Chemistry Reference
In-Depth Information
Chapter 9
Nazmul Islam and Minakshi Das
INTRODUCTION
The study of molecular interactions has been a great challenge from the experimental
as well as theoretical point of view. A lot of endeavors have been made to explain the
nature of bonding and reactivity of molecular systems based on some insightful ideas
and pragmatic rules.
Porphyrins are a group of biologically active heterocyclic organic pigment of both
neutral--most famously as the pigment in red blood cells (RBC), and synthetic origin.
The parent porphyrin is porphine, and substituted porphines are called porphyrins.
The structural properties of porphyrins are closely related to the important role of
these compounds in the biological systems. The porphyrins are usually described as a
union of four modifi ed pyrrole subunits interconnected at their a-C atoms via the me-
thine bridges to form a macro cycle. The macro cycle is referred to cyclic molecules
large enough to accommodate a metal ion in the interior. The macro cycle has 26 p
electrons. As the porphyrins posses (4n + 2) p electrons that are delocalized over the
macrocycle, they certainly satisfy the Hückel's rule for [1] and hence are aromatic in
nature. The macrocycles are highly-conjugated system, thus are deeply colored. Ow-
ing to their high extinction coeffi cients and tunable fl uorescence emission by changing
the central metal ion, porphyrin supramolecular complexes are used as good fl uores-
cent pH sensors [2]. The interesting structures of naturally occurring porphyrins, its
isomer, and substituted analogs have been perfected by nature to give functional dyes
par excellence [3]. The important roles these tetrapyrrolic macrocycles play in vital
biological processes namely, in the photosynthesis the Mg-porphyrins complex, chlo-
rophyll plays an important role, in blood, oxygen transport occurs by Fe-porphyrins
complexes known as hemoglobin and myoglobin, and in the electron transport process
another group of Fe-porphyrins complexes, cytochromes play a vital role. Supramol-
ecules containing zinc-porphyrins have been widely used in host-guest chemistry [4].
The porphyrins are called “Pigments of Life” [5] for their manifold biological sig-
nifi cance. Due to the presence of conjugated
π
-electrons, porphyrins are highly stable
and thus also they are useful in material science as the components in organic metals,
molecular wires, and other devices. Moreover, due to the advent of photodynamic
therapy, porphyrins and their metallocomplexes are used in the treatment of cancer
and dermatological diseases [6]. The porphyrias constitute a heterogeneous group of
diseases, all of which exhibit increased excretion of porphyrin or porphyrin precur-
sors. Some form of porphyrias are inherited, where as others are acquired. Recently
the anticancer drugs are synthesized using several gold-porphyrin complexes [7]. Thus
the interdisciplinary interests on the porphyrins generate a new science relating to the
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