Asymmetrical branching

Important physical property subtleties were noted within the dendrimer subset. For example, dendrimers possessing asymmetrical branch cells (i.e. Den-kewalter type) exhibit a constant density versus generation relationship (Figure 1.20). This is in sharp contrast to symmetrical branch cell dendrimers (Tomalia-type PAMAM) that exhibit a minimum in density between G = 4 and G = 7 (NH3 core) [48, 96]. This is a transition pattern that is consistent with the observed development of container properties described in Figure 1.21. 

Figure 1.20 Comparison of densities as a function of generation for (A) asymmetrical branch cell in DenkewaIter-type dendrimers, (B) symmetrical branch cell in Tomalia-type dendrimers ([densities calculated from experimental hydrodynamic diameters and theoretical, D.A. Tomalia, M. Hall, D.M. Hedstrand, J. Am. Chem. Soc., 109,1601 (1987))...

Fig. 35. Dependence of density on generation for symmetrically branched and asymmetrically branched dendrimers. amu = atomic mass unit. , Denkewalter dendrimers (unsymmetrical branch cells) , Starburst PAMAM dendrimers (symmetrical branch cells)...

Enamides can also be used as precursors for the asymmetric synthesis of amino acids via hydroformylation. Thus, asymmetric branched hydroformylation of vinylsuccinimide and vinylphthalimide upon further oxidation leads to, V-protccted amino carboxylic acids such as alanine. Branched products are obtained with high regioselectivities and good to high optical yields of up to 96% (see Table 7)125. 

Felix, A.M. and Bandaranayake, R.M., Synthesis of symmetrically and asymmetrically branched PEGylating reagents. Theoretical and Applied Science, Journal of Peptide Research, 63(2), 85, 2004... 

P. Burda, C.A. Jakob, J. Beinhauer, J.H. Hegemann, and M. Aebi, Ordered assembly of the asymmetrically branched lipid-linked oligosaccharide in the endoplasmic reticulum is ensured by the substrate specificity of the individual glycosyltransferases, Glycohiology 1999, 9, 617-625. 

Most of the later proposed secondary deposition models use symmetrical morphometric model structures (e.g., 121,125,127-129,144), whereas some others are based on a system of asymmetrically branching airways (e.g., 130,132-134,145-147). Various simulations neglect the existence of alveoli, whereas others add all the alveoli to the last airway generation or add increasing numbers of alveoli to consecutive airway generations within an acinus. However, the influence of using different morphometric data sets is small, and calculated total or regional deposition does not vary considerably from one model to another (see Fig. 6 115,148-150). 

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