Photochemistry involves chemical reactions influenced by light. It has many medical applications including treating skin conditions and cancer. Light affects skin by enabling vitamin D synthesis but also causes damage. Therapeutic non-laser lights are used to treat neonatal jaundice, psoriasis, and vitiligo by inducing photochemical reactions. Lasers can stimulate or destroy cell growth depending on intensity. Photodynamic therapy uses photosensitizers, light, and oxygen to selectively damage diseased cells. It is used to treat cancers and infections. Photochemical tissue bonding also uses photosensitizers and light to suturelessly repair tissue through extracellular matrix bonding.

1. Application of
photochemistry
in medicine

2. What is Photochemistry?
• Composed of two words: photo + chemistry.
• Study of chemical reactions, isomerizations and
physical behavior that may occur under the
influence of visible and/or ultraviolet light.

3. Giacomo Luigi Ciamician
Born: August 25th, 1857 in
Austria
Employer: University of
Bologna
Died: January 2nd, 1922 in
Bologna, Italy
: Father of Modern Molecular Photochemistry
His first photochemistry experiment was published in 1886
and was titled as “On the conversion of quinone to quinol by
light”.
He also made major contributions to the chemistry of pyrroles, the functions
of plant glucosides and alkaloids, and various physical chemical areas such
as spectroscopy.

4. Two fundamental principles:
• First law of photochemistry: The Grotthuss-Draper law, states that light must
be absorbed by a compound in order for a photochemical reaction to take place.
• Second law of photochemistry: The Stark-Einstein law, states that for each
photon of light absorbed by a chemical system, only one molecule is activated for
subsequent reaction.
Quantum yield(Φ):
 The efficiency with which a given photochemical process occurs.
 The number of moles of a stated reactant disappearing, or the number of moles
of a stated product produced, per einstein of monochromatic light absorbed.

5. photochemistry and Medicine
Applications summarized
✔ Effect of light on skin.
✔ Therapeutic treatments using non-laser lights.
✔ Use of lasers.
Neils Finsen, the father of modern photo-medicine.
 Awarded one of the first Nobel prizes in 1903.
 Use of UV light to cure the facial sores by tuberculosis sufferers.

6. Components:
 Light Source
Target Molecules
Photosensitizer
Mechanism:
 Direct (type I) mechanism: The activated photosensitizer may form free
radicals that are reactive to the target molecules and therefore consume
themselves in the reactions.
 Indirect (type II) mechanism: The activated photosensitizer returns to its
ground state after energy transfer to molecular oxygen, and therefore recycles
itself for another round of reaction.

7. Fig. Type I and type II mechanisms of photochemical reactions.

8. 1. Effect of light on skin:
• Light can have both beneficial as well as dramatic effects.
• One beneficial effect is the synthesis of Vitamin D by sunlight.
• One damage of light is that all cell types can become malignant under UV light.
•Tanning represents the reaction of the skin to UV threat.

9. Therapeutic treatments using non-laser lights.
2. Therapeutic treatments using non-laser lights.
i. Neonatal jaundice
• Arises in premature infants.
• Breakdown of RBC produces large amounts of bilirubin, which is water insoluble.
• Enzyme uridine-diphosphoglucuronic glucuronosyltransferase (UDPGT), converts
bilirubin to water soluble form.
• Visible blue light is used for treatment.
• Under influence of light, a photochemical cis- trans isomerisation about one of the double
bonds occurs in the structure.

10. ii. Psoriasis
• The basal layer of the skin overproduces skin cells.
•Upon emerging to the surface they are still viable.
•A photosensitizer psoralen is used which absorbs in the
near UV region.
•Destroys the adjacent skin tissues.
•The term used for this therapy is PUVA treatment.
iii. Vitligo
• Due to loss of melanin in pigmented skin cells.
• Results in white patches.
• PUVA treatment is used.
• PUVA induces repigmentation by varying mechanisms
such as stimulation of melanogenesis.
• Ultraviolet B may be also used but has a few side effects.

11. 3. Use of lasers:
Effects of laser light upon human tissue depends upon the fluence and the presence or absence of sensitiser.
 At low levels(<4 Wcm-2 ), the effect is to stimulate development of cells, a process known as
photochemical bonding.
 At higher energies, the effects are opposite, to destroy or stop cell growth.
 At light levels of 40 Wcm-2 , the light with photosensitizer can lead to tissue destruction termed as
photodynamic therapy.
Photodynamic Therapy:
• Used for treatment of a range cancerous diseases, infections and in ophthalmology.
•Interaction of a non-toxic photosensitizer molecule, visible light and molecular oxygen.
•Offers better selectivity for the uptake of photosensitizer by diseased cells.
• Production of short-lived oxygen radical species.
•Localized cell death via damage to cellular components as proteins, lipids, DNA, etc.

12. Technique of photodynamic therapy

13. Photochemical Tissue Bonding:
• Focuses on tissue repair, aiming to bond tissue edges or surfaces together.
• Sutureless repair using laser welding techniques are available but are not often used to the thermal nature which may create
complications.
• PTB is a non-thermal, rapid and controlled technique.
• PTB shares the same mechanism as PDT.
•The photosensitizers used are xanthenes and napthalmides.
• The extracellular matrix as collagen is the main target molecule.
Fig. Photodynamic Therapy mechansim Fig. Photochemical Tissue Bonding mechansim

14. References:
Chan, B.P., Biomedical
Applications of
Photochemistry, Tissue
Engineering, 2010, 16, 5.
Phillips, D., Light relief:
photochemistry and
medicine, 2010.

15. This Presentation is Prepared by
“Debasish sharma”