Spontaneous generation

Automa-ticity. Special cardiac cells, such as SA and AV nodal cells, His-bundle cells, and Purkinje fibers, spontaneously generate an impulse. This is the property of automaticity. Ectopic sites can act as pacemakers if the rate of phase 4 depolarization or resting membrane potential is increased, or the threshold for excitation is reduced. 

Generation Spontaneous generation of gas bubbles within a homogeneous liquid is theoreticaUy impossible (Bikerman, Foams Theoiy and Industrial Applications, Reinhold, New York, 1953, p. 10). The appearance of a bubble requires a gas nucleus as avoid in the liquid. The nucleus may be in the form of a small bubble or of a solid carrying adsorbed gas, examples of the latter being dust particles, boiling chips, and a solid wall. A void can result from cavitation, mechan-ic ly or acoustically induced. Blander and Katz [AlChE J., 21, 833 (1975)] have thoroughly reviewed bubble nucleation in liquids. 

Selbst-zersetzuog, /. spontaneous decomposition autolysis, -zerstorung,/. self-destruction. -zeugung, /. spontaneous generation, abiogenesis. 

The two most widely accepted mechanisms for the spontaneous generation of radicals from S are the biradical mechanism (top half of Scheme 3.61) first proposed by Flory314 and the Mayo315 or MAH (molecule assisted homolysis) mechanism (lower pari of Scheme 3.61). 

The extent to which anode polarization affects the catalytic properties of the Ni surface for the methane-steam reforming reaction via NEMCA is of considerable practical interest. In a recent investigation62 a 70 wt% Ni-YSZ cermet was used at temperatures 800° to 900°C with low steam to methane ratios, i.e., 0.2 to 0.35. At 900°C the anode characteristics were i<>=0.2 mA/cm2, Oa=2 and ac=1.5. Under these conditions spontaneously generated currents were of the order of 60 mA/cm2 and catalyst overpotentials were as high as 250 mV. It was found that the rate of CH4 consumption due to the reforming reaction increases with increasing catalyst potential, i.e., the reaction exhibits overall electrophobic NEMCA behaviour with a 0.13. Measured A and p values were of the order of 12 and 2 respectively.62 These results show that NEMCA can play an important role in anode performance even when the anode-solid electrolyte interface is non-polarizable (high Io values) as is the case in fuel cell applications. 

ESR spectrum of the RjSn radical (R = 2,4,6-triisopropylphenyl), spontaneously generated upon dissolving distannane RBSn-SnRj in deoxygenated toluene, revealed at -140°C in the solid state the Sn hfcc of 163 Such a radical is more planar than the PhjSn radical ( Sn hfcc = 186.6mT) but less planar than the MejSn radical C Snhfcc = 161.1 mT)3° ... 

Belief in spontaneous generation lived on for years, as it had for centuries. For example, an elderly lady of the writer s early acquaintance complained bitterly that she had been cheated by a merchant who sold her a woolen coat which was of such a quality that it turned entirely into moths when left undisturbed in a closed for some months ... 

John Needham (1713-1781). Similar experiments carried out by an English scientist, John Needham, gave conflicting results. Life developed in Needham s heated closed vessels as well as in the open unheated ones. He therefore believed in spontaneous generation. We shall see later that this result was due to insufficient heating which failed to kill heat-resistant forms of bacteria called spores. But nothing was known about spores at that time. 

In 1775 Lavoisier discovered oxygen and the relation between air and life. This renewed the controversy about spontaneous generation, the objection to Spallanzani s results being that it was the exclusion of air (oxygen) from the flasks which prevented the development of life. 

In spite of these demonstrations, long and bitter controv-ersies still raged. Schroder and von Dusch were not convinced by their own experiments and admitted the possibility that spontaneous generation might occur under natural condi-tions. 

Conaut JB (1953) Pasteurs and Tyndalls Study of Spontaneous Generation. Harvard University... 

Although the work discussed thus far has covered primarily neutral organic radicals, there are many types of cation and anion radicals that are stabilized on the surface. Some of these ion radicals are formed through photochemical processes however, many others are spontaneously generated on a surface. The type of radical ion that is formed depends on the oxidizing or reducing character of particular sites on the surface, as well as on the ionization potential and the electronegativity of the adsorbed molecule. 

Regev, D., Backov, R. and Faure, C. (2004) Gold nanopartides spontaneously generated in onion-type multilamellar vesides bilayers. Particle coupling imaged by cryo-TEM. Chemistry of Materials, 16, 5280-5285. 

If the EDA and CT pre-equilibria are fast relative to such a (follow-up) process, the overall second-order rate constant is k2 = eda c e In this kinetic situation, the ion-radical pair might not be experimentally observed in a thermally activated adiabatic process. However, photochemical (laser) activation via the deliberate irradiation of the charge-transfer absorption (hvct) will lead to the spontaneous generation of the ion-radical pair (equations 4, 5) that is experimentally observable if the time-resolution of the laser pulse exceeds that of the follow-up processes (kf and /tBet)- Indeed, charge-transfer activation provides the basis for the experimental demonstration of the viability of the electron-transfer paradigm in Scheme l.21... 

Electron-transfer activation. The observation of intense coloration upon mixing the solutions of hydroquinone ether MA and nitrogen dioxide at low temperature derives from the transient formation of MA+ cation radical, as confirmed by the spectral comparison with the authentic sample. The oxidation of MA to the corresponding cation radical is effected by the nitrosonium oxidant, which is spontaneously generated during the arene-induced disproportionation of nitrogen dioxide,239 i.e.,... 

It is known that erythrocytes from patients with sickle cell anemia contain various types of abnormal iron deposits [398], which could be the origin of the overproduction of oxygen radicals in these cells. Indeed, Hebbel et al. [399] has showed that sickle erythrocytes spontaneously generate approximately twice as much superoxide as normal erythrocytes. Later on, it has been shown that these cells are also able to generate hydroxyl radicals catalyzed by three types of iron, preexisting free iron, free iron released during oxidative stress, and iron that cannot be chelated with desferrioxamine [400]. 

GEA-3162 spontaneously generates NO when dissolved [93, 94]. GEA-3163 inhibited ADP-induced platelet aggregation and induced a dose-dependent increase in cyclic GMP in platelets. The increase in cGMP was potentiated by phosphodiesterase-5 inhibitor, zaprinast, and inhibited by oxyhemoglobin [94]. 

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