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Figure 11.1 The amphiphilic self-assembly of lipids into bilayers.
1999; Hecht and Frechet 2001a; Kofoed and Reymond 2005; Darbre and Reymond
2006) and catalysts (O'Connor 1987; Lee et al. 1994; Piotti et al. 1999; Schmitzer
et al. 1999; Liu et al. 2000; Hecht and Frechet 2001b; Kono 2002; Murugan et al.
2004; Ambade et al. 2005; Yin et al. 2007), drug delivery vehicles (Liu et al.
2000; Kono 2002; Boas and Heegaard 2004; Al-Jamal et al. 2005; Ambade et al.
2005; Gillies and Frechet 2005; Lee et al. 2005), mediators of DNA transfection
(Choi and Baker 2005; Dufes et al. 2005; Luo et al. 2006; Pietersz et al. 2006;
Gajbhiye et al. 2007; Paleos et al. 2007), and building blocks for biomaterials for
tissue repair (Grinstaff 2002). Many of these areas have been the subject of detailed
reviews in the past and will not be addressed in detail here. In this chapter, we specifi-
cally focus on the impact of amphiphilicity on the structure and self-assembly of den-
drimers, dendrons, and their conjugates in water.
11.2. STRUCTURE
11.2.1. Unimolecular Micelle Analogy
Arborol Dendrimers.
The spherical topology of dendrimers resembles the size
and shape of the Hartley model (1936) of a micellar aggregate formed by surfactant
molecules (Fig. 11.2; Tomalia et al. 1990). Micellar structures have a dynamic,
spherical structure possessing a close-packed, solvent-incompatible core surrounded
by an open, solvent-compatible layer.
This structural model segregates the polar and nonpolar regions by sequestering
the apolar regions within a spherical aggregate displaying polar groups on the
surface to interact with solvent. This resemblance was noted in some of the early
reports on dendrimer chemistry in the mid-1980s. For example, Newkome's group
noted that a hydrophobic dendrimer decorated with polar terminal groups could be
considered as a covalently stabilized, nonequilibrating “unimolecular micelle”
(Kim and Webster 1990) having a critical micelle concentration (CMC) of zero
(Newkome et al. 1985). In 1986 Newkome and coworkers reported an “arborol”
dendrimer based on tris(hydroxymethyl)aminomethane displaying polar hydroxyl
groups at the periphery (Fig. 11.3a; Newkome et al. 1986).
Spherical aggregates (
200-
˚
diameter) of these dendrimers were formed in
aqueous media, and the dendrimers exhibited a nonzero CMC of 2.02 mM. This
