Position-dependent effects

Salwiczek M., Samsonov S., Vagt T., et al. (2009) Position dependent effects of fluorinated amino acids on hydrophobic core formation of a coiled coil heterodimer. Chemistry - A European Journal (2009) in press. 

G. J. Martyna, J. Chem. Phys. (in press, 1996). In this paper, an effective set of molecular dynamics equations are specified that provide an alternative path-integral approach to the calculation of position and velocity time correlation functions. This approach is essentially based on the Wigner phase-space function. For general nonlinear systems, the appropriate MD mass in this approach is not the physical mass, but it must instead be a position-dependent effective mass. 

Jacobsen developed a method employing (pybox)YbCl3 for TMSCN addition to meso-epoxides (Scheme 7.22) [46] with enantioselectivities as high as 92%. Unfortunately, the practical utility of this method is limited because low temperatures must be maintained for very long reaction times (up to seven days). This reaction displayed a second-order dependence on catalyst concentration and a positive nonlinear effect, suggesting a cooperative bimetallic mechanism analogous to that proposed for (salen)Cr-catalyzed ARO reactions (Scheme 7.5). 

In humans, a comparative examination of the positive reinforcing effects of solvents showed that among inhalant-dependent subjects, solvents induced a more intense sensation of pleasant feelings than that induced by alcohol and nicotine in subjects addicted to these substances (Kono et al. 2001). Solvent-dependent subjects reported pleasant feelings comparable to those reported by stimulant-dependent subjects after use of methamphetamine. However,... 

It is possible to distinguish between direct and indirect nOes from their kinetic behavior. The direct nOes grow immediately upon irradiation of the neighboring nucleus, with a first-order rate constant, and their kinetics depend initially only on the intemuclear distance r" indirect nOes are observable only after a certain time lag. We can thus suppress or enhance the indirect nOe s (e.g., at He) by short or long irradiations, respectively, of Ha- a long irradiation time of Ha allows the buildup of indirect negative nOe at He, while a short irradiation time of Ha allows only the direct positive nOe effects of Ha on He to be recorded. 

Fig. 1.3 Effect of a pulsed magnetic field zero. During the gradient pulse, the field gradient of strength g on the phase of a signal becomes position-dependent and a phase shift contribution originating from spins at position is accumulated that is proportional to t. Prior to the gradient pulse, all spins position t and time t. After the gradient pulse...

The solvent dependence of the reaction rate is also consistent with this mechanistic scheme. Comparison of the rate constants for isomerizations of PCMT in chloroform and in nitrobenzene shows a small (ca. 40%) rate enhancement in the latter solvent. Simple electrostatic theory predicts that nucleophilic substitutions in which neutral reactants are converted to ionic products should be accelerated in polar solvents (23), so that a rate increase in nitrobenzene is to be expected. In fact, this effect is often very small (24). For example, Parker and co-workers (25) report that the S 2 reaction of methyl bromide and dimethyl sulfide is accelerated by only 50% on changing the solvent from 88% (w/w) methanol-water to N,N-dimethylacetamide (DMAc) at low ionic strength this is a far greater change in solvent properties than that investigated in the present work. Thus a small, positive dependence of reaction rate on solvent polarity is implicit in the sulfonium ion mechanism. 

On the more positive side, caffeine has been shown not only to have positive mood effects under some circumstances, as discussed above, but more specifically to increase self-reported happiness and feelings of pleasure. This hedonic effect may, however, occur only at relatively low doses.42 More generally, any dose under 300 mg may, depending on other conditions contributing to arousal, promote happiness27 and can even cause euphoria-like symptoms in animals.235 As the dosage increases above 300 mg, levels of contentedness decrease.175... 

Both time- and position-dependent concentration functions can be dealt with by the theory of stochastic processes (Bohacek [1977]). Time functions playing a role in process analysis can be assessed not only by means of information amount M(n)t but also - sometimes in a more effective way -by means of the information flow, J, which is generally given by... 

Irradiation in some cases accelerates the exchange reaction (the positive photocatalytic effect) and in others slows it down (the negative photocatalytic effect). The sign and absolute magnitude of the effect depend on the conditions of experiment and on the past history of a specimen. 

Dopamine produces dose-dependent hemodynamic effects because of its relative affinity for cq-, /Jr, /J2-, and Dr (vascular dopaminergic) receptors. Positive inotropic effects mediated primarily by / -receptors become more prominent with doses of 2 to 5 mcg/kg/min. At doses between 5 to 10 mcg/kg/min, chronotropic and -mediated vasoconstricting effects become more prominent. Especially at higher doses, dopamine alters several parameters that increase myocardial oxygen demand and potentially decrease myocardial blood flow, worsening ischemia in some patients with coronary artery disease. 

Figure 14 shows the circular dichroism spectra for the LB films of p-CDNH C12-H25 including Naph-SOsNa molecules under the initial surface pressure of 30 mN/m. Different induced circular dichro-isms are clearly observed at1 Bbband of naphthalene, depending on the substituted position the negative and positive Cotton effects occur for 1 - and 2-Naph-SOaNa included in the cavity of the CD... 

In addition to modification of calcium influx/efflux, other mechanisms like inhibition of slow inward current [363] and nickel-calcium exchange [364] have been suggested to explain the positive inotropic effects of nickel. Moreover, these inotropic effects might be mediated by an action of nickel on the outside surface of the cardiac cell membrane, where nickel inhibits the ATP-dependent component of the calcium extrusion, thereby causing contraction-enhancement [364, 365],... 

Percolation models differ from the zone-refining model essentially by the absence of mixing in the liquid, giving the liquid position-dependent properties. A simplified account of these models was described in Chapter 8. We will now provide a reasonably comprehensive account which may prove useful to the demanding reader, and then examine some properties of the chromatographic effect in a simple configuration. Let

open volume porosity of the medium, pso, and pliq the density of the solid matrix and melt, respectively, vliq the liquid velocity relative to the matrix, and Cso, and CHq the concentration of element i in the matrix and melt, respectively. Let us rewrite equation (8.3.14) as... 

Another interesting chiral chain end effect is exhibited by the helical polymer block co-polymer, poly(l,l-dimethyl-2,2-di-/z-hexylsilylene)- -poly(triphenylmethyl methacrylate), reported by Sanji and Sakurai (see Scheme 7) and prepared by the anionic polymerization of a masked disilene.333 The helical poly(triphenylmethyl methacrylate) block (PTrMA) is reported to induce a PSS of the same sign in the poly(di- -propylsilylene) block in THF below — 20 °C, and also in the solid state, by helicity transfer, as evidenced by the positive Cotton effect at 340 nm, coincident with a fairly narrow polysilane backbone UV absorption characteristic of an all-transoid-conformation. This phenomenon was termed helical programming. Above 20°C, the polysilane block loses its optical activity and the UV absorption shifts to 310 nm in a reversible, temperature-dependent effect, due to the disordering of the chain, as shown in Figure 45. 

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