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Biomedical subjects

M Marek

Publications and source records attributed to M Marek.

71 records · Page 4Linked to original sources
PubMed1976

In vitro corrosion of dental amalgam phases.

The effects of corrosion on the major phases of dental amalgam microstructure have been studied in vitro on samples prepared by electroplating mercury on Ag3Sn. Tests were made in synthetic saliva, and samples were examined in a scanning electron microscope before and after the exposure. Simple immersion had little effect on the phases. Contact of amalgam with gold increased the probability of deterioation of the gamma2 phase. By anodic polarization beyond the breakdown potential, the gamma2 phase was selectively dissolved. Under conditions simulating a crevice, the gamma2 phase was dissolved and the gamma1 phase suffered morphological changes.

Corrosion↗
PubMed1975

Synchronization in two interacting oscillatory systems.

Nonlinear phenomena arising from the interaction of two oscillating systems of chemical reactions are studied experimentally. The system of two connected flow-through continuous stirred tank reactors (cells) with controlled exchange of reaction mixture is used. The Belousov reaction (oxidation of malonic acid by bromate in sulphuric acid with ceric/cerous ions as catalyst) served as model system. The frequency of oscillations was controlled by change of the reaction temperature. Phenomena such as synchronization of oscillations at a common frequency, synchronization at multiples of a common frequency, rhythm splitting and amplitude amplification were observed, depending on the degree of interaction and the differences in the original oscillation frequencies. Mathematical modelling of the above phenomena failed, probably due to insufficient knowledge of a kinetic model.

Cell Physiological Phenomena↗
PubMed1975

Nonlinear phenomena in oscillatory systems of homogeneous reactions - experimental observations.

An oscillating system of homogeneous chemical reactions (oxidation of malonic acid by bromate with ceric/cerous ion system as the catalyst) was followed experimentally in several reactor configurations, operated both as close and open systems. Characteristic nonlinear phenomena observed include: damped oscillations of concentrations of reaction components in a well stirred batch reactor, undampted oscillations and sudden changes of oscillatory behaviour (jump to a new steady state) in CSTR and interaction of oscillations in the two connected CSTRs. In a tubular reactor stationary and travelling concentration waves were observed. In the system of CSTR connected with a tubular reactor spreading of waves from CSTR to the tubular reactor, and vice versa was followed. In the flow through tubular reactor the formation of non-monotomic concentration profiles was measured, which were basically stationary with respect to changing flow-rate. The course of oscillations in CSTR was simulated on the basis of a semiempirical reaction kinetics model. An attempt to simulate quantitatively the behaviour of the distributed system with the common type of Fickian diffusion balance equations failed. Difficulties in the proper description of the transport of ions through the concentration gradient and the gradient of the electrical potential found in this system are discussed.

Bromates↗
PubMedDate not supplied

Basic studies on heterobifunctional biotin-PEG conjugates with a 3-(4-pyridyldithio)propionyl marker on the second terminus.

Heterobifunctional poly(ethylene glycol) (PEG) derivatives with a biotin terminus have been synthesized and characterized with respect to avidin binding. Unambiguous measurement of biotinyl and pyridyldithiopropionyl end groups was established by selecting suitable assays and introducing necessary modifications. Functional studies on the binding of biotin-PEG conjugates to avidin tetramers revealed much similarity to known biotin-spacer-peptide conjugates with 7-27 atom spacers: dissociation kinetics of the initially formed 4:1 complexes were multiexponential, the complex with 2 ligands per avidin dissociating rather slowly with half-times of approximately 2 days at 25 degrees C. The observed stability of 3:1 and 2:1 complexes with avidin is particularly significant since it allows exploitation of the additional advantages of PEG spacers, i.e. reduced steric strain in biotin-avidin-biotin bridges, reduced nonspecific adsorption of biotinylated probes and markers, and, especially, uncomparable fluorescence intensities of biotin-PEG-fluorophore conjugates as is demonstrated in the accompanying study (second of three papers in this issue).

Avidin↗
PubMedDate not supplied

Biotin-fluorophore conjugates with poly(ethylene glycol) spacers retain intense fluorescence after binding to avidin and streptavidin.

Conventional biotin-fluorophore conjugates with approximately 14 atom spacers lose most of their fluorescence when binding to avidin or streptavidin, as is demonstrated in the present study. This explains the unusual fact that only biotinylated marker enzymes, but not fluorescent biotins, are regularly used in bioanalytic assays. Novel biotin-spacer-fluorophore conjugates are presented that retain intense fluorescence when binding to avidin or streptavidin. Preservation of fluorescence depends upon the use of poly(ethylene glycol) (PEG) spacers, which are shown not to interfere with biotin function. The observed absence of nonspecific interactions may also be attributed to the PEG chain. These novel fluorescent biotins are expected to be excellent new tools in fluorescence microscopy and related techniques.

Avidin↗
PubMedDate not supplied

Biotin-pyrene conjugates with poly(ethylene glycol) spacers are convenient fluorescent probes for avidin and streptavidin.

Conventional biotin-fluorophore conjugates with approximately 14 atom spacers are strongly quenched when bound to avidin or streptavidin, whereas fluorescence becomes insensitive to receptor binding if typical fluorophores are linked to biotin via poly(ethylene glycol) (PEG) chains (Gruber et al., see the second of three papers in this issue). In the present study the antagonism between PEG-PEG repulsion and fluorophore interaction was examined more closely, using biotin-PEG-pyrene conjugates as model compounds. The antagonistic tendencies between hydrophilic PEG chains and hydrophobic pyrene labels were about balanced in the PEG1900 derivative since quenching was approximately 50% in 4:1 complexes with avidin or streptavidin. In contrast, strong quenching and concomitant excimer fluorescence was seen with the biotin-PEG800-pyrene conjugate, providing for a new fluorescence assay to accurately measure avidin and streptavidin concentrations at > or = 40 and > or = 10 nM, respectively. Association/ dissociation kinetics were analyzed from pyrene fluorescence changes, and dissociation constants were deduced. About 3-fold affinities were observed for streptavidin as compared to avidin, and little influence of PEG chain length was seen. All affinities were increased by a factor of approximately 3 when biotin-PEG-tetramethylrhodamine conjugates were used. The observed effect of fluorophore variation upon biotin binding is unexpectedly small; thus, the kinetic/thermodynamic data appear to be representative for biotin-PEG conjugates in general.

Avidin↗
PubMedDate not supplied

New information on high copper amalgam corrosion.

Anodic polarization of "pure" Cu6Sn5 was investigated. Cu6Sn5 is found to be passive in nearly neutral synthetic saliva. Anodic corrosion activity (characteristic for tin) previously reported for a high copper amalgam must result from the presence of the gamma2 phase or tin in the matrix phase.

Chemical Phenomena↗