Reaction coordinates of biomolecular isomerization.
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Transition path sampling has been applied to the molecular dynamics of the alanine dipeptide in vacuum and in aqueous solution. The analysis shows that more degrees of freedom than the traditional dihedral angles, phi and psi, are necessary to describe the reaction coordinates for isomerization of this molecule. In vacuum, an additional dihedral angle is identified as significant. In solution, solvent variables are shown to play a significant role, and this role appears to be more specific than can be captured by friction models. Implications for larger molecules are discussed. (+info)
Charnley low-friction arthroplasty with an autograft of the femoral head for developmental dysplasia of the hip. The 10- to 15-year results.
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Between 1983 and 1988 we carried out 45 Charnley low-friction arthroplasties with autografts from the femoral head in 41 patients for developmental dysplasia of the hip. The preoperative radiographs were assessed for the severity of DDH according to the classifications of Crowe et al, Hartofilakidis et al and Sharp. The postoperative and follow-up radiographs were analysed for coverage of the socket by the graft, for loosening and for the outcome of the fixation of the bone graft. Two patients died (two hips) at four and seven years after THR from causes unrelated to the surgery and were excluded from the final radiological analysis. The mean age of the patients at the time of operation was 46 years 3 months. The autograft of the femoral head covered a mean 26% (16 to 35) of the acetabular component. All the grafts united. Some degree of resorption of the bone graft occurred in 27 patients, and always involved the lateral part of the graft, which was beyond the margin of the socket. After a mean follow-up of 11 years there had been no revisions and 38 patients had no pain or only slight discomfort. One socket migrated and four others were fully demarcated. Our findings indicate that the Charnley LFA with an autograft of the femoral head for DDH remains successful at a follow-up of 15 years. (+info)
Cellular plasma membranes have domains that are defined, in most cases, by cytoskeletal elements. The outer half of the bilayer may also contain domains that organize glycosylphosphatidylinositol (GPI)-linked proteins. To define outer membrane barriers, we measured the resistive force on membrane bound beads as they were scanned across the plasma membrane of HEPA-OVA cells with optical laser tweezers. Beads were bound by antibodies to fluorescein-phosphatidylethanolamine (Fl-PE) or to the class I major histocompatibility complex (MHC class I) Qa-2 (a GPI-anchored protein). Two-dimensional scans of resistive force showed both occasional, resistive barriers and a velocity-dependent, continuous resistance. At the lowest antibody concentration, which gave specific binding, the continuous friction coefficient of Qa-2 was consistent with that observed by single-particle tracking (SPT) of small gold particles. At high antibody concentrations, the friction coefficient was significantly higher but decreased with increasing temperature, addition of deoxycholic acid, or treatment with heparinase I. Barriers to lateral movement (>3 times the continuous resistance) were consistently observed. Elastic barriers (with elastic constants from 1 to 20 pN/microm and sensitive to cytochalasin D) and small nonelastic barriers (<100 nm) were specifically observed with beads bound to the GPI-linked Qa-2. We suggest that GPI-linked proteins interact with transmembrane proteins when aggregated by antibody-coated beads and the transmembrane proteins encounter cytoplasmic barriers to lateral movement. The barriers to lateral movement are dynamic, discontinuous, and low in density. (+info)
Biological microtribology: anisotropy in frictional forces of orthopteran attachment pads reflects the ultrastructure of a highly deformable material.
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Evolutionarily optimized frictional devices of insects are usually adapted to attach to a variety of natural surfaces. Orthopteran attachment pads are composed of hexagonal outgrowths with smooth flexible surfaces. The pads are designed to balance the weight of the insect in different positions and on different materials. In a scanning electron microscopy study followed by freezing-substitution experiments, the ultrastructural architecture of the pad material was visualized. In friction experiments, the interaction was measured between the attachment pad and a polished silicon surface. The inner structure of this material contains distally directed rods, branching close to the surface, and spaces filled with fluid. The specific design of the pad material provides a higher frictional force in the distal direction. Frictional anisotropy is more enhanced at higher normal forces and lower sliding velocities. It is concluded that optimal mechanical functionality of biosystems is the result of a combination of surface structuring and material design. (+info)
Surface characterization of retrieved NiTi orthodontic archwires.
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The structure and morphological condition of retrieved NiTi orthodontic archwires was evaluated and any possible alterations in the surface composition of the alloy following 1-6 months in vivo were characterized. NiTi wires (GAC, German Orthodontics, ORMCO) of various cross-sections were collected through a retrieval protocol and were subjected to multi-technique characterization. Optical microscopy revealed islands of amorphous precipitants and accumulated microcrystalline particles. Micro MIR-FTIR investigation of the retrieved samples demonstrated the presence of a proteinaceous biofilm, the organic constituents of which were mainly amide, alcohol, and carbonate. Scanning electron microscope and X-ray microanalysis showed that the elemental species precipitated on the material surface were Na, K, Cl, Ca, and P, forming NaCl, KCl, and Ca-P precipitates. Increased intra-oral exposure was consistently associated with the presence of a mature film, while evidence of alloy delamination, pitting, and crevice corrosion, as well as a notable reduction in the alloy grain size was observed. Intra-oral exposure of NiTi wires alters the topography and structure of the alloy surface through surface attack in the form of pitting or crevice corrosion or formation of integuments. Further in vivo research is required to resolve the implications of the described ageing pattern in the corrosion resistance of the alloy, the potential for nickel leaching, as well as bracket-archwire friction variants. (+info)
Limits to optimization: fluid dynamics, adhesive strength and the evolution of shape in limpet shells.
