Chemical flip-flop

Figure 14.14 A chemical flip-flop. (a) Snapshot of wave source of the surrounding wave trains, (b) and (c) Snapshots of the wave source taken at 16-s intervals. Notice that they are mirror images. Waves are produced antisymmetrically. (Courtesy of P. De Kepper.)...

Fig. 20. Chemical flip-flop endogeneous wave source and surrounding wavetrains. (a) and (b) Snapshots taken at 16 s time interval. Experimental conditions temperature = 3°C, [KI] = 2.5 X 10" M, [CH2(C00H)2] = 10 M, [CH3COOH] = 2.2M (acetic acid used instead of sulfuric acid), [NaC102] = 2.4 x r Csul i. . m-3...

Bennet, W.F.D., MacCallum, J.L., Hinner, M., Marrink, S.J., Tieleman, D.P. A molecular view of cholesterol flip-flop and chemical potential in different membrane environments. J. Am. Chem. Soc. 2009, in press. 

As the relaxation times for mechanical losses are about 10 6 sec or greater, it is evident that the flow units, which will change their places, are connected with their neighbors by more than one chemical bond. Some chemical binding forces have to be overcome simultaneously otherwise coupled flip-flop-processes will play a dominant role. In phase with the external stress 0 = 0 expjut there is a purely elastic deformation so that... 

An alternative method for excitation of nuclei over a range of chemical shifts is by irradiation with a weak, noise-modulated radio-frequency, instead of with strong r.f. pulses. In one realization of this method, protons were irradiated with repetitive sequences of noise that was truly random,162 and, in another,163 fluorine nuclei were excited by pseudo-random noise generated by amplitude modulation of the r.f. with maximum-length sequences of pulses from a computer or shift register (a series of flip-flop devices connected by feedback loops). With the carrier wave suppressed, the latter process is equivalent to phase modulation of the r.f. by+7r/2 radians when the pulse is turned on, and by —ir/2 radians when it is turned off. This method is identical with that used in most broadband, heteronuclear, noise decouplers, except that greater power is required for decoupling. 

First-order approximation, 450 First-order decay, 18 First-order plot, 18, 35 First-order rate constant, 18, 31, 61 First-Older rate equation, 18, 31, 34 First-order reaaion. 18. 60 Flip-flop problem, 68 Flow methods, 177 Fluorescence quenching, 180 Flux, 134 chemical, 60 Force constant, 294 Force of interaction, intermolecular, 391... 

The question of whether there are other mechanisms leading to CIDNP besides the radical pair mechanism is of central importance because chemical conclusions that are drawn from CIDNP results on the basis of the latter mechanism might of course be entirely wrong with another mechanism being the source of the polarizations. There has been some evidence [67-72] that cross-relaxation in radicals, by which electron spin polarization (C1DEP) is converted into CIDNP, could provide such a mechanism. Depending on whether cross-relaxation occurs by flip-flop transitions (Am = 0) or by double spin flips (Am = 2), opposite or equal phases of CIDEP and CIDNP would result. Since the origin of the electron spin polarizations is usually the triplet mechanism, this cross-relaxational mechanism is sometimes referred to as the triplet mechanism of CIDNP. 

At Stanford, Harden M. McConnell developed a new technique, called spin labelling, based upon EPR spectroscopy. While carbon-centered free radicals are extremely reactive and short-lived, radical oxides of nitrogen, such as NO and NO2, are moderately stable. McConnell noted that nitroxyl radicals (RR N-O) are extremely stable if R and R are tertiary and can be chemically attached to biological molecules of interest. In 1965, he published the concept of spin labeling and, in 1966, demonstrated that a spin-labelled substrate added to a-chymotrypsin forms a covalent enzyme-substrate complex. The EPR signal was quite broad suggesting restricted motion consistent with Koshland s induced-fit model. In 1971, McConnell published a smdy in which spin labelling indicated flip-flop motions of lipids in cell membranes. This was the start of dynamic smdies of cell membranes. 

The cylinder stretching protocol appears to work very well for simple solvent-free membrane models [111, 113, 114], but with more refined models this method suffers from two drawbacks, both related to the equifibration of a chemical potential. First, the cylinder separates the simulation volume into an inside and an outside. If solvent is present, its chemical potential must be the same in these two regions, but for more highly resolved models the solvent permeability through the bilayer is usually too low to ensure automatic relaxation. Second, the chemical potential of lipids also has to be the same in the two bilayer leaflets, and again for more refined models the lipid flip-flop rate tends to be too low for this to happen spontaneously. 

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