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Biomedical subjects

D W Urry

Publications and source records attributed to D W Urry.

At least 109 records · Page 6Linked to original sources
PubMed1983

Synthesis of polypeptide models of elastin. Synthesis and properties of a cross-linked polytetrapeptide.

Synthesis of two copolymers, H-(phi-Pro-Gly-Gly)n-Val-OMe and H-(phi'-Pro-Gly-Gly)n-Val-OMe, where phi is Val or Lys and phi' is Val or Glu is described. Cross-linking between the two copolymers is achieved by a coupling reaction between the epsilon-amino groups of the lysine containing copolymer and delta-carboxyls of the glutamic acid of the other copolymer. The cross-linking reaction was performed during a temperature elicited phase separation with flow orientation of the copolymers. Coacervation of the intermediate polymers is presented, as is the scanning electron micrograph of the insoluble cross-linked product and its calcifiability as determined with electron probe microanalysis. The purity of the key intermediates and polymers is demonstrated by the usual analytical methods. Carbon-13 magnetic resonance spectra of the intermediate monomer units are included to validate the purity of their synthesis.

Cross-Linking Reagents↗
PubMed1982

Intermolecular interactions of gramicidin A' transmembrane channels incorporated into lysophosphatidylcholine lipid systems.

Fluorescence studies are reported on gramicidin A' incorporated into lysophosphatidylcholine phospholipid structures. The shift in the emission maximum during incorporation and the quenching of fluorescence by I- and by acrylamide of the incorporated state obtained after prolonged heating are consistent with the presence of the channel state comprised of two single-stranded beta 6 -helices associated head-to-head (formyl end-to-formyl end). The quantum yield for the incorporated state, when gramicidin A' is within the lipid matrix, is very low and indicates the occurrence of intermolecular Trp-Trp interactions. Possible interactions between channels within the lipid matrix are discussed utilizing Trp-Trp contacts.

Gramicidin↗
PubMed1982

On the relative lipid membrane permeability of Na+ and Ca2+. A physical basis for the messenger role of Ca2+.

Calcium ion is demonstrated to bind in the gramicidin A transmembrane channel at a site that is displaced outward about 1.5 A from that of the sodium ion. The calcium ion is also shown to exchange rapidly with the solution such that the impermeability of the channel to calcium ion is due to the proximity of the lipid which limits solvation energy. Also at its binding site, calcium is shown to be able to draw into binding a peptide carbonyl which is not drawn into coordination by the sodium ion. These results demonstrate aspects of calcium ion transport and interaction which are considered central to its role as a cellular messenger.

Calcium↗
PubMed1982

Irradiation crosslinking of the polytetrapeptide of elastin and compounding to dacron to produce a potential prosthetic material with elasticity and strength.

The poly(tetra peptide), H-(L . Val1-L . Pro2-Gly3-Gly4)n-L . Val-OMe, which is a recurring sequence in tropoelastin the precursor protein of the elastic fiber, has been irradiation crosslinked to produce an elastomeric material with limited strength. When a material such as a Dacron fabric is impregnated by the coacervate phase of the poly(tetrapeptide) prior to irradiation crosslinking at 50 Mrad, the crosslinked product exhibits stress-strain curves with good elastomeric properties and high strength. In addition to the stress-strain curves, the material is characterized by scanning electron microscopy.

Biocompatible Materials↗
PubMed1982

Ion interactions in (1-13C)D-Val8 and D-Leu14 analogs of gramicidin A, the helix sense of the channel and location of ion binding sites.

Ion-induced chemical shifts in the carbonyl carbon resonances of synthesized ad verified (1-13C)D-Val8 gramicidin A and (1-13C)D-Leu14 gramicidin A are utilized in combination with the previously determined location of the ion binding sites of the gramicidin A channel (using the carbonyls of L-residues) to determine that the helix sense of the gramicidin A channel) is left-handed. Having resolved the handedness issue, the location of the ion binding sites (which are fundamental to understanding the mechanism of ion transport) are further delineated with the results indicating two sites separated by just over 20 A. Furthermore, the demonstration that the divalent barium ion interacts at the binding site while not being transported through the channel is used to argue that the mechanism of monovalent vs. divalent cation selectivity is due to the positive image force contribution to the central barrier.

