Nucleation polymerization

Figure 3 Fraction of material in the polymerized state, f, as a function of the mass action variable X relative to its value X at the half-way point f =1/2. Indicated are predictions for the isodesmic and the self-catalyzed nucleated polymerization models. Activation constant of the nucleated polymerization Ka = 10-4.

Figure 6 Examples of compounds that in selective solvents produce biomimetic supramolecular polymers that depending on the conditions exhibit a co-operature intramolecular ordering transition from random to highly ordered helical stacks or a nucleated polymerization transition from oligomeric prenuclei to very long, helical polymeric objects (ten Cate and Sijbesma, 2002 Dankers and Meijer, 2007 van Gorp et al., 2002 Brunsveld, 2001 Jonkheijm, 2005 Hirschberg, 2001).

Nucleated polymerization and other models of fibril formation... 

The actual mechanism of self-association of monomeric protein into amyloid is complex and three mechanisms of stmcture conversion have been proposed (32) In templated assembly, a monomeric native state peptide binds to an existing nucleus. Upon binding, there is a change in the secondary stmcture of the monomer as it is added to the growing chain. Monomer directed conversion involves the presence of a misfolded monomer that templates the stmcture conversion of a native monomer, followed by disassociation and chain formation. The third model is nucleated polymerization, which is the most widely accepted model for the fibril growth. 

The decomposition products of CBA should not affect the rates of nucleation, polymerization, cross-linking, and decomposition of CBA. 

Fig. 5 Models of prion replication, (a) The template assistance model predicts that a PrPSo monomer is more stable than PrPc, but is kinetically inaccessible. In the rare event that a PrPSo monomer is created spontaneously (or provided exogenously), it can template the misfolding of another PrPc molecule by direct interaction. The dashed line shows that the newly created PrPSc monomer can act as another seed to formation of PrPSc. (b) The nucleation polymerization model predicts that barrier to prion protein conversion is the formation of a nucleus in which the protein adopts a PrPSo-like structure. The formation of such a low order aggregate is not favored however, once it has formed, polymerization from a pool of PrPc molecules can take place efficiently. Fragmentation of the polymer increases the number of ends for the recruitment of PrPc monomers...

The 7-Tubulin Ring Complex Nucleates Polymerization of Tubulin Subunits... 

The following fundamental aspects of the colloid chemistry of silica are briefly reviewed in this chapter nucleation, polymerization, and preparation stability of sols surface structure characterization methods sol-gel science gels and powders and uses of silica sols and powders. Silica in biology is not within the scope of this book. Scientists working in this area should soon put together a protocol covering progress done since the publication of Iler s book. 

The classic silica aquasols 5-100 nm in particle diameter are prepared by nucleation, polymerization, and growth in aqueous systems. The particle size range can be extended to at least 300 nm by autoclaving. Silica organosols can be obtained by transferring the aquasols to an organic solvent. 

B. A. Keiser s contribution to this book (the introduction to the section Preparation and Stability of Sols ) constitutes an excellent introduction to silica nucleation, polymerization, and growth in both aqueous and alcoholic systems for the preparation of silica sols. Yoshida s chapter (Chapter 2) focuses on industrial development in the preparation of monodisperse sols from sodium silicate and predicts further progress in the development of silica sols that have shapes other than spherical, such as elongated, fibrous, and platelet. Colloidal silica particles with these shapes show novel properties and open the possibility of new industrial applications. 

Silica Nucleation, Polymerization, and Growth Preparation of Monodispersed Sols... 

Fig. 3. Mechanistic models for formation of PrP-res from PrP. In the heterodimer and autocatalytic nucleated polyerization models, the conformational conversion of PrP to PrP-res is rare unless catalyzed by contact with monomeric or polymeric PrP-res, respectively. In the noncatalytic nucleated polymerization model, the conformational interchange is rapid, but the PrP-res conformer is poorly populated unless stabilized by binding to a preformed, stable PrP-res polymer. See text for further explanation. The wavy lines in the PrP-sen and PrP-res structures designate the flexibly disordered portion of the structure (usually N-terminal residues 23 to - 89), which remains sensitive to PK after conversion to PrP-res.

In the nucleated polymerization type of model, oligomerization/polymerization of PrP is necessary to stabilize PrP-res sufficiently to allow its accumulation to biologically relevant levels. Spontaneous formation of nuclei or seeds of PrP-res is rare because of the weakness of monovalent interactions between PrP molecules and/or the rarity of the conformers that polymerize. However, once formed, oligomeric or polymeric seeds are stabilized by multivalent interactions (Jarrett and Lansbury,Jr., 1993). 

In their various permutations, the two types of models can overlap. For instance, autocatalysis (or templating) of the conformational change in PrP by PrP-res may be a feature of either the heterodimer or nucleated polymerization models (Fig. 3). However, the nucleated polymerization could also occur if PrP rapidly interchanges between high a-helix and... 

Kinetic analyses in the absence of denaturants provided evidence that the conversion process can be separated into two stages, first the binding of PrP-sen to PrP and then a slower conversion of the bound PrP-sen to PrP-res (Bessen et al, 1997 DebBurman et al, 1997 Horiuchi et al, 1999). These observations, and the formation of amyloid fibril polymers by PrP-res, are consistent with an autocatalytic or templated nucleated polymerization mechanism. However, the fact that PrP-res usually induces the conversion of only substoichiometric quantities of PrP-sen in current cell-free reactions makes the reaction less continuous than typical nucleated polymerizations of proteins or peptides. This may be a technical problem rather than a fundamental limitation of the reaction mechanism. On the other hand, since PrP-res forms continuously for long periods of time in vivo, there may be important elements of the mechanism, such as cofactors or microenvironments, that remain to be elucidated (DebBurman et al, 1997 Saborio etal, 1999). 

The third model, nucleated polymerization (NP) (Jarrett and Lans-bury, 1993), predicts that the S- and A-states are in equilibrium in solution, but the Estate is predominant. Soluble A-state protein is stabilized by association with assembled A-state complexes. Thus, the rate-limiting step is formation of a polymerization-competent surface or nucleus rather than conformational conversion. 

SILICA NUCLEATION, POLYMERIZATION, AND GROWTH PREPARATION OF MONODISPERSE SILICA SOLS... 

The nucleation, polymerization and growth, and preparation of monodisperse sols is a focal point of research for many reasons. The nature of our understanding and the ease of formation make the sHica system ideal for studying nucleation and growth of particles in a disperse state. SHica offers many diverse routes to similar products, allowing for customizing of product properties. 

This section contains chapters by Yoshida, Ramsay et al., van Blaaderen and Vrij, Arriagada and Osseo-Asare, Kozuka and Sakka, and Healy. These chapters provide an overview of work in sihca nucleation, polymerization and growth, and preparation of monodisperse sols in a variety of systems. The work presented covers research areas currently under investigation throughout the world. The subjects include aqueous and nonaqueous systems, sUicates and alkoxides, and acid- and base-catalyzed systems and can serve as a basis for understanding sihca nucleation, polymerization, and growth of monodisperse sols. 

---