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Abstract
A kinetic model based on allosteric mechanisms of cooperativity fits the experimentally observed phosphate dependence of hemoglobin reactions. Subunit inequivalence is found to be important in analyzing hemoglobin kinetics. The observed increase in the rate of deoxygenation in the presence of organic phosphates is primarily related to the increased rate of dissociation of the second oxygen molecule.
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Nasal carriage of meticillin-resistant Staphylococcus aureus in medical students. J Hosp Infect 2007; 68:91-2. [PMID: 18069082 DOI: 10.1016/j.jhin.2007.10.008] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/05/2007] [Accepted: 10/11/2007] [Indexed: 11/30/2022]
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Aggregation of polystyrene-poly(methyl methacrylate) diblock copolymers in toluene. Macromolecules 2002. [DOI: 10.1021/ma00066a034] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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Small-angle x-ray scattering study of kinetics of spinodal decomposition in N-isopropylacrylamide gels. PHYSICAL REVIEW. E, STATISTICAL PHYSICS, PLASMAS, FLUIDS, AND RELATED INTERDISCIPLINARY TOPICS 1999; 60:4473-81. [PMID: 11970302 DOI: 10.1103/physreve.60.4473] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/30/1998] [Indexed: 04/18/2023]
Abstract
We present synchrotron-based time-resolved small-angle x-ray scattering (SAXS) measurements of spinodal decomposition in a covalently cross-linked N-isopropylacrylamide gel. The range of wave numbers examined is well beyond the position of the maximum in the structure factor S(q,t). The equilibrium structure factor is described by the sum of a Lorentzian and a Gaussian. Following a temperature jump into the two phase region, the scattered intensity increases with time and eventually saturates. For early times the linear Cahn-Hilliard-Cook (CHC) theory can be used to describe the time evolution of the scattered intensity. From this analysis we found that the growth rate R(q) is linearly dependent on q(2), in agreement with mean-field theoretical predictions. However the Onsager transport coefficient Lambda(q) approximately q(-4), which is stronger than the q dependence predicted by the mean-field theory. We found that the growth rate R(q)>0, even though the wave numbers q probed by SAXS are greater than sqrt[2]q(m) where q(m) is the position of the peak of S(q,t), also in agreement with the mean-field predictions for a deep quench. We have also examined the range of validity of the linear CHC theory, and found that its breakdown occurs earlier at higher wave numbers. At later times, a pinning of the structure was observed. The relaxation to a final, microphase-separated morphology is faster and occurs earlier at the highest wave numbers, which probe length scales comparable to the average distance between crosslinks.
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Abstract
We present dynamic light scattering (DLS) and hydrophobic dye-binding data in an effort to elucidate a molecular mechanism for the ability of gastric mucin to form a gel at low pH, which is crucial to the barrier function of gastric mucus. DLS measurements of dilute mucin solutions were not indicative of intermolecular association, yet there was a steady fall in the measured diffusion coefficient with decreasing pH, suggesting an apparent increase in size. Taken together with the observed rise in depolarized scattering ratio with decreasing pH, these results suggest that gastric mucin undergoes a conformational change from a random coil at pH >/= 4 to an anisotropic, extended conformation at pH < 4. The increased binding of mucin to hydrophobic fluorescent with decreasing pH indicates that the change to an extended conformation is accompanied by exposure of hydrophobic binding sites. In concentrated mucin solutions, the structure factor S(q, t) derived from DLS measurements changed from a stretched exponential decay at pH 7 to a power-law decay at pH 2, which is characteristic of a sol-gel transition. We propose that the conformational change facilitates cross-links among mucin macromolecules through hydrophobic interactions at low pH, which in turn leads to a sol-gel transition when the mucin solution is sufficiently concentrated.
