Georg Schmeer

Biodegradable poly(d,l-lactic acid)-poly(ethylene glycol)-monomethyl ether diblock copolymers: structures and surface properties relevant to their use as biomaterials

To obtain biodegradable polymers with variable surface properties for tissue culture applications, poly(ethylene glycol) blocks were attached to poly(lactic acid) blocks in a variety of combinations. The resulting poly(D,L-lactic acid)-poly(ethylene glycol)-monomethyl ether (Me.PEG-PLA) diblock copolymers were subject to comprehensive investigations concerning their bulk microstructure and surface properties to evaluate their suitability for drug delivery applications as well as for the manufacture of scaffolds in tissue engineering. Results obtained from 1H-NMR, gel permeation chromatography, wide angle X-ray diffraction and modulated differential scanning calorimetry revealed that the polymer bulk microstructure contains poly(ethylene glycol)-monomethyl ether (Me.PEG) domains segregated from poly(D,L-lactic acid) (PLA) domains varying with the composition of the diblock copolymers. Analysis of the surface of polymer films with atomic force microscopy and X-ray photoelectron spectroscopy indicated that there is a variable amount of Me.PEG chains present on the polymer surface, depending on the polymer composition. It could be shown that the presence of Me.PEG chains in the polymer surface had a suppressive effect on the adsorption of two model peptides (salmon calcitonin and human atrial natriuretic peptide). The possibility to modify polymer bulk microstructure as well as surface properties by variation of the copolymer composition is a prerequisite for their efficient use in the fields of drug delivery and tissue engineering.

Non-aqueous electrolyte solutions in chemistry and modern technology

In this paper a brief survey is given of the properties of non-aqueous electrolyte solutions and their applications in chemistry and technology without going into the details of theory. Specific solvent-solute interactions and the role of the solvent beyond its function as a homogenous isotropic medium are stressed. Taking into account Parkers statement1) “Scientists nowadays are under increasing pressure to consider the relevance of their research, and rightly so” we have included examples showing the increasing industrial interest in non-aqueous electrolyte solutions.

Does UV irradiation affect polymer properties relevant to tissue engineering

For most tissue engineering approaches aiming at the repair or generation of living tissues the interaction of cells and polymeric biomaterials is of paramount importance. Prior to contact with cells or tissues, biomaterials have to be sterilized. However, many sterilization procedures such as steam autoclave or heat sterilization are known to strongly affect polymer properties. UV irradiation is used as an alternative sterilization method in many tissue engineering laboratories on a routine basis, however, potential alterations of polymer properties have not been extensively considered. In this study we investigated the effects of UV irradiation on spin-cast films made from biodegradable poly(d,l-lactic acid)–poly(ethylene glycol)–monomethyl ether diblock copolymers (Me.PEG–PLA) which have recently been developed for controlled cell-biomaterial interaction. After 2 h of UV irradiation, which is sufficient for sterilization, no alterations in cell adhesion to polymer films were detected, as demonstrated with 3T3-L1 preadipocytes. This correlated with unchanged film topography and molecular weight distribution. However, extended UV irradiation for 5–24 h elicited drastic responses regarding Me.PEG–PLA polymer properties and interactions with biological elements: Large increases in unspecific protein adsorption and subsequent cell adhesion were observed. Changes in polymer surface properties could be correlated with the observed alterations in cell/protein–polymer interactions. Atomic force microscopy analysis of polymer films revealed a marked “smoothing” of the polymer surface after UV irradiation. Investigations using GPC, 1H-NMR, mass spectrometry, and a PEG-specific colorimetric assay demonstrated that polymer film composition was time-dependently affected by exposure to UV irradiation, i.e., that large amounts of PEG were lost from the copolymer surface. The data indicate that sterilization using UV irradiation for 2 h is an appropriate technique for the recently synthesized Me.PEG–PLA diblock copolymers. However, the study also serves as an example that it is indispensable to control the duration of exposure to UV irradiation for a given biomaterial in order not to compromise polymer properties relevant to tissue engineering purposes.

Conductance of electrolytes in 1-propanol solutions from −40 to 25°C

Conductance data for solutions of LiCl, NaBr, NaI, KI, KSCN, RbI, Et4NI, Pr4NI, Bu4NI, Bu4NClO4, n-Am4NI, i-Am4NI, n-Hept4NI, Me2Bu2NI, MeBu3NI, EtBu3NI, i-Am3BuNI, and i-Am3BuNBPh4 in 1-propanol at −40, −30, −20, −10, 0, 10, and 25°C are communicated and discussed. Evaluation of the data is performed on the basis of a conductance equation that includes a term in c3/2. Single ion conductances at 25 and 10°C are determined with the help of transference numbers to+ (KSCN/PrOH); the data are compared to data estimated by other methods. Ion-pair association constants and their temperature dependence are discussed in terms of contact and solvent separated ion pairs, and the role of non-coulombic forces is shown with the help of an appropriate splitting of the Gibbs energy of ion-pair formation.

