Dendrimers in pharmacy
Dendrimers are like unique molecular architecture with repetitively hyper branched molecules[14,15].
The name dendrimer is originated from Greek words dendron means "tree" and meros indicates "part." A unique difference between conventional linear polymers and dendrimers is that a linear polymer consists of long or short chains of molecules which are revolving like coils and entanglement with each other. A dendrimer molecule consists of a chain of molecules that branched out to from a tree like structure originated from a common center without crisscrossing between each molecule. A dendrimer is typically symmetric around the core, and often adopts a spherical three-dimensional morphology. Structural perfection, mono-dispersibility and highly symmetry are the important features of nano particles
The basic properties of dendrimers molecules are highly governed by the functional groups present on the molecular surface. However there are various examples of dendrimers where the functional group is present internally.[16-18] The dendrimer molecule can be modified to make it more water soluble by introducing charged species or hydrophilic groups in the outer shell of the structure. Other important properties of dendrimers in term of pharmaceutical application include toxicity, crystallinity, tecto-dendrimer formation, and chirality.
Figure 3 - Dendrimer architecture in two(a) and three dimensions(b)(18)
Dendrimers are also classified by generation, which refers to the number of repeated branching cycles that are performed during its synthesis. For example if a dendrimer is made by convergent synthesis (see below), and the branching reactions are performed onto the core molecule three times, the resulting dendrimer is considered a third generation dendrimer. Each successive generation results in a dendrimer roughly twice the molecular weight of the previous generation. Higher generation dendrimers also have more exposed functional groups on the surface, which can later be used to customize the dendrimer for a given application
The conventional polymers are mostly linear in their size and shape. The linearity of these conventional polymers is generally random in nature. The molecular mass of these linear molecules are also not constant and vary randomly. The size and molecular mass of a particular dendrimers is constant and can be controlled during the process of synthesis. Dendrimers are monodisperse macromolecules characterized by particles of uniform size in a dispersed phase. Because of their unique molecular architecture, dendrimers exhibits some significantly improved physical and chemical properties when compared to conventional linear polymers. When dendrimer goes into the solution it form a a tightly packed ball. This packed ball has an immense impact on their rheological properties.(Fig4)
Fig 4:Physical behaviors and rheological properties dendrimers in solution
Lower generation dendrimers are large enough to be spherical but do not form a tightly packed surface, have enormous surface areas in relation to volume(Fig 5).
Fig 5:Structural difference between lower generation and higher generation of Dendrimers
In contrast to linear polymers the intrinsic viscosity of dendrimer solutions does not increase linearly with mass but shows a maximum at a specific generation and then begins to decline. Such behaviors is unlike that of linear polymers. This is likely to be because of the way in which dendrimer shape changes with generation, i.e. lower generations adopt a more open planar-elliptical shape with transition to a more compact spherical shape for higher generations. The presence of many chain ends is responsible for high solubility and miscibility and for high reactivity. In the structure of dendrimer (Fig.6)the molecular density is theoretically highest in the periphery of the dendrimers. It has been suggested that back folding of the terminal branches leads to a more uniform or even reverse density profile (Fig.6). In nature tree-like structures have ability to maximize the exposed surface area, e.g. to maximize the light exposure/number of leaves of a tree(Fig 6)
Fig 6:Tree like structure of dendrimers
In a similar fashion dendritic architecture creates molecules where a large proportion of the groups are exposed at the surface and which can have very high molecular surface to volume ratios (up to 1000 m2/g). The presence of numerous terminal groups in dendrimers improves multiple simultaneous various interactions of surface groups with the solvent, surfaces or other molecules and, as a consequence, dendrimers tend to show high degree of solubility, reactivity, and intense binding.
14.Vogtle, Fritz / Richardt, Gabriele / Werner, NicoleDendrimer Chemistry Concepts, Syntheses, Properties, Applications 2009 ISBN 3-527-32066-0
16. Bifunctional Dendrimers: From Robust Synthesis and Accelerated One-Pot Postfunctionalization Strategy to Potential Applications P. Antoni, Y. Hed, A. Nordberg, D. Nystrom, H. von Holst, A. Hult and M. Malkoch Angew. Int. Ed., 2009, 48 (12), pp 2126-2130doi:10.1002/anie.200804987 PMID 19117006
18.LauraMazzola.Nature Biotechnology 21, 1137 - 1143 (2003).doi:10.1038/nbt1003-1137