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Clusters and fractals in three-dimensional kinetic gelation in the presence of a mobile solvent. ACTA ACUST UNITED AC 1999. [DOI: 10.1088/0305-4470/18/3/011] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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9
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Abstract
Dynamic light scattering was used to follow the tracer diffusion of phospholipid/cholesterol vesicles in aqueous polyacrylamide solutions and compared with the diffusive behavior of polystyrene (PS) latex spheres of comparable diameters. Over the range of the matrix concentration examined (Cp = 0.1-10 mg/ml), the diffusivities of the PS spheres and the large multilamellar vesicles exhibited the Stokes-Einstein (SE) relation, while the diffusivity of the unilamellar vesicles did not follow the increase of the solution's viscosity caused by the presence of the matrix molecules. The difference between the diffusion behaviors of unilamellar vesicles and hard PS spheres of similar size is possibly due to the flexibility of the lipid bilayer of the vesicles. The unilamellar vesicles are capable of changing their shape to move through the entangled polymer solution so that the hindrance to their diffusion due to the presence of the polymer chains is reduced, while the rigid PS spheres have little flexibility and they encounter greater resistance. The multilamellar vesicles are less flexible, thus their diffusion is similar to the hard PS spheres of similar diameter.
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Crossover between Spatially Confined Precipitation and Periodic Pattern Formation in Reaction Diffusion Systems. PHYSICAL REVIEW LETTERS 1996; 77:2834-2837. [PMID: 10062057 DOI: 10.1103/physrevlett.77.2834] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
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Mucin-vesicle interactions in model bile: evidence for vesicle aggregation and fusion before cholesterol crystal formation. Hepatology 1995; 22:856-65. [PMID: 7657293] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/06/2022]
Abstract
Nucleation of cholesterol monohydrate crystals from bile is a critical step in the formation of cholesterol gallstones. Measurement of nucleation in model bile system and the characteristics of the initial nucleus have proven elusive. In this study we have used three separate physical chemical techniques to examine vesicle aggregation and fusion, including dynamic light scattering (DLS), transmission electron microscopy (TEM), and fluorescent biochemical assays. These assays enabled us to quantify the effect of biliary proteins, such as gallbladder mucin, on vesicle fusion and aggregation. In the absence of mucin, fusion is a relatively slow process occurring over 24 hours, whereas physiological concentrations of mucin are able to accelerate almost complete fusion of vesicles within 6 hours. Vesicle fusion and aggregation as characterized by TEM result in the formation of aggregates of multilamellar vesicles and giant fusion bodies associated with a background of mucin. These mucin-vesicle aggregate bodies may represent true nuclei and precede cholesterol monohydrate crystal nucleation. In future studies, these vesicle fusion assays can be used to quantitatively examine the effect of putative pro- and anti-nucleating proteins on the earliest steps of cholesterol crystal nucleation.
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Light-scattering studies in cross-linked gels: Evidence of a microphase separation. PHYSICAL REVIEW. E, STATISTICAL PHYSICS, PLASMAS, FLUIDS, AND RELATED INTERDISCIPLINARY TOPICS 1993; 48:4501-4509. [PMID: 9961132 DOI: 10.1103/physreve.48.4501] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/22/2023]
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Interference of phase separation and gelation: A zeroth-order kinetic model. PHYSICAL REVIEW. E, STATISTICAL PHYSICS, PLASMAS, FLUIDS, AND RELATED INTERDISCIPLINARY TOPICS 1993; 47:4615-4618. [PMID: 9960548 DOI: 10.1103/physreve.47.4615] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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15
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Abstract
The HCl in the mammalian stomach is concentrated enough to digest the stomach itself, yet the gastric epithelium remains undamaged. One protective factor is gastric mucus, which forms a protective layer over the surface epithelium and acts as a diffusion barrier Bicarbonate ions secreted by the gastric epithelium are trapped in the mucus gel, establishing a gradient from pH 1-2 at the lumen to pH 6-7 at the cell surface. How does HCl, secreted at the base of gastric glands by parietal cells, traverse the mucus layer without acidifying it? Here we demonstrate that injection of HCl through solutions of pig gastric mucin produces viscous fingering patterns dependent on pH, mucin concentration and acid flow rate. Above pH 4, discrete fingers are observed, whereas below pH 4, HCl neither penetrates the mucin solution nor forms fingers. Our in vitro results suggest that HCl secreted by the gastric gland can penetrate the mucus gel layer (pH 5-7) through narrow fingers, whereas HCl in the lumen (pH 2) is prevented from diffusing back to the epithelium by the high viscosity of gastric mucus gel on the luminal side.
