Indicator molecule

C7>th e antigen, and I an indicator molecule such as an enzyme, fluorophore, or radioisotope. 

Proton Transfers in More Concentrated Solutions. Measurements with Indicators. The Proton Levels of Indicator Molecules in Dilute Solution. Indicators in More Concentrated Solutions. 

The Proton Levels of Indicator Molecules in Dilute Solution. 

Let us now ask where the vacant proton level must lie, in order that an indicator molecule shall be suitable for use in a very dilute acid solution —where the ratio [Ha0+]/[H20] will be very small compared with unity. According to (216) in order that [BH+]/[B] shall be near unity, obviously J must have a large negative value in other words, the vacant proton level of the molecule B must lie considerably below the occupied proton level of (HaO)+ otherwise, an insufficient crop of (BII)+ ions will be obtained. 

In Sec. 128 it was found that the vacant proton level of indicator 2 lies at 0.192 electron-volt below the occupied level of (HaO)+ in dilute aqueous solution. Using the successive increments listed in the last column of Table 39, we find, counting upward, that the value for indicator 5 is —0.052, referred to the same zero of energy. Proceeding by the same stepwise method to No. 6 we find for the energy of the vacant proton level the positive value +0.038. This still refers to the occupied level of the (II30)+ ion in dilute aqueous solution. It means that work equal to 0.038 electron-volt would be required to transfer a proton from the (H30)+ ion in very dilute solution to the vacant level of No. 6 in the concentrated acid solution in which the measurements were made. A further amount of work would be required to transfer the proton from the occupied level of No. 6 to the vacant proton level of one of the H2O molecules in the same concentrated solution. This is the situation because, as mentioned above, the changing environment has raised the proton level of the (HaO)+ ion relative to that of each of the indicator molecules. 

Figure 6-3. Top Structure of the T6 single crystal unit cell. The a, b, and c crystallographic axes are indicated. Molecule 1 is arbitrarily chosen, whilst the numbering of the other molecules follows the application of the factor group symmetry operations as discussed in the text. Bottom direction cosines between the molecular axes L, M, N and the orthogonal crystal coordinate system a, b, c. The a axis is orthogonal to the b monoclinic axis.

It may also happen that an association equilibrium exists between the luminescent indicator and the quencher. Non-associated indicator molecules will be quenched by a dynamic process however, the paired indicator dye will be instantaneously deactivated after absorption of light (static quenching). Equation 2 still holds provided static quenching is the only luminescence deactivation mechanism (i.e. no simultaneous dynamic quenching occurs) but, in this case, Ksv equals their association constant (Kas). However, if both mechanisms operate simultaneously (a common situation), the Stem-Volmer equation adopts more complicated forms, depending on the stoichiometry of the fluorophore quencher adduct, the occurrence of different complexes, and their different association constants. For instance, if the adduct has a 1 1 composition (the simplest case), the Stem-Volmer equation is given by equation 3 ... 

Next, the indicator dye needs a solvent to interact with the analyte. Pure crystalline indicator dyes might react at the surface but not all indicator would react due to hindered diffusion. Therefore, the indicator is dissolved in a polymer which allows free diffusion of the analyte to and from the indicator molecule. 

The indicator molecule serves to assess the state of health of the cultured cells. The dye neutral red is often used (healthy cells assimilate the dye, dead cells do not). The major drawback to such systems is that they do not reflect the complexities of living animals and, hence, may not accurately reflect likely results of whole-body toxicity studies. Regulatory authorities are (rightly) slow to allow replacement of animal-based test protocols until the replacement system is proven to be reliable and is fully validated. 

Besides inorganic anions, a lot of research effort is focussed on the development of indicator molecules for small organic molecules that possess an anionic and a cationic function such as, e.g., a carboxylate and an ammonium group as in the neurotransmitter y-aminobutyric acid (18, Fig. 7) [78] or an anionic and a neutral... 

Lubbers DW, Opitz N, Speiser PP, Bisson HJ (1977) Nanoencapsulated fluorescence indicator molecules measuring pH and p02 down to submicroscopical regions on the basis of the optode principle. Z Naturforsch 32 133-134... 

Dyes and Indicators. The effects of bromine in dye or indicator molecules, in place of hydrogen, include a shift of light absorption to longer wavelengths, increased dissociation of phenolic hydroxyl groups, and lower solubility (see Dyes and dye intermediates). The first two effects probably result from increased polarization caused by bromine s electronegativity compared to that of hydrogen. 

Lewis acid sites can coordinate with a given indicator molecule to produce an adsorption band identical in position with that produced through proton addition. Even if the indicators used are responsive only to Brpn-sted acids, most basic reagents used to titrate surface acidity (e.g., n-butylamine, pyridine) are strongly adsorbed on surface sites other than Br0nsted acid sites. In this connection, a recent study indicates that adsorption equilibrium is not fully established during titration of silica-alumina with n-butylamine because of the irreversible attachment of amine molecules by adsorption sites at which they first arrive (31). 

In chemically based selectivity, the sensing information is usually obtained from the color reaction between the analyte X and the indicator molecule I forming a colored complex X I, in which either the indicator and/or the conjugate have characteristic absorption... 

The effect of proteins on the dissociation constant of indicators (often called protein error) is well known. It is defined as the difference between the colorimetrically determined values of pH in the presence and in the absence of the proteins in solution, whose pH has been adjusted (usually electrochemically) to its original value. Depending on the type of proteins, their concentration, and on the type of the indicator, it can be as high as 0.8 units of pH. Again, it has its origin in the specific interactions between the indicator molecule and the proteins. 

Both organic and inorganic polymer materials have been used as solid supports of indicator dyes in the development of optical sensors for (bio)chemical species. It is known that the choice of solid support and immobilization procedure have significant effects on the performance of the optical sensors (optodes) in terms of selectivity, sensitivity, dynamic range, calibration, response time and (photo)stability. Immobilization of dyes is, therefore, an essential step in the fabrication of many optical chemical sensors and biosensors. Typically, the indicator molecules have been immobilized in polymer matrices (films or beads) via adsorption, entrapment, ion exchange or covalent binding procedures. 

The polymer materials not only act as supports for the dye and other necessary additives in the sensing phase, providing protective covering for the transduction element polymers also play various roles in chemical sensors. They provide a compatible environment for the indicator molecules, maintaining or improving the appropriate photophysical features (compared to those observed in homogeneous solution) on which the sensing principle is based. In many cases they collect and concentrate the analyte molecules on sensor surfaces. In addition, the polymer can play an important role in the sensitivity and selectivity of an optical sensor, and its interactions with indicator and analyte molecules influence the analytical performance of the device. 

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