Starburst PAMAM

D. Zanini and R. Roy, Practical synthesis of starburst PAMAM a-thiosialoden-drimers for probing multivalent carbohydrate-lectin binding properties, J. Org. Chem., 63 (1998) 3486-3491. 

Electrophoresis is normally run in aqueous media, hence the analytes must be soluble in water. Presently only three types of water-soluble dendrimers have been successfully analyzed using gel electrophoresis techniques. The list includes Starburst PAMAM dendrimers [21], nucleic acid dendrimers [21] and poly(lysine) dendrimers [23, 24] (see Figures 10.2, 10.4 and 10.6). However, in each case appropriate water solubilizing terminal groups are required (i.e. -NH2, -OH or C02H groups) for suitable electrophoretic analysis. 

Bielinska, A., Kukowska-Latallo, J.F., Johnson, J., Tomalia, D.A. and Baker, Jr., J.R. (1996) Regulation of in vitro gene expression using antisense oligonucleotides or antisense expression plasmids transfected using starburst PAMAM dendrimers. Nucleic Acids Res., 24, 2176-2182. 

Starburst PAMAM dendrimers (Fig. 1), a specific class of commercially available dendrimers that have repeating amine/amide branching units, have drawn considerable interest in recent years due to their potential applications in medicine, nanotechnology, and catalysis [3-7]. These dendrimers are readily functionalized to terminate in diverse moieties such as primary amines, carboxylates, hydroxyls, or hydrophobic alkyl chains. Because dendrimer size and end groups can be varied, they are typically named by their generation (Gl, G2, etc.) and exterior functionality (- NH2, - OH). 

Antibodies have been coupled1271 to starburst (PAMAM) dendrimers these reagents offer an attractive approach to the development of immunoassays. These dendritic reagents are stable and retain full immunological activity - both in solution as well as when bound to a solid substrate their analytical sensitivity is equal to or is better than that of the established methods. The dendritic approach to radial partition immunoassays has been shown to possess the best traits of both homogeneous and heterogeneous immunoassay formats. 

CPK models of generations 0-5 (Fig. 27) show that these SPEIs are much more congested and compact than the Starburst PAMAMs. These models indicate that ideally branched generation 5 is forbidden due to dense packing as described and predicted by be Gennes and others [81, 82, 85, 86]. 

These methods were used extensively for structure verification of dendrimers prepared by the divergent initiator core method such as Starburst PAMAM [124] poly(ether) [82], and poly(ethylenimine) dendrimers [2], as well as poly(siloxane), poly(phosphonium), poly (ary lalkyl)ether, poly(arylene) and poly(arylester) dendrimers. In many cases, small-molecule model systems were used for process optimization, defect identification, and stoichiometry studies. 

Presently, defects of the type A and B are the only serious side-reactions encountered in the synthesis of Starburst PAMAM dendrimers (generation = 0-10) by the divergent initiator core method . Defects of the types C, D and E have been minimized to under 3-5 mole% per generation. Polydispersities (Mw/Mn) of 1.0007 have been routinely observed for Starburst PAMAM dendrimers (generation = 4 NH3 core) which has an empirical formula of C465H933N187093 and a calculated molecular weight of 10632, as verified by electrospray mass spectroscopy [169]. 

Fig. 32. 1 3C-NMR spectrum of Starburst PAMAM dendrimers of generation = 7 with assignments for ideal and defective branches (defective branch level ca. 5 mol%)...

Fig. 33. Electrospray ionization mass spectrum of a Starburst PAMAM (generation = 4). Note the last significant peak M = 10632 daltons (degree of polymerization = 93), corresponding to the magic number for ideal structure. The remaining labeled species correspond to M — x(l 14) (defects due to missing monomer units) and the unlabeled species which correspond to M — x(60) (defects due to macrocycle formation)...

Fig. 34. Size exclusion chromatograms (SEC) for Starburst PAMAM dendrimers (generations = 0-8) as a function of elution times (minutes)...

Starburst (PAMAM) (symmetrical branch cell) dendrimers... 

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)...

Clearly, this proposed congestion at the critical branched state M, should exhibit (1) sterically-inhibited reaction rates, (2) sterically-induced stoichiometry, and, quite possibly, (3) a critical phase change due to surface cooperativity (association). These phenomena have been observed experimentally for the Starburst PAMAM dendrimers and may prove to be diagnostic probes for this critical branched state in other dendrimer families. 

Fig. 36. Comparison of change in Starburst PAMAM dendrimer morphology (aspect ratios, J y) as a function of generation...

Fig. 37. Plot of surface area per terminal Z group, A2, as a function of generation for Starburst PAMAM dendrimers. Curves A and B assume 95% and 100%, respectively, of the theoretical number of surface groups...

Fig. 43. Titration curve for Starburst PAMAM dendrimers of generation = 1 with endpoints observed for tertiary and primary amino groups...

Sub-stoichiometric yttrium complexes of Starburst PAMAMs with N(CH2CH2COf)2 surface groups displayed intramolecular exchange reactions as determined by 13C-NMR spectroscopy and 90Y tracer experiments [166]. These and other complexation properties may serve as useful probes for the systematic characterization of dendrimer surfaces. 

Qin et al. (238) used starburst PAMAM dendrimers to transfer genes into a murine cardiac transplantation model. These star-burst dendrimers are a special class of its kind, which are highly branched spherical polymers... 

Figure 1.20. Synthesis of an amine-terminated polyamide dendrimer by a divergent route (the Starburst PAMAM system). Adapted from Frechet and Hawker (1996).

Acetylcholinesterase (AChE, type V-S from electric eels), butyrylcholinesterase (pseudocholinesterase from horse serum, E.C. 3.1.1.8), and glutaraldehyde were obtained from Sigma (St. Louis, MO). Organophosphorus hydrolase C 3.1.8.1 OPH, phosphotriesterase) was obtained as described previously (77). Amino-terminated Starburst (PAMAM) dendruner (generation 4) was obtained from Aldrich (Milwaukee, WI). ProbeOn Plus microscope slides were obtained from Fisher Biotech (Pittsburgh, PA). 

---