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The porphyrin handbook - Kadish K.M.

Kadish K.M. The porphyrin handbook - Academic press, 2000. - 368 p.
Download (direct link): kadishsmishgulilard2000.djvu
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signal, attenuated by co-infusion of inhibitor of nitric oxide synthase
L-NMMA or L-arginine (Figure 19).
The detection of a signal for nitric oxide in response to
acetylcholine provides further evidence, in man, that the endothelium-
derived relaxing factor is nitric oxide. Functional studies suggest that
alteration in the L-arginine/nitric oxide pathway occurs in hypertension,
diabetes, and hypercholesterolemia, and that decreased production of
nitric oxide might be a link between risk factors and atherogenesis. The
ability to assess local generation of nitric oxide with the porphyrinic
sensor should facilitate exploration of the role of this mediator in
physiological or pathophysiological processes.
Acetylcholine Time, s
1.0 nmol/min lOnmol/min
Figure 19. In vivo measurement of NO in humans.
44/Porphyrin-Based Electrochemical Sensors
251
III. Potentiometric Porphyrinic Sensors for Detection of Ions
Most potentiometric anion-selective electrodes are based on anion
exchangers such as quaternary ammonium salts. The selectivity pattern of
these sensors correlates with anion lipophilicity. Highly hydrated anions
such as fluoride, bicarbonate and chloride are difficult to monitor due
to significant interference from more lipophilic anions like perchlorate,
salicylate and nitrate which may be present in the analyzed sample. The
anions can be classified according to their lipophilicity resulting in
the classical Hofmeister series (CIO 4 > SCN " > salicylate > I " > NOJ >
Br > NO2 >C1 > HCO3 >F ).92
In order to improve the practical application of potentiometric
sensors, much effort must be devoted to developing membrane materials
whose selectivity deviates from the Hofmeister series. Chloride- and
nitrite-selective electrodes based on lipophylic vitamin ,2 derivatives
have been reported by Simon.93 These electrodes were the first anion-
selective electrodes of which selectivity sequence did not obey the
Hofmeister series. Since then, a number of anion-selective liquid-
membrane electrodes have been developed using a variety of hosts:
metallocenes, diphosphonium dication salts, diquartemary ammonium
dication salts, bisthiourea derivatives, metalloporphyrins, lipophylic
macrocyclic polyamines, cytosine-dependent triamine and
metallophtalocyamines.9498 Among this large variety of compounds studied,
the metalloporphyrins seem to be the most promising for the preparation
of effective ion-selective potentiometric sensors.13- 16."-113
Metalloporphyrins, one carbon larger but structurally analogous to the
corrin ring of vitamin l2, exhibited unique anion ionophore properties
when incorporated into a polymer membrane. Mn(III), Co(III), Ru(II), and
Sn(IV) porphyrin-based ion-selective sensors exhibit high selectivity for
thiocyanate, nitrite/thiocyanate, thiocyanate and salicylate,
respectively.15-100-107-110-114 The nature of the
metalloporphyrin-anion interaction mechanism correlates with the charge
of the central metal ion. Metal(II) porphyrins serve as neutral carriers
and metal(IV) porphyrins as charged carriers.110 Metal(III) porphyrins
serve as either neutral or charged carriers depending on the existence
and number of strongly bound axial ligands.111 Lipophylic ionic
additives, either cationic (tetraphenylborate salts for charge carriers)
or anionic (tetraphenylborate salts for charge carriers), can be used to
determine the dominant mechanism within the polymer membrane responsible
for the potential distribution across the membrane. The lipophilic ionic
carriers also reduce the membrane resistance, decrease counter-ion
interference and enhance selectivity.
The selectivity toward anions can be significantly changed by altering
the structure of the porphyrin (Table 4). Lipophilic derivatives of
vitamin B12 (so-called cobusters such aquocyano [heptakis (2-
phenylethyl)] cobyrinate were the first compounds applied as components
in liquid-membrane electrodes. Based on the exisiting knowledge on the
coordination chemistry of vitamin 12 and related corrin compounds, it
was suggested that the cobyrinates behave as positively charged
ionophores.13'14'93 The anion-selectivity sequence induced by cobyrinates
changed significantly with their structure and was in sharp contrast to
the usual selectivity pattern exhibited by classical anion-exchangers
(Hofmeister series). As a logical consequence of these observations, the
screening for ionophoric substances was extended to the structurally
related class of metalloporphyrins. Among the different compounds tested
were expanded porphyrins and hindered porphyrin structures.15'118
The potentiometric response of membranes doped with metalloporphyrins
involves the direct interaction of the anion as an axial ligand. So,
changes in the porphyrin structure, both in terms of initial ligation as
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