Vesicle polymerisation

Substrate retention and ion permeability are important features of vesicles. Hence, vesicles polymerised from phospholipids retain labelled glucose... 

Vesicle polymerisation. There are several options to achieve polymerisation in/or of vesicles (Paleos, 1990) ... 

After more than 20 years, Walde et al. (1994) returned in a way to coacervate experiments, although using other methods. Walde (from the Luisi group) repeated nucleotide polymerisation of ADP to give polyadenylic acid, catalysed by polynucleotide phosphorylase (PNPase). But instead of Oparin s coacervates, the Zurich group used micelles and self-forming vesicles. They were able to demonstrate that enzyme-catalysed reactions can take place in these molecular structures, which can thus serve as protocell models. Two different supramolecular systems were used ... 

The experiments were carried out using Ci4-phosphatidylcholine (PC) vesicles. The biochemical reaction which was planned to occur in the vesicles was the aforementioned RNA polymerisation reaction involving the enzyme polynucleotide phosphorylase (PNPase), which Oparin and co-workers had used many years ago in their work on coacervates. PNPase and added ADP then form oligonucleotides in the vesicles. 

Fig. 10.7 RNA synthesis in vesicles. Membrane permeability can be regulated by choosing the correct chain length of the fatty acids in the phospholipids. Short chains (a) make the bilayer so unstable that even large molecules such as proteases can enter the vesicle interior and damage the polymerase. Carbon chains which are too long (b) prevent the entry of substrate molecules such as ADR RNA polymerisation in the vesicle occurs only with C14 fatty acids (c)...

In the case of vesicles, there is an obvious attraction in the utilisation of these systems for delivery and targeting of molecules and there has been much interest in using vesicle systems for drug delivery. Attempts have also been made to polymerise vesicles to give them added structural rigidity and stability, but these efforts have not been particularly successful so far. 

The length of the microtubule cylinder may measure upto 10,000 A, but the diameter is usually 180-250 A. The polymerisation of tubulins to microtubules takes place at 37°C in presence of Mg ions, endogenous cofactors such as GTP and microtubule-associated proteins (MAPs). The depolymerisation of microtubules occurs at temperatures lower than 37°C and in presence of Ca ions. The assembly and disassembly of microtubules proceeds in a nucleated fashion and is associated with a number of cellular functions. The formation of microtubules are required to control various cell activities such as cytoplasmic movement, cell division, cell shape and substrate and vesicle transport etc. Thus, interruption of the microtubulin assembly by a chemotherapeutic agent would result in several cellular dysfunctions leading to death of the parasites. Several drugs are known to bind with tubulin and block its polymerisation into microtubules. This results in gradual disappearance of microtubules from the cells. Consequently cytoplasmic movement and transport of nutrients are disturbed. These abnormal conditions cause death of the cell [59,60]. 

The structure-activity importance of synthetic pyridinium-based polymers synthesised via the ring-opening metathesis polymerisation (ROMP) of norbornene has been reported these polymers were expected to directly affect the bacterial membrane [24-26]. The interaction of these polymers with a phospholipid membrane model has been investigated using the fluorescent dye, calcein, embedded in unilamellar vesicles. 

The second involves polymerising micelles or vesicles formed from a surfactant which is itself the monomer. This amounts to polymerising the micellar or vesicular envelope. 

Polymerisable surfactants constitute another important area of polymerisation in disperse media. Over the past fifteen years, a number of studies have been made of surfactant assemblages possessing a polymerisable group. The aim of these studies is to fix the structure of the initial assemblages in such a way as to obtain stable aggregates with controllable size, rigidity and permeability. These systems could be applied where the non-polymerisable assemblages had proven inadequate as a result of their limited lifetime. The studies concerned two types of self-assembled structures vesicles and micellar systems. 

Control of Reactivity in Polymerised Vesicles. Certain reactions can be controlled by means of vesicles. Depending on its chemical nature, a substrate will be localised at a preferred site on the vesicle. This site will differ from that of the transition state of the reaction product. Such a relocation of compounds during reaction can be useful in catalysis or for a product separation. 

When vesicles are polymerised, the interaction between substrate and vesicle is modified. We therefore expect different reactivities for monomer vesicles and their polymerised counterparts. This hypothesis has been borne out experimentally with regard to hydrolysis and aminolysis of nitrophenylesters. Polymerisation allows better control of reactivity by localising the substrate at different vesicular sites. 

Encapsulation of Colloidal Particles. Note also that polymerised vesicles constitute an ideal medium for the formation of small and uniform colloidal particles of metals or oxides. These can be used as catalysts. For example, ultraviolet irradiation of vesicles formed from polymerisable surfactants and containing potassium tetrachloroplatinate (K2PtCl4) results in the formation... 

Fig. 6.10. Different interactions between polymerised and non-polymerised vesicles [6.7]...

In the photochemical conversion of solar energy, it may prove possible to use colloidal semiconductors dispersed in polymerised vesicles. 

In this case, vesicles favour energy transfer between these two species and also oppose any back transfer between the reduced relay R and the oxidised sensitiser S+. Hence the utility of polymerised vesicles, which effectively immobilise the semiconductor. 

Fig. 6.11. Possible ways of releasing substances encapsulated in mixed polymerised vesicles [6.1]...

We should also mention the possibility of controlling delivery of encapsulated substances by successive polymerisation and depolymerisation reactions. For example, a programmed opening of partially polymerised mixed liposomes has been achieved through redox splitting of lipids containing sulfur-sulfur bonds -S-S-. The formation of stable and porous polymerised vesicles was confirmed by transmission electron microscopy. 

Topochemical effect. This is a consequence of the organisation of surfactant molecules induced by the micellar state. Although micelles are relatively labile aggregates and less organised than vesicles, topochemical effects may nevertheless influence the polymerisation process. 

Figure II - 39. Polymerisation of vesicles by means of UV-radiadon or AIBN.

Here Vp is the volume fraction of polymer (related to the conversion), X is the number average degree of polymerisation of the polymer, x is the Flory-Huggins interaction parameter between the monomer and the polymer, R is the gas constant and T the temperature. Um is the molar volume of the monomer, y is the particle-water interfacial tension and To is the radius of the unswollen micelles, vesicles and/or latex particles. [M]a is the concentration of monomer in the aqueous phase and [M]a,sat the saturation concentration of monomer in aqueous phase. Figure 3.3 shows the contributions of the different terms of Equation 3.10 to the Vanzo equation. For a more detailed discussion see also Section 4.2 and Figure 4.5. 

Recently, Jung et al. (2000) described the polymerisation in vesicles, leading to different types of morphologies including hollow particles. 

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