PubMed Health⌕ Search

Biomedical subjects

Holger Frey

Publications and source records attributed to Holger Frey.

9 recordsLinked to original sources
PubMed2006

Synthesis of multiarm star poly(glycerol)-block-poly(2-hydroxyethyl methacrylate).

Well-defined multiarm star block copolymers poly(glycerol)-b-poly(2-hydroxyethyl methacrylate) (PG-b-PHEMA) with an average of 56, 66, and 90 PHEMA arms, respectively, have been prepared by atom transfer radical polymerization (ATRP) of HEMA in methanol by a core-first strategy. The hyperbranched macroinitiators employed were prepared on the basis of well-defined hyperbranched polyglycerol by esterification with 2-bromoisobutyryl bromide. Polydispersites M(w)/M(n) of the new multiarm stars were in the range of 1.11-1.82. Unexpectedly, with the combination of CuCl/CuBr(2)/2,2'-bipyridyl as catalyst, the polymerization conversion can be driven to maximum values of 79%. The control of CuCl catalyst concentration is also very important to achieve high conversion and narrow polydispersity. The absolute M(n) values of the obtained multiarm star polymers were in good agreement with the calculated ones, and the highest M(n) values of the multiarm star copolymer is around 10(6) g/mol. Kinetic analysis shows that an induction period exists in the polymerization of HEMA. After this induction period, a linear dependence of ln ([M](0)/[M](t)()) on time was observed. Due to the star architecture, the viscosity of the obtained multiarm star PHEMA is much lower than that of linear PHEMA.

Biocompatible Materials↗
PubMed2005

Controlled crystallization of CaCO(3) on hyperbranched polyglycerol adsorbed to self-assembled monolayers.

The formation of biominerals by living organisms is governed by the cooperation of soluble and insoluble macromolecules with peculiar interfacial properties. To date, most of the studies on mineralization processes involve model systems that only account for the existence of one organic matrix and thus disregard the interaction between the soluble and insoluble organic components that is crucial for a better understanding of the processes taking place at the inorganic-organic interface. We have set up a model system composed of a matrix surface, namely, a self-assembled monolayer (SAM), and a soluble component, hyperbranched polyglycerol. The model mineral calcium carbonate displays diverse polymorphism. It could be demonstrated that the phase selection of calcium carbonate is controlled by the cooperative interaction of the SAM and hyperbranched polyglycerol of different molecular weights (M(n) = 500-6000 g/mol) adsorbed to the SAM. Our studies showed that hyperbranched polyglycerol is adsorbed to polar as well as to nonpolar SAMs. This effect can be related to its highly flexible structure and its amphiphilic character. The adsorption of hyperbranched polyglycerol to the SAMs with different surface polarities resulted in the formation of aragonite for alkyl-terminated SAMs and no phase selection for carboxylate-terminated SAMs.

Adsorption↗
PubMed2005

Soluble oligoaramide precursors--a novel class of building blocks for rod-coil architectures.

A new synthetic route is described that allows the reversible conversion of the inherently insoluble oligo-p-benzamides into soluble materials through the formation of imidoyl chlorides. Syntheses of the corresponding dimer, trimer, and tetramer are reported; these compounds can easily be purified by crystallization and are accessible on the multigram scale. Structural proof was obtained by single-crystal X-ray structures of the trimer and tetramer precursors. They can be selectively functionalized into amides or esters at the terminal carboxylic acid group followed by hydrolysis of the imidoyl chlorides to the parent amides. This new class of compounds gives access to strongly aggregating rigid rodlike materials in few synthetic steps, as is demonstrated by the preparation of poly(ethylene glycol)-co-oligo(p-benzamide) rod-coil block copolymers.

Journal Article↗
PubMed2005

Relationship between the structure of amphiphilic copolymers and their ability to disturb lipid bilayers.