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Limpets are commonly found on wave-swept rocky shores, where they may be subjected to water velocities in excess of 20 m s(-1). These extreme flows can impose large forces (lift and drag), challenging the animal's ability to adhere to the substratum. It is commonly thought that the conical shape of limpet shells has evolved in part to reduce these hydrodynamic forces while providing a large aperture for adhesion. This study documents how lift and drag actually vary with the shape of limpet-like models and uses these data to explore the potential of hydrodynamic forces to serve as a selective factor in the evolution of limpet shell morphology. At a low ratio of shell height to shell radius, lift is the dominant force, while at high ratios of height to radius drag is dominant. The risk of dislodgment is minimized when the ratio of height to radius is 1.06 and the apex is in the center of the shell. Real limpets are seldom optimally shaped, however, with a typical height-to-radius ratio of 0.68 and an apex well anterior of the shell's center. The disparity between the actual and the hydrodynamically optimal shape of shells may be due to the high tenacity of limpets' adhesive system. Most limpets adhere to the substratum so strongly that they are unlikely to be dislodged by lift or drag regardless of the shape of their shell. The evolution of a tenacious adhesion system (perhaps in response to predation) has thus preempted selection for a hydrodynamically optimal shell, allowing the shell to respond to alternative selective factors. (+info)
Hydrodynamics, shell shape, behavior and survivorship in the owl limpet Lottia gigantea.
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On wave-swept rocky shores, limpets are subjected to water velocities in excess of 20 m s(-1), which may impose large hydrodynamic forces. Despite the extreme severity of this flow environment, predictions from conical models suggest that limpets' shells are typically far from the optimal shape that would minimize the risk of dislodgment, a deviation that is allowed by the high tenacity of the limpets' adhesive system. In this study, we test this conclusion using an actual limpet. The shell of Lottia gigantea differs substantially from the hydrodynamic optimum in that its apex is displaced anteriorly to form a plough, which is used to defend the limpet's territory. The hydrodynamic effects of this shape are similar to those observed in conical models: the animal experiences an increased lift when facing into the flow and a decreased lift when the flow is at its back. However, neither effect has a substantial impact on the risk of dislodgment. When the animal is stationary, its adhesion to the substratum is very strong, and its risk of being dislodged is small regardless of its orientation to the flow and despite its sub-optimal shape. In contrast, when the animal is crawling rapidly, its adhesion is substantially decreased, and it would probably be dislodged by rapid flow even if the shell were shaped optimally. The risk of dislodgment by waves is therefore functionally independent of shell shape. In essence, despite the extremely high water velocities to which this species is subjected, its shell has had the 'permission' of the flow environment to respond to other selective factors, in particular those associated with its aggressive, territorial behavior. The result is a shell that is both a potent territorial weapon and a functional (albeit less than optimal) hydrodynamic shape. (+info)
Influence of ceramic and stainless steel brackets on the notching of archwires during clinical treatment.
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The surface topography of 100 clinically used archwires of stainless steel, beta-, or nickel-titanium were investigated that had contacted either ceramic or stainless steel brackets. One group consisted of two sets: 60 wires with no treatment records accessed to bias analyses, and 40 wires for which extensive clinical records were available, half of which were used with ceramic or stainless steel brackets. A control group consisted of two sets: 30 unused wires comprised of five round and rectangular wires of each alloy, and four wires that were ligated and immediately removed from patients' mouths. After ultrasonic cleaning, each wire was inspected under an optical and/or a scanning electron microscope. Notches were categorized with regard to frequency, patterns, and severity, and mapped as a function of wire aspect (lingual, facial, and occlusal/gingival) and anatomical regions (molar, premolar, canine, and incisor). From these data the average severity of notch patterns and a notching index were derived. Although no recognizable defect patterns were observed in the control group, seven basic patterns were recognized for each wire cross-sectional shape in the clinically used wires. These wires appeared most damaged on their lingual aspect and least damaged on their facial aspect. With regard to anatomical regions, notching was prevalent in the anterior regions and sparse in the molar regions. The notch activity and the severity were nearly three times greater from ceramic brackets than from stainless steel brackets. Over one-third of all notches documented in ceramic bracket cases had severity numbers of 3 and penetrated at least one-quarter of each wire's dimension, However, over two-thirds of all notches documented in stainless steel bracket cases had severity numbers of 1. From these tabulations a theory of notch formation was proposed in which vertical movement from tooth or wire during mastication caused fretting wear, and horizontal movement during orthodontic procedures such as space closure, tipping, or bodily movement caused sliding wear. (+info)