Micelles dendritic

Recently, several articles have been published dealing with water soluble dendrimers, due to their outstanding structure and properties which are similar to natural micellar systems, e. g., liposomes. Scheme 17 shows such an example of a dendritic micelle containing a hydrophobic inner core surrounded by a hydrophilic layer of carboxylate groups. The most important difference to natural micelles is that dendritic micelles are covalently bound structures and not dyn-... 

In contrast to dendrimers built up from aliphatic chains, polyphenylene dendrimer micelles possess shape and size persistent cavities due to their rigid scaffold which strongly depends on the type of dendrimer. In this case a selective incorporation of guest molecules, e.g., fluorescent dyes, should be possible, dependent on the size of the guest molecule and the cavity of the host. The non-covalent uptake of dyes with an appropriate size thus allows the investigation of their interactions within the dendritic micelle. In our case we made the second-generation polyphenylene dendrimer 48, which bears 16 carboxy-functions at the periphery, by starting from a tetrahedral core and an appropriately... 

To estimate how many dye molecules fit into the dendritic micelles, UV-titra-tion experiments have been employed. In comparison with the spectra of a pure pinacyanol chloride solution in water, the peaks of the absorption maxima of the dye in the presence of the dendrimer are shifted bathochromically due to solvatochromic effects, which indicates the incorporation of the dye within the branches of the dendrimer. At dye-to-dendrimer molar ratios higher than 4 1, in addition to the bathochromic shifts, hypsochromically shifted peaks start to appear, indicating that the dendrimer is not incorporating further dyes. We interpret this as an incorporation of up to four dyes within the branches of the dendrimer. This observation correlates with the calculated available space within the dendrimer, obtained from the molecular simulations. Further studies of the interactions of the dyes within the dendritic micelle are in progress. 

Fig. 8. Structure of dendritic micelle with a porphyrin core, 33...

The structure and properties of water soluble dendrimers, such as 46, is, in itself, a very promising area of research due to their similarity with natural micellar systems. As can be seen from the two-dimensional representation of 46 the structure contains a hydrophobic inner core surrounded by a hydrophilic layer of carboxylate groups (Fig. 12). However these dendritic micelles differ from traditional micelles in that they are static, covalently bound structures instead of dynamic associations of individual molecules. A number of studies have exploited this unique feature of dendritic micelles in the design of novel recyclable solubilization and extraction systems that may find great application in the recovery of organic materials from aqueous solutions [84,86-88]. These studies have also shown that dendritic micelles can solubilize hydrophobic molecules in aqueous solution to the same, if not greater, extent than traditional SDS micelles. The advantages of these dendritic micelles are that they do not suffer from a critical micelle concentration and therefore display solvation ability at nanomolar... 

Aathimanikandan SV, Savariar EN, Thayumanavan S. Temperature-sensitive dendritic micelles. J Am Chem Soc 2005 127 14922-14929. 

Recently, we are also able to perform liquid-liquid extractions of water-soluble dyes from water to organic solvents like toluene using the unimolecular dendritic micelles as extractants. These guest-host properties of the modified poly(propylene imine) dendrimers make use of the basic nature of the interior of the dendrimer. Therefore, it is possible to transfer dyes like Bengal Rose into toluene. Although simple tri-octylamines are capable of transferring these dyes as well, it is evident from the experiments that unimolecular dendritic inverted micelles are much more selective, probably due to the shielding effect of the apolar shell [70]. 

It is also of interest to study the self-assembly of these inverted unimolecular dendritic micelles into vesicles in water at pH = 1 and at the air-water interface. In these vesicles, the dendrimers adopt a distorted conformation, far from spherical. These dendritic amphiphiles form a novel class of vesicle-forming surfactants [71] and introduce new perspectives towards a better understanding of the structure-property relation of amphiphiles on the one hand and dendrimers on the other hand. 

Figure 23 Schematic representation of the vesicle formation of the fifth generation inverted unimolecular dendritic micelles...

Figure 5. Preparation and structure of dendritic micelles, ((K02C)16-[G-4])2-[Cl, 20.

The concept of having layers of reactive functionalities or sites in the interior of dendritic macromolecules has been elegantly demonstrated recently by Newkome s group.30 The synthesis of dendritic micelles, containing layers of triple bonds to which metal clusters can be attached,... 

Stevelmans S, vanHest JCM, Jansen J et al (1996) Synthesis, cheu cterization, and guest-host properties of inverted unimolecular dendritic micelles. J Am Chem Soc 118 7398-7399... 

However, it is not only from an architectural viewpoint that dendrimers can be contrasted to organized assemblies. but micellar stabilization and organization of species in uncharacteristic environments are also suggested. Thus, stmctural attributes of branched frameworks ean be employed for such applications as aqueous solubilization of inherently water-insoluble species as well as molecular ordering based on noneovalent interaetions, such as //-bonding and ionic associations. The advent and current status of several dendritic micelles are herein ehronicled. 

D. Bhadra, S. Bhadra, N.K. Jain, PEGylated lysine based copoly-meric dendritic micelles for solubilization and delivery of arte-mether, J. Pharm. Pharm. Sci. 8 (2005) 467 82. 

Initially we studied the interactions of laccase with [G2]-PEG5k-[G2] in aqueous media. In all cases investigated the copolymer is added as dry powder to the buffered solution of the enzyme in order to direct it towards enzyme surface sorption instead of forming empty linear-dendritic micelles. 

Investigation of the possible applications of dendrimers has been limited by their lack of ready availability [14]. It is anticipated that dendrimers may be used as a polymeric matrix for catalysts, as a membrane for gas separation, and as part of the diagnostic kit for MRI since many groups can be added to the core or to the periphery. The unique structure and solubility characteristics of dendrimers suggest that their use as dendritic micelles may be important. In addition, they may form interesting and useful monolayers [14]. 

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