Nonionic amphiphiles and particularly block copolymers of ethylene oxide and propylene oxide (Pluronics) cause pronounced chemosensitization of tumor cells that exhibit multiple resistance to antineoplastic drugs. This effect is due to inhibition of P-glycoprotein (P-gp) responsible for drug efflux. It was suggested that the inhibition of P-gp might be due to changes in its lipid surrounding. Indeed, high dependence of P-gp activity on the membrane microviscosity was demonstrated [Regev et al. (1999) Eur. J. Biochem. 259, 18-24], suggesting that the ability of Pluronics to affect the P-gp activity is mediated by their effect on the membrane structure. We have found recently that adsorption of Pluronics on lipid bilayers induced considerable disturbance of the lipid packing [Krylova et al. (2003) Chemistry 9, 3930-3936]. In the present paper, we studied 19 amphiphilic copolymers, including newly synthesized hyperbranched polyglycerols, Pluronic and Brij surfactants, for their ability to accelerate flip-flop and permeation of antitumor drug doxorubicin (DOX) in liposomes. It was found that not only bulk hydrophobicity but also the chemical microstructure of the copolymer determines its membrane disturbing ability. Copolymers containing polypropylene oxide caused higher acceleration of flip-flop and DOX permeation than polysurfactants containing aliphatic chains. The effects of copolymers containing hyperbranched polyglycerol "corona" were more pronounced, as compared to the copolymers with linear poly(ethylene oxide) chains, indicating that a bulky hydrophilic block induces additional disturbances in the lipid bilayer. A good correlation between the copolymer flippase activity and a linear combination of copolymer bulk hydrophobicity and the van der Waals volume of its hydrophobic block was found. The relationship between the structure of a copolymer and its ability to disturb lipid membranes presented in this paper may be useful for the design of novel amphiphilic copolymers capable of affecting the activity of membrane transporters in living cells.

Adsorption↗
PubMed2004

Synthesis and supramolecular association of immobilized NCN-pincer platinum(II) complexes on hyperbranched polyglycerol supports.

Pertosylation of hyperbranched polyglycerol (M(n)=2000; M(w)/M(n)=1.3) followed by partial displacement of the tosyl groups with carboxylic acid functionalized NCN-pincer platinum(II) complexes [PtI-2,6-(NMe(2)CH(2))(2)C(6)H(2)-4-COOH], resulted in covalent attachment of the NCN-pincer complexes to the polyglycerol. These functionalized hyperbranched macromolecules have been characterized by (1)H, (13)C, and (195)Pt NMR, UV-visible, and IR spectroscopy. The presence of Pt and I atoms renders them directly visible by transmission electron microscopy (TEM) without staining procedures, which offers images of associated hyperbranched macromolecules. TEM micrographs show disk-shaped structures with a small size-distribution (15-20 nm), and characteristic core-shell ring structures. The thickness of the corona observed in TEM could be correlated with the substitution degree with pincer platinum moieties.

Carbon↗
PubMed2002

Hyperbranched molecular nanocapsules: comparison of the hyperbranched architecture with the perfect linear analogue.

The results of a comparative study of hyperbranched, partially esterified polyglycerols with their linear polyglycerol analogues with respect to molecular encapsulation and phase transfer are reported. Two hyperbranched polyglycerol samples with molecular weights of 3000 and 8000 g/mol (Mw/Mn = 1.3), respectively, have been partially esterified using palmitoyl chloride. The same modification was applied to the structurally analogous linear polyglycerol (3000 g/mol). A detailed UV-vis study correlated with viscosity experiments demonstrated that only the hyperbranched core-shell structures form "nanocapsules", leading to the encapsulation of polar guest molecules. The results underline the crucial role of the hyperbranched topology and the resulting solution conformation for supramolecular guest encapsulation and phase transfer. The unusually compact ("collapsed") structure assumed by the hyperbranched core-shell amphiphiles in apolar media is responsible for the formation of a hydrophilic compartment, capable of irreversibly taking up guest molecules.

Journal Article↗
PubMed2002

Dendritic polyglycerol: a new versatile biocompatible-material.

Polyglycerol represents the first hyperbranched polymer that can be prepared in a controlled synthesis. It is characterized by the combination of a stable, biocompatible polyether scaffold, high-end group functionality and a compact, well-defined dendrimer-like architecture. These characteristics can be used to generate new materials properties and for biomedical applications to molecularly amplify or multiply effects or to create extremely high local concentrations of drugs, molecular labels, or probe moieties. Therefore, dendritic polyglycerols are expected to lead to new strategies for 'molecular medicine'. In this brief summary, the current state of the art in polyglycerol research is given, focusing on applications in life sciences.

Biocompatible Materials↗