Diffusion Of Large Molecules Research Articles

Positions of cations and molecules in zeolites with the mordenite-type framework VII dehydrated cesium mordenite

Natural mordenite was ion-exchanged with CsCl solution and dehydrated at 300°C. The crystal structure [ a 18.194(19) b 20.470(17) c 7.506(9): Cs 7 4Al∼ 8Si∼ 40O 96] probably belongs to space group P2 1cn but was refined in the average space group Cmcm. Site occupancies for Cs are site II 3.8, IV 1.9, VI 1.8 atoms. Combined occupancies of sites IV and VI are less than 4 in dK-, dRb- and dCs- varieties. Cations larger than 1.3Å have not been found in site I. Cations in site VI block diffusion of large molecules in the main channel, and conversion of Na-mordenite from small-port to large-port variety by “decationization” towards H-mordenite might be explained by emptying sites IV and VI while up to 4 Na could remain in site I.

Membrane alterations during cornification of mammalian squamous epithelia: A freeze‐fracture, tracer, and thin‐section study

Tight junctions (zonulae occludentes) create a pericellular barrier to the diffusion of large molecules in non-keratinizing mammalian epithelia. However, in cornifying epithelia such as the epidermis, the importance of tight-junctional elements versus secreted intercellular lipid for barrier function is uncertain. In an attempt to resolve this question, we compared membrane structure in the stratum granulosum and stratum corneum of epidermis, esophagus, and vagina of newborn and adult humans and mice under both normal and various experimental conditions. We incubated pieces of epidermis in organ culture and infused tissues with lanthanum or horseradish peroxidase in vivo and in vitro. All were processed for electron microscopy of freeze-fracture replicas or thin sections. Lanthanum seeped outward to the stratum granulosum in all tissues examined--further apical migration was halted by lamellar-body contents in skin. A similar pattern of intercellular lamellar lipid deposition and membrane structure occurred in all epithelia studied. Freeze-fracture replicas of these obstructive regions revealed occasional, incomplete junctional strands (particularly in moist epithelia) and abundant lamellar material, but complete zonulae occludentes were never encountered. A possible relationship between moisture and tight junction formation was further suggested by organ culture experiments during which brief incubations stimulated an increase in the number of junctional strands and diminished numbers of lamellar bodies. We conclude that, in the epithelia studied, the deposition of secreted lamellar body contents forms the barrier to water-soluble tracer loss: tight-junctional elements are either absent or too fragmentary to constitute an effective barrier.

Diffusion of large molecules in polymers: a measuring technique based on microdensitometry in the infra-red

A technique based on infra-red microdensitometry has been developed for studying the centre-of-mass translational diffusion of large additive molecules in polymeric matrices. In this technique the diffusion broadening with time of a known initial concentration profile of the additive within the bulk polymer is monitored. The shape of the concentration profile after diffusion broadening readily yields D, the diffusion constant; it may also yield the concentration dependence of D where this is significant. The technique is found to be reasonably flexible, and applicable to a number of polymer—additive combinations. Values of D in the range 10 −5 to 10 −10 cm 2/sec and initial additive concentrations of down to 0.3% may be studied. The technique is viable over a wide range of temperatures and pressures. Some results obtained by this technique are presented, for the case of various linear long-chain amides diffusing in low density polyethylene.

Technique for the study of diffusion of large molecules in polymers based on infrared microdensitometry.

THE diffusion of small molecules (in the gaseous, vapour or liquid phases) through polymeric matrices has been the subject of much study1, and the techniques for such studies are, by and large, well established. Similar studies in the case of large molecules (broadly speaking, those forming solids at room temperature) in polymers, including polymeric self-diffusion, have been more rare2–5. Existing techniques for measurements of the diffusion of large molecules in synthetic polymers rely mainly on radioactive labelling of the diffusants. Nuclear magnetic resonance (NMR) has been used5, but is limited to measurements of diffusion in the melt, and also to relatively high diffusion rates5. Measurements involving radioactively labelled diffusants have two major shortcomings, in addition to the necessity of having to obtain the required labelled material : they fail with diffusants which are surface active with respect to the polymer2, and they may involve significant interfacial resistance which the method does not reveal2,3.

