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1. Porphyrins
S. A. Muhammad, PhD

2. Learning objectives
• At the end of this lecture, students should be able to:
 identify different porphyrins and their role.
 enumerate the properties of porphyrins.
 identify biomedical application of porphyrins

3. Porphyrins
• Porphyrins are ring-shaped molecules that
undergo a series of chemical changes to
produce heme.
• Heme is an important component of the
protein haemoglobin, which carries oxygen in
red blood cells.
• Porphyrins are formed by the union of four
pyrrole rings through methenyl bridges.
• They usually contain a metal ion linked to the
nitrogen atoms of the pyrrole rings.
• The biologically important porphyrins are
typically present as conjugated proteins

4. Example of some porphyrin-containing compounds
Protein Function
Haemoglobin Transport of oxygen in body
Myoglobin Storage of oxygen in muscle
Cytochrome c Involve in electron transfer in ETC
Cytochrome P450 Hydroxylation of xenobiotic
Catalase & peroxidase Detoxification of hydrogen peroxide
Tryptophan pyrrolase Oxidation of tryptophan

5. Pyrrole and Porphyrin
Pyrrole- the structural unit of porphyrin
 Rings are labeled I, II, III, and IV.
 Substituent positions on the rings are labeled 1, 2, 3, 4, 5, 6, 7, and 8.
 The methyne bridges (=HC—) are labeled α, β, γ, and δ.
 The numbering system used is that of Hans Fischer.

6. Porphyrins
• Fischer proposed a shorthand formula in which the methyne bridges
are omitted.
Uroporphyrin III (A -acetate=—CH2COOH; P -propionate =—CH2CH2COOH)

7. Porphyrins
Uroporphyrin (A- acetate; P- propionate)
They are found in urine but they are not restricted to urine.

8. Porphyrins
Coproporphyrins (M-methyl; P- propionate)
 They were first isolated from faeces but can also be found in urine

9. Addition of iron to protoporphyrin to form heme
(V- vinyl = -CH=CH2)
Protoporphyrins are found in blood and tissues.

10. Absorption spectra of porphyrins
• Porphyrins are strongly colored compounds.
• The variety of available colors is apparently unlimited although there
is an observable inclination towards red and purple.
• Porphyrins contain intense absorptions in the visible region.
• Even more intense (ten times and more) is the Soret band found in
the near UV located between 350 and 450 nm, which is attributed to
the electronic transitions between atoms of carbons of the
tetrapyrrolic cycle.

11. Biomedical application of porphyrins
• Porphyrins are used as diagnostic & therapeutic agents because of
their photophysical properties, such as the long wavelength of
emission & absorption, high singlet oxygen quantum yield & low in
vivo toxicity.
• The tunability of porphyrins to absorb in the therapeutic window
(600–800 nm) has allowed for their use in photo-medicine.

12. Porphyrin conjugated systems

13. Application in Photodynamic or sonodynamic therapy
Photodynamic therapy (PDT)- is a 2 stage treatment that combines
light energy with a drug (photosensitizer) to destroy cancerous and
precancerous cells after light activation.
Photothermal therapy (PTT)- minimally invasive that mainly relies on
an optical absorbing agent (photosensitizer), which can absorb energy
and convert it into heat.
In sonodynamic therapy, ultrasound is used in place of light.

14. PDT mechanism
(a) Profile of PDT treatment
(b) Generation of excited states &
reactive oxygen species (ROS)

15. Imaging Application
Porphyrins as contrast agents.
• Porphyrin is used as contrast agent in MRI- this technology is based on the absorption of
pulses of radiofrequency by a body when it is positioned in a magnetic field.
• A perfect contrast agent is one that is able to affect radiofrequency pulses and enhance
image contrast.
• Porphyrins are important in medical imaging field due to low toxicity, high tumor uptake,
and the ability to form complexes with metals.
• Highly stable metal complexes as magnetic resonance imaging contrast agents can be
administered to patients to enhance the contrast between the disease & healthy tissues.
• E.g., insertion & binding of Mn into porphyrin for the development of Mn-based contrast
agent

16. Application in Drug delivery
• The delivery of drug to the target tissue is vital in disease treatment.
• Porphyrin-based metal-organic frameworks have tunable porous structure that allow high
drug loading ability, adaptable functionality, and biodegradability.
• This porphyrin-based material can deliver the drug at a controllable rate.
• The application of metalloporphyrin in targeted delivery of drugs or biologics may reduce
the dose of drug and limit complications of toxicity and non-specific delivery.

17. Illustration of sonoactivable doxorubicin-loaded porphyrin-
phospholipid-liposome for cancer treatment
• Wang et al. developed a porphyrin-
based nanodelivery system for SDT.
• The US induced lipid oxidation and
efficient disruption of liposomes to
release loaded doxorubicin.
• Upon exposure to low intensity US, it
increased Dox nuclear subcellular
location and cytotoxicity in cancer cells
in vitro and suppressed tumour growth
in animal model.

18. Application as a Biosensor
• A biosensor is a device that measures biological/chemical reactions by
generating signals that is proportional to the concentration of the analyte.
• It can be used in the determination of ferrochelatase in bone marrow.
• Ferrochelatase catalyses the insertion of the ferrous ion into the porphyrin
cavity for the synthesis of heme.
• Human genetic defect affecting this enzyme may result in a disease known as
erythropoietic protoporphyria

19. Application as Biosensor
• Determination of ferrochelatase activity can be achieved with the use of
porphyrins, ferrous ion substrates, and the spectrophotometric measurements
of the synthesized heme. This method can detect a very low amount of
protoheme formed.
• Another assay to measure ferrochelatase activity is by monitoring the
disappearance of porphyrin because under these conditions porphyrin and
heme show an isosbestic (a specific wavelength or frequency at which the
total absorbance of a sample does not change during a chemical reaction)
point which indicates the conversion of substrate into product.

20. References
• Huang, H., Song, W., Rieffel, J., & Lovell, J. F. (2015). Emerging applications of porphyrins
in photomedicine. Frontiers in Physics, 3, 23.
• Imran, M., Ramzan, M., Qureshi, A. K., Khan, M. A., & Tariq, M. (2018). Emerging
applications of porphyrins and metalloporphyrins in biomedicine and diagnostic magnetic
resonance imaging. Biosensors, 8(4), E95.
• Tsolekile, N., Nelana, S., & Oluwafemi, O. S. (2019). Porphyrin as diagnostic and
therapeutic agent. Molecules, 24(14), 2669.
• Wang, X.; Yan, F.; Liu, X.;Wang, P.; Shao, S.; Sun, Y.; Zheng, H. (2018). Enhanced drug
delivery using sonoactivatable liposomes with membrane-embedded porphyrins. Journal of
Controlled Release, 2018, 286, 358–368