Size exclusion chromatography dendrimers

Purify the derivatized dendrimer using gel filtration (size exclusion chromatography) on a desalting column or through use of ultrafiltration spin-tubes (for G-4 and above). For smaller dendrimers, the derivatives may be purified by repeated precipitation from a meth-anolic solution by addition of ethyl acetate, dioxane, or benzene. The SPDP-dendrimer may be dried by lyophilization (if in water or buffer) or by solvent evaporation in vacuo (if the precipitation method was used). 

Purify the modified dendrimer away from reactants and reaction by-products using dialysis or size exclusion chromatography. 

Purify the biotinylated dendrimer by diluting it with an equal volume of water and then using dialysis, ultrafiltration, or size exclusion chromatography. 

Purify the biotin-dendrimer using size exclusion chromatography on a desalting matrix or by use of ultrafiltration (e.g., centrifugal concentrators). 

Purify the DTPA-dendrimer using dialysis, size exclusion chromatography, or spin-tube concentrators having a molecular weight cutoff of 5,000 Daltons. 

The monodispersed nature of dendrimers has been verified extensively by mass spectroscopy, size exclusion chromatography, gel electrophoresis and electron microscopy (TEM). As is always the case, the level of monodispersity is determined by the skill of the synthetic chemist, as well as the isolation/purification methods utilized. 

It should be noted that discrepancies amidst available data may result from different methods of determination. For example, selected hydrodynamic radii of PAMAM dendrimers of Table 14.1 were obtained by two different experimental methods for generations 1 through 4 by dilute solution viscometry, while for generations 5 through 10 by size exclusion chromatography. 

To characterize dendrimers, analytical methods used in synthetic organic chemistry as well as in macromolecular chemistry can be applied. Mass spectrometry and NMR spectroscopy are especially useful tools to estimate purity and structural perfection. To get an idea of the size of dendrimers, direct visualization methods such as atomic force microscopy (AFM) and transmission electron microscopy (TEM), or indirect methods such as size exclusion chromatography (SEC) or viscosimetry, are valuable. Computer aided simulation also became a very useful tool not only for the simulation of the geometry of a distinct molecule, but also for the estimation of the dynamics in a dendritic system, especially concerning mobility, shape-persistence, and end-group disposition. 

For PAMAM dendrimers, the molecular dimensions were determined by size-exclusion chromatography and the dimensions of PPl dendrimers were determined by SANs data for the high-generation PPl dendrimers are not available. 

The architecture of hypeibranched polymers and dendrimers is connected with difficulties in determining molar mass. Many of the common characterization techniques—e.g. size exclusion chromatography (SEC)—used for polymers are relative methods where polymer standards of known molar mass and dispersity are needed for calibration. Highly branched polymers exhibit a different relationship between molar mass and hydrodynamic radius than their linear counterparts. 

In the case of ruthenium(II)-bis(terpyridine) dendrimers (Fig. 6.50) of stepwise increasing dendrimer size, an influence of dendrimer size on the reversibility of redox processes could be deduced from size-exclusion chromatography and from the electrochemical properties [95]. 

A chromatographic technique frequently used in polymer and dendrimer analysis is size exclusion chromatography (SEC) [13], which is often also called gel permeation chromatography (GPC). It is a straightforward method of determining the relative molar mass, the molar mass distribution, and the polydispersity index (PDI) with compatively modest consumption of material and time. 

SEC by itself is not an absolute MW determination method but the analysis of the elution peak has been used extensively for estimating the molecular purity of dendrimers. If the shape of the elution peak of a size exclusion chromatography experiment is Gaussian, the total dispersion, oT of the curve is given by the sum of squares [47] ... 

A related approach, termed dendrimer-supported combinatorial chemistry (DCC), has been disclosed by Kim et al. [23], It uses dendrimers as soluble supports (see Fig. 7). In this case the reactions are performed in solution and the dendrimeric intermediates are isolated/purified by size exclusion chromatography. The strategy was validated by the preparation of a 3 x 3 x 3 combinatorial library using the Fischer indole synthesis. 

Masamune and coworkers reported a divergent stepwise synthesis of siloxane dendrimers as shown in Scheme 15155. The third generation dendrimer was the largest obtained. Characterization techniques included 1H, 13C and 29Si NMR spectroscopy, mass spectroscopy and size-exclusion chromatography. 

The purification of all the dendritic intermediates was obtained by SEC (size exclusion chromatography), by simply charging the raw reaction mixtures in DMF or DMA (dimethylacetamide) on a Sephadex LFi-20 column and eluting the dendrimers with 15 mm runs. The intermediates were always obtained with FiPLC purity >90% and could be characterized by various analytical methods such as NMR and FiPLC/MS. 

These dendrimers have branch cell segments which are approximately one-half the dimensions of the PAMAM series (5 A versus 10 A) and possess chemical linkages which have much greater chemical and physical stability. A comparison of theoretical molar masses, number of terminal groups, CPK predicted diameters, and measured diameters (size exclusion chromatography, SEC) as a function of generation is shown in Table 3. 

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