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Biomedical subjects

H Kunz

Publications and source records attributed to H Kunz.

At least 19 recordsLinked to original sources
PubMed2003

Mechanical model of normal and anomalous diffusion.

The overdamped dynamics of a charged particle driven by an uniform electric field through a random sequence of scatterers in one dimension is investigated. Analytic expressions of the mean velocity and of the velocity power spectrum are presented. These show that above a threshold value of the field normal diffusion is superimposed to ballistic motion. The diffusion constant can be given explicitly. At the threshold field, the transition between conduction and localization is accompanied by an anomalous diffusion. Our results exemplify that, even in the absence of time-dependent stochastic forces, a purely mechanical model equipped with a quenched disorder can exhibit normal as well as anomalous diffusion, the latter emerging as a critical property. Via another interpretation, as the motion of a particle on an inclined rough surface, our results are relevant for the problem of segregation by flow.

Journal Article↗
PubMed2001

Application of Ugi reactions in synthesis of divalent neoglycoconjugates: evidence that the sugars are presented in restricted conformation.

[reaction: see text]. The Ugi reaction has been used to prepare divalent galactose derivatives. NMR analysis shows that a divalent neoglycoconjugate, where the glycopeptides are bridged by a terephthaloyl group, is an 83:17 mixture of two conformers; the amide groups of the major isomer have E-anti conformations. The spatial relationship and the relative orientation of the sugars are restricted, which may have consequences for the recognition of this and related structures in biological systems.

Carbohydrate Conformation↗
PubMed1999

Modeling circadian rhythm generation in the suprachiasmatic nucleus with locally coupled self-sustained oscillators: phase shifts and phase response curves.

Circadian rhythm generation in the suprachiasmatic nucleus was modeled by locally coupled self-sustained oscillators. The model is composed of 10,000 oscillators, arranged in a square array. Coupling between oscillators and standard deviation of (randomly determined) intrinsic oscillator periods were varied. A stable overall rhythm emerged. The model behavior was investigated for phase shifts of a 24-h zeitgeber cycle. Prolongation of either the dark or the light phase resulted in a lengthening of the period, whereas shortening of the dark or the light phase shortened the period. The model's response to shifts in the light-dark cycle was dependent only on the extent of the shift and was insensitive to changes in parameters. Phase response curves (PRC) and amplitude response curves were determined for single and triple 5-h light pulses (1000 lux). Single pulses lead to type 1 PRCs with larger phase shifts for weak coupling. Triple pulses generally evoked type 1 PRCs with the exception of weak coupling, where a type 0 PRC was observed.

Algorithms↗
PubMed1998

Effects of glycosylation on fragments of tumour associated human epithelial mucin MUC1.

The glycodecapeptide AcPAPGS(alpha GalNAc)T(alpha GalNAc)APPA and the C-terminal glycohexapeptide AcS(alpha GalNAc)T(alpha GalNAc)APPA have been synthesized by applying the N-terminal Fmoc group in combination with the heptyl ester cleavable by lipase-catalyzed hydrolysis at pH 7. The solution conformation of these MUC1-related synthetic glycopeptides and the control, non-glycosylated decapeptide AcPAPGSTAPPA have been investigated using NMR spectroscopy. The structural studies indicate that the glycohexapeptide has a folded structure in solution. For this molecule, unrestrained molecular dynamics has been used to confirm the presence of the observed solution through-space connections. The results indicate that the non-globular nature of MUC1 is due to both protein core sequence and the effect of carbohydrate.

Amino Acid Sequence↗
PubMed1998

Synthesis of cyclic dipeptide templates, their incorporation into peptides and studies on their conformational and biological properties.

This study investigated the diastereoselective synthesis of three dipeptide templates 1, 2 and 3, which may be regarded as conformationally restricted analogs of H-Gly-Xaa-OH, in which Xaa constitutes an aromatic amino acid. Bond formation between alpha-C of Gly and the aromatic moiety was achieved by proton-catalyzed intramolecular electrophilic aromatic substitution. The absolute configuration of the dipeptide templates was determined by single-crystal X-ray crystallography or by nuclear Overhauser enhancement measurements. A protective group strategy was elaborated to allow their incorporation into peptide sequences by liquid phase as well as by solid-phase peptide synthesis. The templates were used to generate an enkephalin analog 15, a modified peptidic neurokinin antagonist 20 and two dermorphin derivatives (24 and 33). Molecular dynamic simulations with 15 and 20 revealed the preference for a turn-like motif for 15. The biological activity, as investigated by respective receptor binding and functional assays, was strongly diminished with all four derivatives, indicating that their receptor-relevant molecular geometries lie outside the examined conformational space.

