Pre-transitions

Lipid assemblies of the lamellar type, such as lipid bilayers, can feature a true phase transition in which the topology does not change. Upon cooling, the bilayer goes from the fluid phase to the gel phase. In the fluid phase, the acyl chains are disordered, in the sense that there is enough free volume around the chains to allow for chain conformation variations. In the gel phase, the acyl chains are more densely packed and believed to be ordered in an all-trans (straight) configuration. For very pure systems, at temperatures below this sharp gel-to-liquid phase transition, there are several other states and distinct transitions detectable (pre-transition, ripple phase, etc.). These phases will not be reviewed here. In biomembranes, many type of lipids (and other molecules) occur, and it is known that for this reason the gel-to-liquid phase transition is... 

Michael-aldol for 2-mercaptobenzaldehydes and maleimides. Use of catalyst 166 provided a variety of fused heterocycles in high yield and high enantiomeric ratios (Scheme 44). The authors propose that the chiral catalyst simultaneously activates the thiol and the maleimide via Brpnsted base and acid interactions. It was proposed that the pre-transition state arrangement of the catalyst and substrates determines the stereochemical outcome. 

Fig. 10 Ishikawa s proposed pre-transition state for the Michael addition of glycines to unsaturated esters...

Fig. 11 Kunz s hypothesized pre-transition state for the asymmetric hydrocyanation of imines promoted by a novel paracyclophane imine catalyst...

When a phase transition occurs from a pure single state and in the absence of wettable surfaces the embryogenesis of the new phase is referred to as homogeneous nucleation. What is commonly referred to as classical nucleation theory is based on the following physical picture. Density fluctuations in the pre-transitional state result in local domains with characteristics of the new phases. If these fluctuations produce an embryo which exceeds a critical size then this embryo will not be dissipated but will grow to macroscopic size in an open system. The concept is applied to very diverse phenomena ... 

The morphologies of the large ionic clusters observed in these simulations rather suggest free chain end folding to produce rudimentary lattice structure as a possible pre transitional mechanism. 

A further application of the coplanar cell configuration showed in Fig. 3.1(c) concerns the study of the time dependence of the photocurrent following carrier excitation by means of a short pnlse of illnmination. This transient photodecay technique enables the examination of the interaction of initially free carriers with varions localized states. In principle, the decay of photocnrrent measured in this manner should (in the absence of recombination effects and phenomena associated with drift close to the surface of a thin film) correspond to the behavior in the initial pre-transit regime of a TOF pnlse. Becanse it allows measurements to be performed on very thin films under conditions appropriate to their nse in many device applications, and because the photocurrent may be examined over several decades of time withont the complications associated with carrier extraction, the techniqne has become rather popular over recent years. 

Fig. 35. Scheme of the "ripple structure of bi layers of saturated phospholipids found between the pre-transition and main-transition of the membrane 86)... 

The value of the pre-exponential factor is very important in Frenkel s theory of pre-transition states and heterophase fluctuations [10]. However, simple qualitative arguments appear to show that the effects considered by... 

In3+, Sn4+) but these are less ionic in character than corresponding compounds of pre-transition metals. 

In solution, post-transition metals form stronger complexes than with pre-transition metals. Lower oxidation states... 

Phase transition is an important property of membranes. Below the phase transition temperature, lipids are tilted and highly ordered. They are in their solid or "gel" state. Increasing the temperature leads to a pre-transition, characterized by periodic undulations and straightening of the hydrocarbon chain. Further increase of the temperature causes the main phase transition. Above the main phase transition temperature, lipids are fluid or "liquid crystalline." Figure 3 shows the phase diagram for the interaction of water with a lipid as well as its inferred arrangements in a model membrane (5). Phase transitions in membranes and membrane models have been extensively studied by spectroscopic techniques and by differential scanning calorimetry. 

Figure 3. Schematic representation of a phospholipid-water phase diagram. The temperature scale is arbitrary and varies from lipid to lipid. For the sake of clarity phase separations and other complexities in the 20-99% water region are not indicated. Structures proposed for the phospholipid bilayers at different temperatures are shown on the right-hand side. At low temperature, the lipids are arranged in tilted one-dimensional lattices. At the pre-transition temperature, two-dimensional arrangements are formed with periodic undulations. Above the main phase, transitions lipids revert to one-dimensional lattice arrangements, separated somewhat from each other, and assume mobile liquid-like conformations.

In any case the spin-Peierls transition is driven by one-dimensional pre-transitional structural fluctuations [46]. Such fluctuations start to develop at some temperature TF above TsP. The effect of these critical fluctuations is to induce a local pairing of the spins which leads to an observable deviation of the magnetic susceptibility x below TF from the general Bon-... 

There are occasions when specimens exhibit a paramorphotic texture, i.e. one that reflects order inherited from the parent phase. At a molecular level, the transition between liquid crystalline phases typically occurs via a route that requires the minimum instantaneous rearrangement of molecules. Because textures are dictated by molecular order, the immediate post-transition texture may not easily be distinguishable from its pre-transition counterpart. Stable textures that are characteristic of the new phase may require a long time, sometimes months, to form. Transitions between highly otdered smectics are especially likely to favour paramorphoses. 

In the case of 53, a chair-like pre-transition state conformer 53a is thought to be involved, in which the bulky aryl and palladium groups are pseudo equatorial. This gives rise to the energetically most favourable equatorial conformer 53b, which is transformed into 54a (Scheme 5.6.16). Under identical conditions, in the presence of vinyltri-n-butyltin, the intermediate 53a is intercepted by vinyl transfer (Scheme 5.6.16) showing that this inter-molecular anion capture 54b is significantly faster than the 6-exo-trig cyclization-anion capture. 

Figure 8.7 Proposed pre-transition-state assembly for the catalytic hydrocyanation of N-allylbenzaldimine.

Remarkably, MCT describes well the key glassy features around the fi -relaxation for both G and G" without essentially any adjustable parameters the plateau Gp and minimum G values are taken from the experiments, whereas the additional mode coupling parameters (usually referred to as a, b, and B) assume constant values for all test systems [225,256]. It is also worth noting that MCT describes the pre-transitional approach to the glass for hard spheres [238], and it was successfully applied well within the glassy state for ultrasoft colloids [225]. 

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