PH, physiological

Rigal-Verneil, D., Paul-Buclon, C., Sampoux, Ph. Physiologic de la comee. In Encyclopedic Medico-Chirurgicale - Ophtalmologie, p. 9. Editions Techniques, Paris, 21020-C-10 (1990). 

A number of factors affect the rate of methyl ketone production, including temperature, pH, physiological state of the mould, and the ratio of the concentration of fatty acid to the dry weight of spores (Adda et ai, 1982). Fan et al. (1976) found that both resting spores and fungal mycelium are capable of producing methyl ketones. The rate of production of methyl ketones does not depend directly on the concentrations of FFA precursors indeed high concentrations of FFAs are toxic to P. roqueforti. 

Responsive polymers combine the properties of a chemical sensor and of an actuator. The change of degree of swelling can be stimulated by a change in properties of the environmental phase (autonomous system, e.g., pH, physiological variables) or it can be triggered by an additional force, for example, for a T-sensitive polymer by heating. 

A limitation of the ap and tt descriptors is the specificity of the atom typing, e.g., benzoic acid and phenyltetrazole would not be perceived as very similar, even though carboxylates and tetrazoles are both anions at physiological pH. 

These compounds are present in almost all mam malian cells where they are believed to be involved in cell differentiation and proliferation Because each ni trogen of a polyamine is protonated at physiological pH (7 4) putrescine spermidine and spermine exist as cations with a charge of + 2 +3 and + 4 respectively... 

Loperamide. 4-(4-Chlorophenyl)-4-hydroxy-A7,A/-dimethyl- 7, 7-diphenyl-l piperidinebutanamide monohydrochloride [43552-83-5] (Imodium) (14) is practically iasoluble ia water (0.002%) at physiological pH. Its crystals are not affected by light and it is not hygroscopic. It may be synthesized (12). 

Immersion electrodes are the most common glass electrodes. These are roughly cylindrical and consist of a barrel or stem of inert glass that is sealed at the lower end to a tip, which is often hemispherical, of special pH-responsive glass. The tip is completely immersed in the solution during measurements. Miniature and microelectrodes are also used widely, particularly in physiological studies. Capillary electrodes permit the use of small samples and provide protection from exposure to air during the measurements, eg, for the determination of blood pH. This type of electrode may be provided with a water jacket for temperature control. 

Thus nicotinoids that have the highest insecticidal action have the highest piC and, consequently, exist largely in the ionized form at physiological pH. This produces the anomaly that the compounds that are most highly ionized react most rapidly with the receptor protein, yet they are less able to penetrate through the ionic barrier surrounding the insect nerve synapse. 

Dmg distribution into tissue reservoirs depends on the physicochemical properties of the dmg. Tissue reservoirs include fat, bone, and the principal body organs. Access of dmgs to these reservoirs depends on partition coefficient, charge or degree of ionization at physiological pH, and extent of protein binding. Thus, lipophilic molecules accumulate in fat reservoirs and this accumulation can alter considerably both the duration and the concentration—response curves of dmg action. Some dmgs may accumulate selectively in defined tissues, for example, the tetracycline antibiotics in bone (see Antibiotics,tetracyclines). 

In another attempt to relate degree of ionization with antibacterial activity, the effect of pH of the medium on the antibacterial activity was studied (27,28). Activity increased with increase in pH only up to the point at which the dmg was 50% ionized, and then decreased. The interpretation of this was that sulfonamides penetrate the bacterial cell in the unionized form, but once inside the cell, the equiUbrium between ionized and unionized forms is reestabhshed, and the activity is due to the ionized form. For optimum activity, a sulfonamide should have a p that provides half-dissociation at the physiologic pH in the area where it is absorbed. This observation also provided an explanation of the paraboHc relationship between piC and MIC (24). 

Other Radioprotective Chemicals. The bis-methylthio- and methylthioamino-derivatives of 1-methylquinolinium iodide and l-methylpyridinium-2-dithioacetic acid provide reasonable protection to mice at much lower doses than the aminothiols, which suggests a different mechanism of action (139). One of these compounds, the 2-(methylthio)-2-piperidino derivative of the l-methyl-2-vinyl quinolinium iodide (VQ), interacts with supercoUed plasmic DNA primarily by intercalation. Minor substitutions on the aromatic quinolinium ring system markedly influence this interaction. Like WR-1065, VQ is positively charged at physiological pH, and the DNA-binding affinities of VQ and WR-1065 appear to be similar. 

Zirconium is generally nontoxic as an element or in compounds (97,98). At pH normally associated with biological activity, zirconium chiefly exists as the dioxide which is insoluble in water and in this form zirconium is physiologically inert. 

Nisin acts bactericidally primarily against gram-positive bacteria. It acts best at acid pH and is almost insoluble at physiological pH. Nisin and probably all lantibiotics appear to permeabilize the bacterial ceU membrane to release small molecules, resulting in an immediate coUapse of the membrane... 

Thromboxane A2 is a potent platelet aggregating agent and vasodilator which undergoes rapid hydrolysis under physiological conditions (ti/2 32 sec. at pH 7 and 37°C). The synthesis of stable analogs was of interest for biological studies of this potent but evanescent prostanoid. 

Schopf and Lehmann found that lobelanine could be synthesised by keeping at 25° a mixture of glutardialdehyde, methylamine hydrochloride and benzoylacetic acid in a buffered solution. The best yield was obtained at pH 4-5, and appeared to be complete in forty hours. At pH 7 or 9, 11 or 13, the yield was very small. This synthesis under physiological conditions is represented as occurring in accordance with the following scheme —... 

A characteristic of all the above reactions is that the yield of the aldoliza-tion product depends on the pH of the reaction mixture (324), the maximum yield usually occurring near pH 7. Such reactions have been carried out in vitro in dilute aqueous buffer under so-called physiological conditions, i.e., conditions attainable in the living cell. Although this oversimplified technique for the study of alkaloid biogenesis is now being abandoned in favor of experiments in vivo with labeled precursors, such reactions are still of interest to organic chemists. 

Consequently, biochemists conducting in vitro experiments were limited in their choice of buffers effective at or near physiological pH. In 1966, N.E. Good devised a set of synthetic buffers to remedy this problem, and over the years the list has expanded so that a good selection is available (Figure 2.18). 

Typical values for pAlg are in the range of 9.0 to 9.8. At physiological pH, the a-carboxyl group of a simple amino acid (with no ionizable side chains) is completely dissociated, whereas the a-amino group has not really begun its dissociation. The titration curve for such an amino acid is shown in Figure 4.7. 

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