3. Poriphyrins and Photo Systems

8/2/2019 3. Poriphyrins and Photo Systems

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O SORPHYRINES


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Por h rins

Porphyrins (pronounce) are tetrapyrroles. They consist of four

weakly aromatic pyrrole (pronounce) rings joined by methene bridges.

Porphyrin is a heterocyclic macrocycle made from 4 pyrrole subunits

n e on oppos e s es pos on roug me ne r ges = - .

The extensive conjugated system makes the compound chromatic, hence

,

The macrocycle has 22 pi electrons, 18 of which are active in the

con u ated s stem.

These are the central groups of biologically imp molecules such as

Hemoglobin, Myoglobin, Chlorophyll, Cytochromes, etc


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Uses of Porphyrins

Because of the unique chemistry of porphyrins, they are able to perform

As a metal binder (ligands)

s a so ar ce convert g t or c em ca energy

As an oxygen transport medium (hemoglobin)

As an electron transfer medium (conducting polymers)

Gene regulation

Drug metabolism

Iron metabolism

Hormone synthesis


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As the basic building block of hemoglobin

Heme a cross-coupled

or h rin used in the lar er

molecule hemoglobin


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Cytochrome C a molecule responsible for transporting an electron used to

provide energy to the organism.

These molecules are identical, or very similar, for related species of plants or

animals.


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Chlorophyll

A green compound found in leaves and green stems of plants.

It occurs in cell organelles called chloroplasts, which are absent in

animals.

Chlorophyll is the molecule that traps this 'most elusive of all powers

and is called a photoreceptor.

The basic structure of a chlorophyll molecule is a porphyrin ring,

coor na e o a cen ra a om.

This is very similar in structure to the heme group found in

, ,

chlorophyll it is magnesium.


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Chlorophyll a and chlrophyll b

The most important pigment in plants is chlorophyll.

,

chlorophyll b (chl b)

Chloro h ll is com osed of two arts; the first is a or h rin rin

with magnesium at its center, the second is a hydrophobic phytol tail

The ring has many delocalized electrons that are shared between

several of the C, N, and H atoms; these delocalized electrons are very

important for the function of chlorophyll.

The tail is a 20 carbon chain that is highly hydrophic and stabilizes the

molecule in the hydrophobic core of the thylakoid membrane.


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Structurally CH3

group is present in chl a where chl b has a CHO group.

Chlorophyll a and b absorb different wavelengths better than others.

chl a absorbs best at 450 and 680 nm

chl b absorbs best at 500 and 640 nm

chlorophyll a is directly involved in the redox reactions of the

light reactions, chl b functions as an accessory pigment

Accessory pigments absorb light and pass the energy from the

g o e c a n e reac on cen er

Other accessory pigments can be present such as xanthophylls and

.

The most well known carotenoid is beta-carotene which absorbs

different wavelen ths than the chloro h lls.


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Photosystems

,

photosystems.

Photosystem I optimally absorbs photons of a wavelength of 700 nm.

Photosystem II optimally absorbs photons of a wavelength of 680 nm.

Photosystem II uses light energy to oxidize two molecules of water into

one molecule of molecular oxygen.

The 4 electrons removed from the water molecules are transferred by an

electron transport chain to ultimately reduce 2NADP+ to 2NADPH.

During the electron transport process a proton gradient is generated

across the thylakoid membrane.

This proton motive force is then used to drive the synthesis of ATP.

s process requ res , , cytoc rome , erre ox n- +

reductase and chloroplast ATP synthase.


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Photosystem I Photosystem II

Contains the chloro h ll-a molecule P700 Contains the chloro h ll-a molecule P680

Absorbs light maximally at 700 nm Absorbs light maximally at 680 nm

Receives energy from photons, fromassociated accessory pigments in its

antenna system, and from the electron

Receives energy from photons and fromassociated accessory pigments in its

antenna system.

transport chain from Photosystem II.

Uses the ener from li ht to reduce Uses the ener from li ht to oxidise water

NADP+ to NADPH + H+, or simply to

power a proton pump (plastoquinone, or

PQ).

molecules, producing protons (H+) and O2

as well as passing an electron to the

electron transport chain.

o p o osys ems are requ re or

noncyclic electron transport

n y o osys em par c pa es n cyc c

electron transport,


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Chlorophyll A and B

absorb li ht mostl in the

red andblue regions of the

s ectrum

Carotene and xanthophyll

a sor g t rom ot er

regions and pass the energy

to chlorophyll

3. Poriphyrins and Photo Systems

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