Biological transfer models approach

Several models have been developed in the last decade aimed at describing the emission of volatile organic compounds (VOCs) from indoor materials. These models may be broadly distinguished with respect to their conceptual background (physical-mass transfer models and/or empirical-statistical models) as well as their ability to describe different emission profiles. Physical models are models based on principles of physics and chemistry, whereas the empirical models do not necessarily require fundamental knowledge of the underlying physical, chemical and/or biological mechanisms. Many models used in the indoor air quality field in practice are hybrid models, in which aspects of both physical and empirical approaches are combined. 

The problem of linking atomic scale descriptions to continuum descriptions is also a nontrivial one. We will emphasize here that the problem cannot be solved by heroic extensions of the size of molecular dynamics simulations to millions of particles and that this is actually unnecessary. Here we will describe the use of atomic scale calculations for fixing boundary conditions for continuum descriptions in the context of the modeling of static structure (capacitance) and outer shell electron transfer. Though we believe that more can be done with these approaches, several kinds of electrochemical problems—for example, those associated with corrosion phenomena and both inorganic and biological polymers—will require approaches that take into account further intermediate mesoscopic scales. There is less progress to report here, and our discussion will be brief. 

A (more modem) approach to the membrane potentials observed in biology is to take account not only of the liquid junction (Nemst-Planckian) potential aspects, but also to model the net potential difference across the membrane as a bielectrode. On each side of the membrane it is supposed that (differing) electron-transfer reactions occur. The observed potential is the difference of these, plus IR components, of active electronic and ionic potential differences across the membrane. 

A remarkable property of the hydroxamates is the pronounced specificity for ferric iron and even very simple model compounds like acethy-droxamic acid have stability constants approaching 1030 (Table 1). Ferrous iron is bound only relatively weakly and this discrepancy in the avidity for the two oxidation states is powerful evidence that the ferric hydroxamates do not function biologically, as do the hemes, by alternate oxidation and reduction. On the other hand these properties provide a simple means of pick up, transfer and release of iron at the point of demand in living cells. 

In order to understand these complex metabolic interactions more fully and to maximize the information obtained in these studies, we developed a detailed kinetic model of zinc metabolism(, ). Modeling of the kinetic data obtained from measurements of biological tracers by compartmental analysis allows derivation of information related not only to the transient dynamic patterns of tracer movements through the system, but also information about the steady state patterns of native zinc. This approach provides data for absorption, absorption rates, transfer rates between compartments, zinc masses in the total body and individual compartments and minimum daily requirements. Data may be collected without disrupting the normal living patterns of the subjects and the difficulties and inconveniences of metabolic wards can be avoided. 

There are many studies of the transfer of electrons from enzymes to substrates, across biological membranes, to (or from) electrodes from (or to) substrates, between adsorbed molecular dyes and semiconductor particles, within synthetic films and nano-scale arrays, within molecular wires , and so on. Only a few, general comments will be offered on these topics here. The basic physics of molecular electron transfer does not change with the scale of the system, as long as identifiable molecular moieties are present with at least partly localized electronic configurations. The nature of the properties observed, the experimental probes available, and the level of theoretical treatment that is useful may be very different. Different approaches, different limiting models are used for extended arrays (or lattices) of very strongly coupled moieties. 

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