Chemistry Reference
In-Depth Information
As shown in Figure 13.10, we often functionalize dendrimers using sequential
addition of isothiocyanates. The degree of dendrimer functionalization with each
component is dictated by the number of equivalents of each isothiocyanate that are
added. Characterization is performed using matrix assisted laser desorption ionization
(MALDI) time of flight mass spectrometry (MS). Although MALDI provides accu-
rate information regarding the number of end groups that have been added, MS (and
NMR) cannot provide information about the relative locations of dendrimer end
groups. EPR spectroscopy of spin-labeled dendrimers provides this information.
The synthesis strategy for functionalization of the G(4)-PAMAM dendrimer with
4-isothiocyanato 2,2,6,6-tetramethylpiperidine N-oxide (NCS-TEMPO) is shown in
Figure 13.15. We chose to partially functionalize the dendrimer with a spin label
while leaving the remaining terminal amino groups unreacted and to add primary,
secondary, tertiary, and aromatic and carbohydrate isothiocyanates as the second
coupling partner. The degree of dendrimer functionalization with each component
was controlled by using different equivalents of isothiocyanates and was verified
using MALDI (Samuelson et al. 2004; Walter et al. 2005).
The EPR spectra for the series of dendrimers bearing varying amounts of TEMPO
but with the remaining PAMAM primary amines unreacted are provided in
Figure 13.16. Dendrimers bearing only a few nitroxides have considerably sharper
EPR spectra than dendrimers with higher nitroxide loadings. This is because spin
labels that are close to one another in space induce significantly more line broadening
in their EPR spectra than the isolated spins. Placing more spins on the dendrimer
forces them to reside in proximity, causing an increase in line broadening.
Line-broadening effects for nitroxide-functionalized dendrimers that have
unreacted primary amines and line-broadening effects when a second isothiocyanate
is reacted with the dendrimer do not vary significantly. One clear indication of
Figure 13.15 Synthesis of 2,2,6,6-tetramethylpiperidine N-oxide (TEMPO) functionalized
dendrimers.
