Chemistry Reference
In-Depth Information
k
q
H
2
O
Mobile aqueous
phase
O
2
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
k
q
dend
O
O
O
O
O
O
O
O
Immobile
phase
Pd porphyrin
k
q
H
2
O
>
k
q
dend
Low density, but
restricted dynamics
FIGURE 14.10
Cartoon illustrating the effect of a hydrophobically folded dendrimer on
oxygen diffusion and phosphorescence quenching rate.
14.7 EFFECT OF THE DENDRIMER OUTER LAYER
As mentioned earlier, it is critically important that quenching properties of phospho-
rescent probes do not change in the presence of blood plasma proteins and other
biological macromolecules. Quenching constants of all carboxylate-terminated por-
phyrin dendrimers (Figure 14.8), however, were found to be highly sensitive to albumin.
For example, k
q
of gen 2 Fr
echet-type porphyrin dendrimer in albumin solution (2% by
mass) is five times lower (30mmHg
1
s
1
) than that in the albumin-free aqueous buffer
(151mmHg
1
s
1
).
4
On the contrary, quenching of porphyrin dendrimers modified
with oligoethyleneglycol residues (PEG350, average MW
350) appeared to be
unaffected by serum proteins. Thus, k
q
of the PEGylated analog of gen 2 Fr
ΒΌ
echet-
type porphyrin dendrimer was found to have the same value (130mmHg
1
s
1
)inthe
presence and in the absence of albumin [75].
It is important to distinguish between the effect of hydrophobic dendrons and the
effect of external PEG residues. While the latter also contribute to the protection from
quenching, especially at early dendritic generations, the net effect of PEGs is very
small compared to the shielding by dendritic branches. Moreover, the PEG effect
rapidly levels off with extension of linear chains. This notion was supported, for
example, by oxygen quenching measurements in a series of star-shaped amphiplilic
diblock copolymers with Pd porphyrin cores [79]. It was shown by this study that
4
The value of the quenching constant for PdP-(Fr
2
OH)
8
in the aqueous buffer was probably affected by
aggregation, as revealed by the broadened absorption spectra.
