Chemistry Reference
In-Depth Information
(a)
(b)
SO
3
Na
HO
2
C
CO
2
H
NaO
3
S
SO
3
Na
N
N
N
N
Pd
N
Pd
NaO
3
S
SO
3
Na
N
N
N
NaO
3
S
SO
3
Na
HO
2
C
CO
2
H
SO
3
Na
FIGURE 14.3
Simple water-soluble derivatives of phosphorescent Pd porphyrins used
originally as complexes with serum albumin for oxygen measurements by phosphorescence
quenching: Pd-meso-tetracarboxyphenylporphyrin (PdTCPP) (a) and polysulfonato derivative
of Pd tetraphenyltetrabenzoporphyrin (Green 2W).
In order to avoid the need for prebinding to albumin, phosphorescent chromo-
phores have to be made water soluble in the physiological pH range. Unfortunately,
porphyrins modified with simple ionic groups (SO
3
,CO
2
,NH
3
þ
) or neutral
hydrophilic residues, such as small oligoethyleneglycols [30], are still prone to
aggregation. In addition, when injected systemically, porphyrins, as well as other
small-molecule luminescent probes (e.g., Ru(bpy)
3
þ
-like complexes) [31], become
unevenly distributed throughout the tissue, where they bind to many biological
targets. Local environments of probe molecules bound to different proteins are highly
heterogeneous, resulting in unpredictable distributions of oxygen quenching con-
stants k
q
(Eq. 14.2). As a result, the response of the phosphorescence to oxygen cannot
be interpreted quantitatively.
The problem of keeping the diffusional accessibility of the phosphorescent
chromophore to oxygen in biological environments constant throughout the studied
object and the calibration unchanging, is by far the biggest challenge in the design of
phosphorescent oxygen probes. Moreover, this problem is inherent in all methods
relying on kinetics of oxygen diffusion. To overcome this difficulty, design of a
molecular oxygen sensor entails construction of a well-defined microenvironment
around the phosphorescent chromophore in order to isolate it from interactions with
other molecules, except for oxygen. Dendritic encapsulation [3,4] arguably provides
one of the most straightforward ways to construct monodisperse, well-defined
molecular jackets around luminescent chromophores [32,33].
14.4 PHOSPHORESCENT CORES OF DENDRITIC
OXYGEN PROBES
The key photophysical properties required for biological oxygen sensing are
strong absorption, preferably in the near-infrared region of the spectrum (from
