Reassociation process

The narrow molecular weight distributions accomplished by the supported catalysts were attributed to the absence of any organoaluminium co-catalyst dissocia-tion/reassociation processes at the heterogenized active neodymium centers. Furthermore, the order of the grafting sequence seemed to have minor implications for the catalyst performance. Control experiments have been conducted to explain the lower activity [0.9 (47) and 1.1 kg-PBD molNd h (48)] of the supported neodymium catalyst. Accordingly, an increase of the catalyst concentration (48) and use of a nonporous silica support (49) suggested that monomer diffusion and accessibility of the Nd centers are limited by the relatively small mesopores [dp = 2.4 (47) and 2.5 nm (48), after grafting]. 

Huppertz and Smiddy (2008) demonstrated that application of high pressure (250-300 MPa) led to an initial rapid micellar disruption this was followed by a partial reversal of the high-pressure-induced reassociation of micellar fragments. Partial internal cross-linking of casein micelles by transglutaminase prior to pressure treatment considerably slowed down both the disruption and reassociation processes. 

Computation in the gas phase did not locate a concerted TS, but indicated instead that the reactions proceed by dissociation-reassociation. The reassociation process has no barrier, whereas the dissociation has a very small one (2.4-2.6 kcal/mol). For the oxy anion, inclusion of a Li+ counterion resulted in a concerted process with a barrier of about 12 kcal/mol. Since this more closely approximates solution conditions, it suggests that a concerted mechanism is feasible in solution. The dissociation mechanism is favored for both nitrogen and carbon. 

Stereochemical non-rigidity has been observed in the platinum(0) diene complex H2C=CHSiMe2)20 Pt(PBu 3)] and the activation parameters have been measured268. it was concluded that the vinylsiloxane ligand was undergoing a facile dissociation-reassociation process, although a related non-dissociative mode involving rotation about the alkene C-C axis could not be completely dismissed. 

The reassociation process is dependent on various factors. We will emphasize here those aspects related to more practical situations such as short oligonucleotide annealing and/or hybridization, especially in solution at neutral pH. The factors involved in other types of hybridization, e.g., filter (or membrane) hybridization and in situ hybridization, are similar, at least qualitatively, to those for hybridization in solution. 

Fig. 1. Two models to describe the process of transcription through nucleosomes. The spooling model is taken from Studitsky et al. [89]. The RP (RNA polymerase) is shown to cause octamer displacement from the DNA that is being transcribed. The octamer is transferred to the DNA that was previously transcribed which occurs in a series of eight steps. The disruptive model is taken from van Holde et al. [3]. The octamer is shown to be disrupted in a series of steps (A-E) in which the two H2A, H2B dimers are displaced by the RNA polymerase and subsequently shown to reassociate after the polymerase has passed.

The model of Fig. IB is taken from a review by van Holde et al. [3] which I refer to as the disruptive model. In this model the polymerase causes conditions (step A) which promote not only the displacement of the entry site H2A, H2B dimer from DNA, but also from the H3, H4 tetramer (step B). As a result of this disruption, the polymerase is free to transcribe through the tetramer alone without a general displacement from its associated DNA (step C). The H2A, H2B dimer is now free to reassociate to the vacated entry site (step D) to re-establish contacts with both the DNA and the H3, H4 tetramer. As transcription proceeds into the exit site H2A, H2B dimer, these proteins are now displaced from both the DNA and the H3, H4 tetramer in a similar manner as the entry site H2A, H2B dimer (step E). A positive feature with regard to this model is that by displacement of H2A, H2B, the polymerase is able to transcribe the DNA with half the histones displaced prior to transcription. Therefore both models, spooling and disruptive , describe mechanisms which would favorably enhance the process of transcription. Support for the disruptive model comes from the substantial in vivo information which suggests that nucleosomes undergo substantial disruption during transcription, as was described in the previous section. Of particular note are those observations which indicate that a discrete population of H2A, H2B... 

Starch retrogradation has been used to describe changes in physical behavior following gela-tinization. It is the process that occurs when starch molecules reassociate and form an ordered stmcture. Under favorable conditions, a crystalline order appears and physical phase separation occurs. 

Complete denaturation of DNA leads to separation of the two complementary strands. If a solution of denatured DNA is cooled quickly, the denatured strands remain separated. However, if the temperature is held for some time just below Tm (a process known as annealing), the native double-stranded structure can be reformed. An important tool for studying DNA has been the measurement of the kinetics of reassociation of separated strands of relatively short DNA fragments.72 556 557... 

Maltodextrins may be manufactured either by acid or by acid-enzyme processes. Maltodextrins produced by acid conversion of starch from dent com contain a high percentage of linear fragments, which may slowly reassociate into insoluble compounds causing haze in certain applications. 

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