges were prepared from mono-alkenyl beta-CDs by the cross-metathesis reaction (homodimerization) in the presence of 2(nd)-generation Grubbs catalyst under sonochemical conditions. By these improved protocols CD dimers could be obtained in gram amounts to prepare stable adducts of bis-CDs with contrast agents (CAs) containing gadolinium(iii) chelates. In the case of Gd(iii) chleate "G-1" the inclusion complexes were found to be 2 to 3 orders of magnitude more stable than that formed by beta-CD (K(ass) = 4.3 x 10(4) M(-1)vs 8.0 x 10(2) M(-1)). Relaxivity increased as well by factors of 3 and 4, viz. from 9.1 mM(-1) s(-1) (beta-CD) to 27.7 and 35 mM(-1) s(-1).     

A Carborane‐Derivative “Click” Reaction under Heterogeneous Conditions for the Synthesis of a Promising Lipophilic MRI/GdBNCT Agent

In this study, the Huisgen reaction has been used to functionalise a carborane cage with a lipophilic moiety and a 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid (DOTA) ligand to obtain a new Gd boron neutron‐capture therapy (BNCT)/magnetic resonance imaging (MRI) agent. The introduction of the triazole units has been accomplished under both heterogeneous conditions, by the use of a Cu‐supported ionic‐liquid catalyst, and homogeneous conditions. The ability of the Gd complex of the synthesised ligand to form stable adducts with low‐density lipoproteins (LDLs) has been evaluated and then MRI has been performed on tumour melanoma cells incubated in the presence of a Gd‐complex/LDL imaging probe. It has been concluded that the high amount of intracellular boron necessary to perform BNCT can be reached even in the presence of a relatively low‐boron‐containing LDL concentration.     

How Many Are Too Many? Managing OnLine Growing Pains

The Physical Chemistry OnLine Consortium (PCOL) is dedicated to enhancing undergraduate physical chemistry by encouraging faculty and challenging students to conduct short-term (~4 weeks) Webbased, mathematically sophisticated activities. PCOL projects involve students and faculty at geographically dispersed institutions. In the fall of 2000, PCOL sponsored three very different projects involving faculty and 175 students from 16 different institutions. This paper discusses strategies for dealing with the growing pains that our successful Web-based educational endeavor has encountered.Presented at the ACS Division of Chemical Education sponsored symposium on web-assisted learning in chemistry at its 221st national meeting in San Diego, CA April 1–5, 2001.     

Recent advances in the synthesis of cyclodextrin derivatives under microwaves and power ultrasound

We describe improved syntheses of CD derivatives, like 6^I-monotosyl, 6^I-monoazido-6^I -monodeoxy, 6^I-monoamino-6^I-monodeoxy-βCD and 2^I-O-mono(methylamino)alkyl-βCDs, that were carried out under US or MW, to great advantage in terms of yield, purity and reaction time. In the search for more efficient procedures to prepare monosubstituted CDs, we applied MW and/or US to some fundamental preparations such as those of 6^I-amino-6^I-deoxy-βCD and a series of 2-monoaminomethyl-βCD derivatives.     

Cohomologie des groupes et corps d'invariants multiplicatifs tordus

We give a cohomological calculation of the unramified Brauer group of a field of invariants under a twisted multiplicative action of a finite group. Received: October 13, 1992; final version: July 3, 1996     

Microstructure and transport properties of YBa2Cu3O7−δ films produced by laser ablation from a BaF2/Y2O3/CuO target

The effects of varying the temperature and duration of the post-deposition anneal in watersaturated oxygen were investigated for YBa₂Cu₃O_7−δ films of varying thickness. The films were produced by laser ablation from pressed powder targets consisting of BaF₂,Y₂O₃, and CuO mixtures. This technique produces superconducting films with a highly textured surface. The films were fabricated on SrTiO₃ substrates and were analyzed with X-ray diffraction, scanning electron microscopy, and temperature dependent resistivity. Critical current density (J_c) measurements were performed in magnetic fields up to 1 T. For film thickness on the order of 900 nm, completely c-axis oriented films were obtained with a 60 min anneal at 850°C. Thinner films required less annealing, either shorter times or lower temperatures, to achieve similar results, indicating that the optimal annealing conditions are dependent on film thickness.     

Central sets and substitutive dynamical systems

In this paper we establish a new connection between central sets and the strong coincidence conjecture   for fixed points of irreducible primitive substitutions of Pisot type. Central sets, first introduced by Furstenberg using notions from topological dynamics, constitute a special class of subsets of N$\mathbb{N}$ possessing strong combinatorial properties: Each central set contains arbitrarily long arithmetic progressions, and solutions to all partition regular systems of homogeneous linear equations. We give an equivalent reformulation of the strong coincidence condition in terms of central sets and minimal idempotent ultrafilters in the Stone–?ech compactification βN$\beta \mathbb{N}$. This provides a new arithmetical approach to an outstanding conjecture in tiling theory, the Pisot substitution conjecture  . The results in this paper rely on interactions between different areas of mathematics, some of which had not previously been directly linked: They include the general theory of combinatorics on words, abstract numeration systems, tilings, topological dynamics and the algebraic/topological properties of Stone–?ech compactification of N$\mathbb{N}$.     

Monolithically integrated bacteriorhodopsin-GaAs field-effect transistor photoreceiver

We have applied the large photovoltage developed across a layer of selectively deposited bacteriorhodopsin to the gate terminal of a monolithically integrated GaAs-based modulation-doped field-effect transistor, which delivers an amplified photoinduced current signal. The integrated biophotoreceiver device exhibits a responsivity of 3.8 A/W. The optoelectronic integrated circuit is achieved by molecular-beam epitaxy of the field-effect transistor's heterostructure, photolithography, and selective-area bacteriorhodopsin electrodeposition.     

Physical Chemistry Online: Maximizing Your Potential

The Physical Chemistry On-Line (PCOL) consortium has developed and conducted a series of short-term projects for use in the physical chemistry curriculum. The projects involve faculty and students from geographically dispersed institutions, are short in duration (~4–6 weeks), and use email and the World Wide Web for communication and information distribution. They are designed to enhance physical chemistry at colleges and universities that may have limited resources available for physical chemistry by offering an alternate pedagogical approach. This paper will highlight the motivations of the participants, outline the specific projects used to date, and provide some evaluation of the pedagogical effectiveness of the approach.     

Direct NMR Spectroscopic Observation of a Lanthanide-Coordinated Water Molecule whose Exchange Rate Is Dependent on the Conformation of the Complexes

A large difference in the exchange rate of the coordinated water molecule is seen in solution for the diastereoisomers M and m of a europium(III ) complex with an octadentate, neutral macrocyclic ligand (k_ex( M )<k_ex( m )). This finding is of high relevance to the design of more efficient contrast agents for magnetic resonance imaging.