Crystallography, X-Ray↗
PubMed1997

Glycosyl azides as building blocks in convergent syntheses of oligomeric lactosamine and Lewis(x) saccharides.

Oligosaccharides containing type 2 lactosamine repeating units, e.g. neo-lacto-octaose and trimeric Lewis(x) derivatives, are constructed using neo-lactosamine azide building blocks. The azido group provides a favorable protection of the anomeric position which is stable to versatile protecting group manipulations and glycosylation reactions. On the other hand, glycosyl azides can be converted into glycosyl fluorides via a 1,3-dipolar cycloaddition with di-tert-butyl-acetylenedicarboxylate and subsequent treatment of the resulting N-glycosyl triazoles with hydrogen fluoride-pyridine complex. Activation of the lactosamine fluorides with Lewis acids affords the possibility to extend the oligosaccharide chain with disaccharide units. Suitable protecting group combinations within the galactose and the glucosamine portion of the lactosamine unit enable selective deprotection reactions and, subsequently, chain extension or branching, e.g. to yield Lewis(x) structures.

Amino Sugars↗
PubMed1995

Chemical and enzymatic synthesis of glycopeptides.

Progress recently made in the synthesis of biologically relevant N- and O-glycopeptides is illustrated by examples. In this context, developments in the preparation of complex saccharide side chains and in the subsequent coupling to peptide portions is described. Special emphasis is given to the synthesis of Lewis antigen-type structures. Furthermore, modern methods in solid phase peptide syntheses utilizing glycosylated building blocks are presented. Recent advances in glycopeptide syntheses employing enzymatic methods in deprotection steps as well as in peptide/saccharide chain elongation are reported.

Amino Acid Sequence↗
PubMed1994

Synthesis of glycopeptides and neoglycoproteins containing the fucosylated linkage region of N-glycoproteins.

N-Glycoproteins fucosylated in the core region occur in tumor membranes and virus envelopes. Partial structures of such N-glycoproteins containing fucosylated chitobiosyl asparagine conjugates were synthesized using the allyloxycarbonyl (Aloc) and the tert-butyl ester protecting groups in the peptide portion. As the alpha-fucosidic bond of the conjugates revealed to be very sensitive to acids when carrying ether-type protecting groups, a method for exchanging the protecting groups of the fucose portion of saccharides was developed. Conjugates containing O-acetyl protected fucose proved to be stable against acids used in glycopeptide syntheses. These methods were applied in the synthesis of a fucosyl chitobiose hexapeptide with the partial sequence of a leukemia virus envelope glycoprotein. The glycopeptide was coupled to bovine serum albumin yielding a neoglycoprotein which contains a glycoconjugate of exactly specified structure.

Amino Acid Sequence↗
PubMed1994

[Laboratory diagnosis of parvovirus B 19 infection: comparison of ten commercial IgG- and IgM-antibody tests].

Seven ELISAs, two westernblots and one immunofluorescence assay for serological detection of parvovirus-B19-infection were compared with regard to their sensitivity and specificity. All ten assays are commercially available and use recombinant proteins (VP1, VP2) from bacterial and eukaryotic expression systems and synthetic viral peptides as antigens. The ELISA assays comprise indirect ELISA and mu-capture systems. All assays were tested for their sensitivity with follow-up sera from 21 patients with acute parvovirus B19 infections. The specificity was analysed by 194 sera from pregnant women and from patients with acute other virus infections or rheumatoid factors, but without acute parvovirus infection. Only three ELISAs, both westernblots and the immunofluorescence assay can be recommended for the IgG-measuring (prevalence). For the detection of an acute parvovirus infection, especially during pregnancy, two ELISAs show good IgG- and IgM-results, one ELISA is less recommendable and all other assays show only a poor IgM-specificity.

Antibodies, Viral↗
PubMed1993

A new method of anomeric protection and activation based on the conversion of glycosyl azides into glycosyl fluorides.

Glycosyl azides provide reliable anomeric protection stable to conditions for hydrolytic removal of ester groups, for reductive opening or release of acetalic diol protection, for the introduction of ether-type protection, and for glycosylation processes. The utility of this anomeric protection is further enhanced as glycosyl azides may be converted into glycosyl fluorides, which can be activated for glycosylation reactions. To this end, glycosyl azides have been subjected to 1,3-dipolar cycloaddition with di-tert-butyl acetylenedicarboxylate. On treatment with hydrogen fluoride-pyridine complex the N-glycosyl triazole derivatives directly give glycosyl fluorides.

Azides↗