Biological Transport↗
PubMed1982

Dielectric relaxation studies of ionic processes in lysolecithin-packaged gramicidin channels.

Dielectric permitivities have been determined for suspensions of lysolecithin packaged malonyl gramicidin channels over the frequency range of 5 kHz to 900 MHz and under conditions of approximately equimolar concentrations (approximately 10mM) of channels and salts. The salts were lithium chloride, sodium chloride and thallium acetate. A relaxation process unique to the thallium acetate-channel system was observed which on analysis gave rise to a relaxation time at 25 degrees of 120 msec. The permitivity data, as well as a comparison of binding constants, indicate that the relaxation process results from TI+ being bound within the channel and more specifically from an intrachannel ion translocation with a rate constant of approximately 4 x 10(6) sec-1 and with an energy of activation of less than 6.7 kcal/mole. These data compare favorably with data from conductance studies on planar bilayers and with ion and carbon-13 nuclear magnetic studies on the lysolecithin packaged malonyl gramicidin channels which combine to indicate that the relaxation process is due to the jump of the thallium ion across a central barrier.

Circular Dichroism↗
PubMed1982

Location of monovalent cation binding sites in the gramicidin channel.

Six syntheses of gramicidin A have been carried out, each with 90% 13 C enrichment of a single carbonyl carbon these being the formyl, Val-1, Trp-9, Trp-11, Trp-13, and Trp-15 carbonyl carbons. Each gramicidin A was incorporated as the channel state into phospholipid structures, and the chemical shift of the carbonyl carbon resonance was monitored by 13C NMR as a function of ion concentration. Plots of Na+- and Tl+-induced chemical shifts as a function of carbonyl location in the channel indicate two symmetrically related binding sites centered at the tryptophan carbonyls and separated by 23 A. The absence of ion-induced chemical shifts for the formyl and Val-1 carbonyl carbon resonances indicates that there is no binding site midway through the channel but rather a central free-energy barrier for ion transit through the channel. Ion induced chemical shifts of the tryptophan carbonyl carbon resonances at 100 mM Na+ verify that the tight binding constant (Kbt congruent to 70 M-1), observed with 23Na NMR, results from binding within the channel. This observation and the lateral, triangular distribution of the coordinating Trp-9, -11, and -13 carbonyls combine to provide an experimental demonstration that the carbonyls of the walls of the channel directly coordinate the ion, successfully competing with the polar solvent. With the binding sites verified and localized, it is possible to conclude that the transport mechanism for Na+ is well represented by the case of the two-site model [D. W. Urry, Venkatachalam, C. M., Spisni, A., Läuger, P. & Khaled, M. A. (1980) Proc. Natl. Acad. Sci. USA 77, 2028--2032].

Binding Sites↗
PubMed1982

Synthesis and characterization of 1-(13) C-D X Leu12, 14 gramicidin A.

The 13C-D-Leu12, 14 gramicidin A was synthesized by the solid phase method incorporating 13C-D-leucine in positions 12 and 14 with about 25 and 50% enrichment, respectively. The pentadecapeptide was removed from the resin by ethanolamine treatment, with the N-protecting group (Boc) still on. After removal of the protecting group, the peptide was formylated and purified by preparative t.l.c. to obtain 13C-D-Leu12, 14 gramicidin A in a very pure state in an overall yield of about 12.5%. The peptide was then thoroughly characterized by HPLC which gave one single peak with the same retention time as that of Val1-gramicidin A of the natural gramicidin mixture. The CD spectra of the synthetic and the HPLC purified natural Val1-GA were obtained and found to be identical, indicating the optical purity of the sample. The synthetic GA was characterized by 13C n.m.r. spectrum and compared with that of natural GA. Single channel conductance parameters of the synthetic GA were determined and found to be indistinguishable from those of natural Val1-GA in lipid bilayer membranes and the mean channel lifetime was found to be as reported earlier by others.