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Dynamic behaviour of semidilute solutions and gels of poly(methacrylic acid). POLYMER 1992. [DOI: 10.1016/0032-3861(92)90899-8] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
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17
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Profound increase in viscosity and aggregation of pig gastric mucin at low pH. THE AMERICAN JOURNAL OF PHYSIOLOGY 1991; 261:G827-32. [PMID: 1719823 DOI: 10.1152/ajpgi.1991.261.5.g827] [Citation(s) in RCA: 50] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
Epithelial mucins are glycoproteins of very large molecular weight that provide viscoelastic and gel-forming properties to mucus, the jellylike protective layer covering epithelial organs. In the mammalian stomach the mucus gel layer protects the underlying epithelial cells from HCl in the lumen. We report here that pig gastric mucin undergoes a 100-fold increase in viscosity in vitro when pH is lowered from 7 to 2. Sedimentation velocity and dynamic light-scattering measurements revealed the formation of extremely large aggregates at low pH consistent with the observed increase in viscosity. Aggregation of mucin at low pH was prevented by increasing the ionic strength, suggesting the involvement of electrostatic interactions. Trypsin digestion and thiol reduction, but not enzymatic removal of neuraminic acid, prevented aggregation at low pH. This implies that the peptide core rather than the oligosaccharide side chains of the molecule is involved in the aggregation of mucin at low pH. Increased aggregation and viscosity at low pH were also observed in a solvent made to mimic the ionic composition of gastric juice, indicating the physiological relevance of our findings. Our observations suggest that one mechanism of gastric protection may be the ability of gastric mucin to undergo aggregation with a marked increase in viscosity at low pH.
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Quasielastic light-scattering study of the movement of particles in gels. PHYSICAL REVIEW LETTERS 1987; 59:684-687. [PMID: 10035844 DOI: 10.1103/physrevlett.59.684] [Citation(s) in RCA: 38] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
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Erratum: Raman spectroscopy of supercooled water [J. Chem. Phys. 76, 2221 (1982)]. J Chem Phys 1982. [DOI: 10.1063/1.444462] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
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Abstract
Raman spectra of 1,2-dimyristoyl-sn-glycero-3-phosphocholines specifically deuterated in the 2 chain at one of positions 3, 4, 6, 10, 12, and 14 have been obtained as a function of temperature. The frequencies of the CD2 vibrational stretching modes depend on the position of the labeled CD2 group, being maximum at position 3 of the acyl chain and then decreasing until they become constant beyond position 6. This frequency dependence is interpreted in terms of the inductive effect of the charge distribution of the acyl chain carboxyl group. In both gel and liquid-crystal phases, the Raman line widths depend on the position of the CD2 group, being minimum at position 6 and increasing toward both ends of the hydrocarbon chain. The width of the CD stretching bands abruptly increases at the phase transition temperature. The magnitude of the increase depends upon the position of the label, increasing almost linearly up to position 10 and then decreasing for positions 12 and 14. The spectra for the CD2 group at position at position 3 and the terminal CD3 group are almost the same in both phases. These results are interpreted in terms of the effects of hydrocarbon chain organization on the vibrational modes.
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Abstract
We report the first Raman spectroscopic study of the glycosaminoglycans chondroitin 4-sulfate, chondroitin 6-sulfate and hyaluronic acid, both in solution and in the solid state. To aid in spectral identification, infrared spectra were also recorded from films of these samples. Vibrational frequencies for important functional groups like the sulfate groups, glycosidic linkages, C-OH and the N-acetyl group can be identified from the Raman spectra. Certain differences in the spectra of the different glycosaminoglycans can be interpreted in terms of the geometry of the various substituents, while other differences can be related to differences in chemical composition.
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Kinetics of co-operative ligand binding in proteins: the effects of organic phosphates on hemoglobin oxygenation. J Mol Biol 1976; 103:89-126. [PMID: 957427 DOI: 10.1016/0022-2836(76)90054-1] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
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