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porphyrins. Notably, examples of porphyrin dendrimers based on 5,15-diarylporphyrins
are well known [62,63].
14.5 POLYGLUTAMIC PD PORPHYRIN DENDRIMERS
Polyglutamic Pd porphyrin dendrimers [64-66] made up the first generation of
dendritic oxygen probes. To practitioners in field these probes are known as Oxyphors
R2 and G2, which over the years have been used in many biological studies [67]. The
idea to use dendritic encapsulation to protect the triplet states of Pd porphyrins from
oxygen quenching was inspired by the pioneering work of Aida et al. [68], who
showed that dendritic wedges, attached to the meso-aryl groups on Zn tetraarylpor-
phyrin, were capable of “caging” the core and shielding it from quenchers of
fluorescence. As we began working on the synthesis of polyglutamic Pd porphyrin
dendrimers, reports from the Diederich group [62,63] revealed that Newkome-type
poly(ester-amide) dendrons, which are similar to polyglutamates in composition,
indeed created hydrophobic local environments around encapsulated Fe porphyrins in
aqueous solutions. Approximately in parallel with our first publication [64], Balzani
and coworkers reported attenuation of oxygen quenching of luminescence in a series
of dendritic Ru bypyridines [69], confirming that dendritic encapsulation indeed
could be a viable way to build phosphorescent oxygen probes with controllable
quenching parameters.
The choice of
L
-glutamic acid as a building block was largely motivated by its
accessibility and biological compatibility. Dendritic polyglutamates resemble in
composition heme-proteins, and yet are unlikely to be enzymatically degraded or
targeted by the immune system in vivo. On the other hand, even if partial decom-
position of polyglutamates were to occur, glutamic acid monomers are unlikely to
show toxicity. In addition, ease of modification of peripheral carboxyls with various
functional groups was attractive from the point of view of fine-tuning the solubility of
the construct, depending on the intended application.
Although convergent synthesis of polyglutamic dendrons had been already
published by Twyman et al. [70], in our original work we followed the divergent
path. On the one hand, the presence of strongly colored porphyrin cores facilitated
chromatographic purifications of intermediate dendrimers; on the other hand, it was
unclear if higher generation dendrons (
3) would be accessible by the convergent
method. Later, however, at least for preparation of lower generation dendrimers [71],
we switched entirely to the convergent route, especially after developing a high-yield
synthesis of gen 2
3
triglutamic dendron using the CDMT coupling chemistry [72].
The synthesized polyglutamic Pd porphyrin dendrimers [65] are depicted in
Figure 14.5. The compounds are described by the general formula PdP-(Glu
n
OH)
4
,
where n
¼
>
1-4 (dendrimer generation) and Glu
n
OH is the glutamic layer. Further
3
Here and throughout the text we use abbreviations “gen 1,” “gen 2,” etc., to designate dendritic
generations.