Amino Acids↗
PubMed1982

Calcium binding to a calcifiable matrix: 43Ca NMR binding studies on the polypentapeptide of elastin.

Calcium-43 nuclear magnetic resonance (NMR) has been used to characterize the binding of calcium ion to the polypentapeptide of elastin. The change in longitudinal relaxation time, T1, from 1000 msec to 15 msec at 38 mM CaCl2, on going from D2O to coacervate concentration at 20 degrees C, is dramatic. At 20 degrees C, the binding constant decreases with increasing calcium ion concentration over the range of 40 mM to 250 mM with apparent binding constants of Ktb congruent to 35 M-1 and Kwb congruent to 7M-1. On raising the temperature to 38 degrees C, there is a significant decrease in calcium interaction, as seen in the large increase in T1 from 15 msec at 20 degrees C to 350 msec at 38 mM CaCl2. This correlates with an inverse temperature transition for the polypentapeptide and results in a concentration dependence of the calcium interaction which shows positive cooperativity. Since the polypentapeptide has been shown to calcify massively when exposed to normal serum dialysates, these calcium interactions are of particular interest as the initiating process for calcification of this component of the elastic fiber.

Binding Sites↗
PubMed1981

Single channel properties of D-Leu2-gramicidin A. Side chain modulation of channel lifetime.

The channel forming properties of synthetic gramicidin A and DLeu2-gramicidin A were compared in black lipid membranes. The most probable single channel conductance was identical for both derivatives but in each case a distribution of smaller channel sizes was observed. However, the lifetime of the channel formed by DLeu2-gramicidin A was considerably shorter than for gramicidin A. The DLeu2 substitution is considered to interfere with the head to head hydrogen bonding which forms the conducting dimer, thus destabilizing the dimeric structure of the channel and reducing the lifetime. This represents the first demonstration of side-chain modulation of channel lifetime.

Biological Transport↗
PubMed1981

Compounding of elastin polypentapeptide to collagen analogue: a potential elastomeric prosthetic material.

The polypentapeptide, H-(L . Val1-L . Pro2-Gly3-L . Val4-Gly5)n-L . Val-OMe, which is the most common recurring sequence within the elastic fiber, is demonstrated to be elastomeric when irradiation cross-linked but to have limited strength. On irradiation compounding with a collagen analogue, such as Dacron, stress-strain studies show the product to have an elastic modulus greater than that of fibrous aortic elastin and similar to that of aortic wall. In addition, the compounded product has the requisite strength. Of the 40, 50 and 60 MRAD cross-linked polypentapeptide-Dacron products, those derived from the larger doses of 50 and 60 MRAD exhibited somewhat better elastomeric properties. The unstretched and stretched products were characterized by scanning electron microscopy which demonstrated the importance of a fabric weave with a uniform extension. In general irradiation cross-linking has the advantage of being able to produce larger quantities of elastomeric material and compounding to a collagen analogue provides the required strength.

Collagen↗
PubMed1981

Conformation characterization of cyclopentapeptide, L.Val-L.Pro-Gly-L.Val-Gly: a repeating analogue of elastin.

The cyclopentapeptide, L.Val1-L.Pro2-Gly3-L.Val4-Gly5, was synthesized and its conformational characterization was carried out using n.m.r. and theoretical energy calculations. The n.m.r. studies indicated the existence of a cis Val1-Pro2 peptide bond in water and a very strong intramolecular H-bond between the val1 NH and Gly3 C=O groups. This H-bond forms a beta-turn (type II) placing Val4 and Gly5 residues within the turn. Two minimum energy conformations were derived, one of which agrees very well with the solution conformation.

Amino Acid Sequence↗