Working Model Of Chemistry

[Jeana Rodia]

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alpha(S2)-Casein (alpha(S2)-CN) comprises up to 10% of the casein fraction in bovine milk. The role of alpha(S2)-CN in casein micelles has not been studied in detail in part because of a lack of structural information on the molecule. Interest in the utilization of this molecule in dairy products and nutrition has been renewed by work in 3 areas: biological activity via potentially biologically active peptides, functionality in cheeses and products, and nutrition in terms of calcium uptake. To help clarify the behavior of alpha(S2)-CN in its structure-function relationships in milk and its possible applications in dairy products, this paper reviews the chemistry of the protein and presents a working 3-dimensional molecular model for this casein. The model was produced by threading the backbone sequence of the protein onto a homologous protein: chloride intracellular channel protein-4. Overall, the model is in good agreement with experimental data for the protein, although the amount of helix may be over-predicted. The model, however, offers a unique view of the highly positive C-terminal portion of the molecule as a surface-accessible area. This region may be the site for interactions with kappa-carrageenan, phosphate, and other anions. In addition, most of the physiologically active peptides isolated from alpha(S2)-CN occur in this region. This structure should be viewed as a working model that can be changed as more precise experimental data are obtained.

Creating working models for Class 12 science can be a fascinating way to understand and apply various scientific principles. Here are 15 ideas spanning physics, chemistry, and biology: Physics Chemistry Biology @howtofunda

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Geometrical shapes are everywhere in chemistry. Before you carry on reading, grab a pen and paper or your smartphone and make a note of all the instances of shapes, angles, volumes and areas that students may encounter in pre-16 chemistry.

Students spend a considerable amount of time in their chemistry lessons looking at or drawing 2D representations of a 3D world. From states of matter to organic reaction mechanisms, the richness of three dimensions is often reduced to the plane of the page.

Simple covalent molecules: valence shell electron pair repulsion (VSEPR) rules go beyond the scope of this article, but this simple balloon exercise can help familiarise students with molecular shapes. You can tie balloons at their nozzles to model linear, trigonal planar, tetrahedral, octahedral and even trigonal bipyramidal (PF5) bonding arrangements.

An appreciation of surface area and volume is important in understanding reaction rates and the particular properties of nanoparticles. Whether discussing marble chips reacting with hydrochloric acid or nanoparticles, these particles will often be modelled as small cubes with a side of length l. One of the advantages of modelling particles as cubes is that each of the six sides is identical so calculating the surface area, volume and the ratio is simpler than for other 3D shapes.

The TLVs are guidelines to be used by professional industrial hygienists. The values presented in this book are intended for use only as guidelines or recommendations to assist in the evaluation and control of potential workplace health hazards and for no other use (e.g., neither for evaluating or controlling community air pollution; nor for estimating the toxic potential of continuous, uninterrupted exposures or other extended work periods; nor for proving or disproving an existing disease or physical condition in an individual). Further, these values are not fine lines between safe and dangerous conditions and should not be used by anyone who is not trained in the discipline of industrial hygiene. TLVs are not regulatory or consensus standards.

Threshold Limit Values (TLVs) refer to airborne concentrations of chemical substances and represent conditions under which it is believed that nearly all workers may be repeatedly exposed, day after day, over a working lifetime, without adverse health effects.

Those who use the TLVs MUST consult the latest Documentation to ensure that they understand the basis for the TLV and the information used in its development. The amount and quality of the information that is available for each chemical substance varies over time.

Chemical substances with equivalent TLVs (i.e., same numerical values) cannot be assumed to have similar toxicologic effects or similar biologic potency. In this book, there are columns listing the TLVs for each chemical substance (that is, airborne concentrations in parts per million [ppm] or milligrams per cubic meter [mg/m3]) and critical effects produced by the chemical substance. These critical effects form the basis of the TLV.

The TWA concentration for a conventional 8-hour workday and a 40-hour workweek, to which it is believed that nearly all workers may be repeatedly exposed, day after day, for a working lifetime without adverse effect. Although calculating the average concentration for a workweek, rather than a workday, may be appropriate in some instances, ACGIH does not offer guidance regarding such exposures.

The concentration that should not be exceeded during any part of the working exposure. If instantaneous measurements are not available, sampling should be conducted for the minimum period of time sufficient to detect exposures at or above the ceiling value.

ACGIH believes that TLVs based on physical irritation should be considered no less binding than those based on physical impairment. There is increasing evidence that physical irritation may initiate, promote, or accelerate adverse health effects through interaction with other chemical or biologic agents or through other mechanisms.

Special consideration should also be given to the application of the TLVs in assessing the health hazards that may be associated with exposure to a mixture of two or more substances. A brief discussion of basic considerations involved in developing TLVs for mixtures and methods for their development, amplified by specific examples, is given in Appendix E.

When workers are exposed to air contaminants at temperatures and pressures substantially different than those at 25C and 760 torr, care should be taken in comparing sampling results to the applicable TLVs. For aerosols, the TWA exposure concentration (calculated using sample volumes not adjusted to conditions at 25C and 760 torr) should be compared directly to the applicable TLVs published in the TLVs and BEIs book. For gases and vapors, there are a number of options for comparing air-sampling results to the TLV, and these are discussed in detail by Stephenson and Lillquist (2001). One method that is simple in its conceptual approach is 1) to determine the exposure concentration, expressed in terms of mass per volume, at the sampling site using the sample volume not adjusted to conditions at 25C and 760 torr, 2) if required, to convert the TLV to mg/m3 (or other mass per volume measure) using a molar volume of 24.4 L/mole, and 3) to compare the exposure concentration to the TLV, both in units of mass per volume.

A number of assumptions are made when comparing sampling results obtained under unusual atmospheric conditions to the TLVs. One such assumption is that the volume of air inspired by the worker per workday is not appreciably different under moderate conditions of temperature and pressure as compared to those at 25C and 760 torr (Stephenson and Lillquist, 2001). An additional assumption for gases and vapors is that absorbed dose is correlated to the partial pressure of the inhaled compound. Sampling results obtained under unusual conditions cannot easily be compared to the published TLVs, and extreme care should be exercised if workers are exposed to very high or low ambient pressures.

Application of TLVs to work schedules markedly different from the conventional 8-hour day, 40-hour workweek requires particular judgment to provide protection for these workers equal to that provided to workers on conventional workshifts. Short workweeks can allow workers to have more than one job, perhaps with similar exposures, and may result in overexposure, even if neither job by itself entails overexposure.

The Brief and Scala model is easier to use than some of the more complex models based on pharmacokinetic actions. The application of such models usually requires knowledge of the biological half-life of each substance, and some models require additional data. Another model developed by the University of Montreal and the Institute de Recherche en Sante et en Securite du Travail (IRSST) uses the Haber method to calculate adjusted exposure limits (Brodeur et al., 2001). This method generates values close to those obtained from physiologically based pharmacokinetic (PBPK) models.

TLVs for gases and vapors are established in terms of parts of vapor or gas per million parts of contaminated air by volume (ppm), but may also be expressed in mg/m3. For convenience to the user, these TLVs also reference molecular weights. Where 24.45 = molar volume of air in liters at 25C and 760 torr, the conversion equations for gases and vapors [ppm:mg/m3] are as follows:

Additional copies of the TLVs and BEIs book and the multi-volume Documentation of the Threshold Limit Values and Biological Exposure Indices, upon which this book is based, are available from ACGIH. Documentation of individual TLVs is also available. Consult the ACGIH website (acgih.org/store) for additional information and availability concerning these publications.

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