Some implications of eyring's theory on the diffusion of large molecules

Some implications of eyring's theory on the diffusion of large molecules

Diffusion of thiodipropionic esters and hydroxybenzophenones in isotactic polypropylene

The diffusion of members of two series of organic compounds in the liquid state in isotactic polypropylene has been studied between 80–110°. The substances examined were: dimethyl 3, 3′-thiodipropanoate; di- n-hexyl 3, 3′-thiodipropanoate, di- n-dodecyl 3, 3′-thiodipropanoate, di- n-hexadecyl 3, 3′-thiodipropanoate; 2-hydroxy-4-methoxybenzophenone, 2-hydroxy-4- n-butoxybenzophenone, 2-hydroxy-4- n-octoxybenzophenone, 2-hydroxy-4- n-dodecoxybenzophenone, 2-hydroxy-4- n-octadecoxybenzophenone. The diffusion coefficients are reported. A linear dependence of the logarithm of the frequency factor D 0 and of activation energy E 4 upon the number of carbon atoms present in the alcoholic radical was found. The experimental data suggest that the mechanism of the diffusion of large molecules in crystalline polymers is analogous to that assumed for small molecules in amorphous polymers.

Neoplasm Transplantation Inhibition by Uninvolved Lymph Tissue

HETEROLOGOUS transplantation of human neoplasms into the cheek pouch of the syrian hamster (Mesocricetus auratus) was first reported in 1951 (ref. 1). The uniqueness of the pouch as a site for transplantation results from the alymphatic structure and the very slow diffusion of large molecules out of the pouch area2. The large mass of loose connective tissue which comprises most of the pouch may be responsible for limiting the diffusion of transplantation antigens. Its selection as a favourable site for tumour transplantation has been based not only on its immunological responses, but also on its simplified structure, the ease with which transplants vascularize, the accuracy with which growth rates and regressions can be measured, and the fact that repeated eversion of the pouch for observation of transplants creates little or no trauma to host tissue or transplant.

EVIDENCE FOR HIGH ENERGY STORAGE INTERMEDIATES IN BIOLUMINESCENCE

Abstract— The bioluminescent enzyme from Photobacterium fischeri is normally activated in vifro by reaction with FMNH2 and O2. in the presence of a long‐chain aldehyde. Emission from enzyme intermediates in this reaction continues for several seconds, and if the mixture is frozen just after initiation of the reaction, this presumptive emission may be delayed until the system is warmed again. Light is then emitted in a fashion analogous to thermo‐luminescent emission, with a maximum intensity at — 10°C. The experiments described here show that the total amount of light which is emitted under these conditions no longer depends so much upon aldehyde, a relatively high quantum yield being obtained both with and without aldehyde.It is further shown that bioluminescence may be activated by light, populating it is believed, the same state which is responsible for the emission in the case of the FMNH2‐induced emission. The light‐induced reaction does not depend on flavin in the enzyme preparations, nor does the activation spectrum resemble that of a flavoprotein. Activation may be carried out in the solid state at temperatures down to at least — 100°C, and so does not involve the diffusion of large molecules. It is proposed that energy storage takes place by charge separation, and that the excited state from which emission takes place is associated with charge recombination.

The Cuticle of Wool

Friction between wool fibres is smaller than friction between other fibres. This is explained by a thin fibre coating, the epicuticle, which was discovered by means of the electron microscope. The epicuticle is resistant to chlorine, acids, and enzymes. Alkaline treatment perforates it. The cuticle below the epiouticle consists of an intermediate layer, the exocuticle, and the scale substance proper, the endocuticle. The epicuticle prevents diffusion of large molecules, e. g. –those of dyes. The Allwörden reaction and staining tests for damage are explained by the existence of the epicuticle. It is easily damaged by mechanical treatments. Its removal gives an increase in fibre friction and thereby anti–felting properties. The importance of the frietional properties of fibres for spinning and wearing qualities is stressed.