A quantum chemical approach to the water assisted neutral hydrolysis of ethyl acetate and its derivatives

A quantum chemical approach to the neutral water assisted hydrolysis of ethyl acetate and its halogenated derivatives in the gas phase is presented. Calculations of a cluster containing one ester and two water molecules were accomplished with the progam Gaussian 94 at the semi-empirical AM1 and the abinitio RHF/3-21G and RHF/6-31+G(d) computational levels. For each method the Gibbs energies of several key structures along the reaction path were calculated after full geometry optimization. For the proposed first reaction step a transition state with tetrahedral structure at the carbonyl carbon of the ester was found, thus corroborating experimental studies. The computed Gibbs energy of activation of the first reaction reveals a similar influence of polar substituents on this reaction as found by experiments in the liquid phase.

The influence of hydrophobic solvation on the alkaline hydrolysis of ethyl esters of polar substituted 2-methylpropionic acids in water

The rate constants of the alkaline hydrolysis of 2-methylpropionic acid ethyl ester, α-hydroxy, α-bromo, α-cyano, and α-nitro-2-methylpropionic acid ethyl ester have been measured in water between 5°C and 45°C with the help of an appropriate conductance bridge developed in our laboratory. The influence of the polar substituents on the rate constant, a powerful probe for discussing two-particle interactions in solutions, is characterized by an intramolecular structure parameter of the activated complex and a dielectric parameter of its surroundings. The dependence of the reaction rate on the structure parameter is compared to that of substituted acetic acid derivatives. The smaller dielectric parameter of 2-methylpropionic acid esters reveals the increasing hydrophobic solvation in comparison to acetic acid derivatives.

Electrical conductivity of reverse micelles in supercritical carbon dioxide

The cond. of water/ClPFPE-NH4/scCO2 systems is studied over a wide range of pressures and droplet vol. fractions at 35° and at 2 const. water-to-surfactant molar ratios, W0 = 10.3 and 18.7, resp. The results are interpreted according to 3 different models for low, intermediate, and high concns. of water and surfactant. Information is obtained about a possible bulk-like behavior of water, about the approx. size of the micelles, about micellar percolation, and about changes of CO2-water droplet interactions with pressure.

Hydration of molecular anions with oxygen sites: a Monte Carlo study

Abstract Equilibrium properties of molecular model water (in pure phase and in electrolyte solutions) are calculated with the help of spatial pair correlation functions, starting from classical molecular pair interactions. Simulations were performed for a new simple water model derived from combined quantum-mechanical and statistical calculations in a SCF/SSOZ scheme. Ion hydration of molecular anions with oxygen sites such as nitrate and perchlorate ions is of special interest when compared with hydration structures of spherical atomic anions like the halide ions. The water structure around the different solutes, and their influence on solution properties, is discussed.

Investigations on Substituent and Solvent Effects of Solvolysis Reactions. VIII. The Influence of Water and Nonaqueous Solvents on the Imidazolysis of 4-Nitrophenyl Acetate

The aminolysis reaction of 4-nitrophenyl acetate with imidazole was investigated in water, acetonitrile, propylene carbonate, and 1,4-dioxane at different temperatures. the observed rates can be evaluated with the help of a reaction mechanism consisting of two competitive reaction paths; the first is the bimolecular nucleophilic substitution of the phenol moiety by imidazole; the second path is performed via general base catalysis by a second imidazole molecule. The rate constants of the bimolecular reaction are quantitatively correlated with the four-parameter Taft–Kamlet equation for solvent effects on chemical reactions. A qualitative interpretation of the data shows that the two reaction paths are influenced to the same extent by the solvent.

Untersuchungen zum Substituenten- und Lösungsmitteleinfluß auf Solvolysereaktionen

The influence of polar substituents on the hydrolysis of acetic acid esters is in perfect agreement with the predictions of an electrostatic model of the activated complex both for alkaline and neutral hydrolysis. Acid hydrolysis involves the counteraction of two polar effects and is not accessible to this model. In agreement with this result, the steric parameter Es of Tafts linear energy relationship is not helpful for the discussion of steric effects of polar substituents in ester hydrolysis reactions.