Dissociation spontaneous

Although extraction of lipids from membranes can be induced in atomic force apparatus (Leckband et al., 1994) and biomembrane force probe (Evans et al., 1991) experiments, spontaneous dissociation of a lipid from a membrane occurs very rarely because it involves an energy barrier of about 20 kcal/mol (Cevc and Marsh, 1987). However, lipids are known to be extracted from membranes by various enzymes. One such enzyme is phospholipase A2 (PLA2), which complexes with membrane surfaces, destabilizes a phospholipid, extracts it from the membrane, and catalyzes the hydrolysis reaction of the srir2-acyl chain of the lipid, producing lysophospholipids and fatty acids (Slotboom et al., 1982 Dennis, 1983 Jain et al., 1995). SMD simulations were employed to investigate the extraction of a lipid molecule from a DLPE monolayer by human synovial PLA2 (see Eig. 6b), and to compare this process to the extraction of a lipid from a lipid monolayer into the aqueous phase (Stepaniants et al., 1997). 

Generated by spontaneous dissociation of the dimer. In equilibrium with dimer. 

Tertiary alkyl halides, JR3CX, undergo spontaneous dissociation to yield a carbocation, K3C+, plus halide ion. Which do you think reacts faster, (CH3)3CBt or H2C=CHC(CH3)2Br Explain. 

Jn light of the fact that tertiary alkyl halides undergo spontaneous dissociation to yield a carbocation plus halide ion (Problem 10.40) propose a mechanism for the following reaction ... 

Figure IT.9 An energy diagram for an S l reaction. The slower, rate-limiting step is the spontaneous dissociation of the alkyl halide to give a carbocation intermediate. Reaction of the carbocation with a nucleophile then occurs in a second, faster step.

Spontaneous dissociation of the protonated alcohol occurs in a slow, rate-limiting step to yield a carbocation intermediate plus water. 

Much evidence has been obtained in support of the El mechanism. For example, El reactions show first-order kinetics, consistent with a rate-limiting spontaneous dissociation process, l- urthermore, El reactions show- no deuterium isotope effect because rupture of the C—H (or C—D) bond occurs after the rate-limiting step rather than during it. Thus, we can t measure a rate difference between a deuterated and nondeuterated substrate. 

The S il reaction occurs when the substrate spontaneously dissociates to a carbocation in a slow rate-limiting step, followed by a rapid reaction with the nucleophile. As a result, SN1 reactions are kinetically first-order and take place with racemization of configuration at the carbon atom. They are most favored for tertiary substrates. Both S l and S 2 reactions occur in biological pathways, although the leaving group is typically a diphosphate ion rather than a halide. 

El reaction (Section 11.10) A unimolecular elimination reaction in which the substrate spontaneously dissociates to give a carbocation intermediate, tvhich loses a proton in a separate step. 

With N2O5 in CCI4, there is spontaneous dissociation ... 

The molecules retain their essential character In all cases studied. This implies that the spontaneous dissociation observed under certain circumstances for H2O must be a cooperative phenomenon involving more than one H2O molecule. 

The theory of electrolytic dissociation was not immediately recognized universally, despite the fact that it could qualitatively and quantitatively explain certain fundamental properties of electrolyte solutions. For many scientists the reasons for spontaneous dissociation of stable compounds were obscure. Thus, an energy of about 770kJ/mol is required to break up the bonds in the lattice of NaCl, and about 430kJ/mol is required to split H l bonds during the formation of hydrochloric acid solution. Yet the energy of thermal motions in these compounds is not above lOkJ/mol. It was the weak point of Arrhenius s theory that this mismatch could not be explained. 

This hydrogen chloride-perchloric acid complex spontaneously dissociates with explosive violence. 

Besides the diradical oxygen molecule, there are known some relatively stable oxygen radicals discovered by Goldschmidt.96 Because they are dissociated by light,97 their spontaneous dissociation as measured by the color has probably been overestimated. 

These findings led to the conclusion that the regulation of membrane anchored proteins has to be achieved by mechanisms other than spontaneous dissociation. In principal, binding to an escort protein or de-S-acylation may induce dissociation of the lipoproteins out of the membrane structure. 

N-diazeniumdiolates spontaneously dissociate at physiological pH to release nitric oxide (NO) by stable first order kinetics with half-lives ranging from 2 s to 20 h [209, 210]. They are blessed with many attributes that make them an especially attractive starting point for designing solutions to important clinical problems, namely they are stable as solids, have structural diversity, a controlled rate of release of NO on hydrolysis, and a rich derivatization chemistry that facilitates targeting of NO to specific sites of need, a critical goal for therapeutic uses of a molecule with natural bioeffector roles in virtually every organ [208]. 

MAHMA NONOate spontaneously dissociates in a pH-dependent, first-order process with a half-life of 1 min at 37 °C. In vivo, examination in anesthetised rats showed, that MAHMA NONOate had both platelet inhibitory and vasodepressor effects [50]. Like other NONOates, MAHMA NONOate inhibited collagen-induced and ADP-induced rat platelet aggregation in a concentration-dependent manner [51]. ODQ, a soluble GS inhibitor, caused only small influence on the concentration-response curve to MAHMA NONOate, indicating that cGMP-independent mechanisms play a crucial role. A potential target of MAHMA NONOate induced inhibition was the sarco-endoplasmic reticulum calcium-ATPase of the platelet dense tubular system... 

The k term corresponds to spontaneous dissociation and the k2 term arises from cyanide-assisted dissociation ... 

It is appropriate at this point to discuss the various approaches used to describe the stability of complexes. In order to be able to detect a species, it must be stable in the time scale of the measurements. In other words, during the experiments other competing metals or ligands present in the medium as contaminants or reagents should not substitute the metal or ligand. Furthermore, the experimental conditions should be such that the species does not spontanously dissociate. Both conditions could be satisfied when the complex has the appropriate thermodynamic and kinetic properties. These properties are determined in part by the experimental conditions. 

Some reactions are catalysed or induced by ultraviolet light and oxygen. The effect of this is twofold (i) some species will spontaneously dissociate and (ii) new species may be formed. It is therefore clear that wrong results could be obtained. 

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