What Is Photosensitization? Definition, Symptoms & Examples
In the presence of photodynamic agents, skin (especially areas exposed to light and lacking substantial protective hair, fur, or pigmentation) becomes more susceptible to ultraviolet light. Sunburn and photodermatitis are not the same as photosensitization since all of these disorders cause pathologic skin changes without the involvement of a photodynamic agent.
When photons react with a photodynamic agent in photosensitization, unstable, high-energy molecules are formed. These high-energy molecules react with skin substrate molecules, releasing free radicals that increase membrane permeability in outer cells and lysosomes. Damage to the outer cell membranes causes cellular potassium to leak out and cytoplasmic extrusion to occur. Damage to the lysosomal membrane allows lytic enzymes to enter the cell. Skin ulceration, necrosis, and edema are also possible outcomes. The length of time between exposure to a photodynamic agent and the onset of clinical symptoms is determined by the type of agent, its dosage, and the amount of sunlight exposure.
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The source of the photodynamic agent is usually used to classify photosensitization. The three forms of photosensitivity are main (type I) photosensitivity, aberrant endogenous pigment synthesis (type II) photosensitivity, and hepatogenous (secondary, type III) photosensitivity. Idiopathic (type IV) photosensitivity has been identified as a fourth group.
Photosensitizing agents may come in a variety of forms, including those that are fungal or bacterial in origin. Plant-derived compounds, on the other hand, are the most common causes of photosensitivity in animals. Photosensitization may affect any animal, but cattle, sheep, goats, and horses are the most commonly affected.
Photosensitivity has been recorded in 25 percent to 90 percent of patients receiving demethylchlortetracycline, 20 percent of patients receiving doxycycline, 7% of patients receiving methacycline, and just a few patients receiving minocycline in people belonging to Tetracycline photosensitivity group.
Primary Photosensitization and Secondary Photosensitization
When a photodynamic agent is ingested, injected, or absorbed through the skin, it causes primary photosensitization. After the animal is exposed to ultraviolet light, the agent enters the systemic circulation in its native form, causing skin cell membrane damage.
Example of Photosensitizer - Hypericin is an example of a primary photosensitizing agent.
In horses, primary photosensitization from other plants like buckwheat (Fagopyrum toxicosis), which contains many toxins similar to hypericin in St. John's wort, is uncommon. Photosensitization (furocoumarin toxicosis) caused by spring parsley, Bishop's plant, and Dutchman's breeches are more common in sheep, cattle, and pasture-raised swine.
Photosensitivity in Sheep
Sporadic photosensitization in sheep occurs in conjunction with the grazing of a variety of plant species, including grasses, cereals, and legumes, but it is uncommon. Below is a specimen of how photosensitivity in sheep affects their body.
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Mechanism of Tetracycline Phototoxicity
Seven clinically used tetracyclines were studied to see what factors were relevant in the phototoxicity process (TC). The TCs' clinical phototoxicity, photochemical degradation rates, and in vitro phototoxicity were all qualitatively related but not quantitatively. In vitro, phototoxicity was partly oxygen-dependent, and singlet oxygen could be involved. The observed differences between the in vivo action spectrum and the absorption spectrum of demethylchlortetracycline may be due to the contribution of photoproducts to the phototoxic process. The absorption of UVA radiation by TC contributes to at least two key processes, according to a mechanistic model for in vivo phototoxicity.
(i) development of one or more photo products that photosensitizer through absorption of visible radiation; (ii) photosensitization of biologic molecules by the drug to induce phototoxicity.
Talaporfin has been studied to treat Port-Wine Stain and Benign Prostatic Hyperplasia.
Clinical Signs of Photosensitization
Photosensitivity can develop within a few days of sheep moving to biserrula pastures, but it can take weeks to develop. The following are the first signs that an animal could be photosensitized:
Restlessness
Head shaking
Rubbing
seeking out of shade.
Swelling of the head, eyelids, mulesed area (tail), backline, and muzzles are early signs of photosensitization. These are all the areas of the body that are the most visible. If left untreated, these areas can become reddened and inflamed, with the overlying skin dying and peeling away, revealing raw tissue underneath. Lambs may lose the tips or both of their ears in extreme cases, there may be a general split in the fur, and sheep may appear lame due to inflammation of the coronets.
Methylene Blue Photosensitizer
Methylene blue (MB) is a hydrophilic phenothiazine derivative also known as methylthioninium chloride. It's a photosensitizer that absorbs light at 660 nanometers. This limit is well within the emission range of most low-level laser therapy diode lasers. These lasers are often available at hospitals that treat patients with head and neck cancer, or they can be purchased for a low price. MB is barely activated by ambient light. As a result, health effects from environmental light exposure are unlikely. Furthermore, MB is a low-cost photosensitizer. Because of its effectiveness against a wide spectrum of pathogens, including bacteria, fungi, and viruses, MB is used in antimicrobial photodynamic therapy (APDT) and as a potent PDT medication for local treatment of periodontal diseases.
Photosensitization in Sheep
Swelling of the ears, eyelids, mulesed area (tail), backline, and muzzles are early signs of photosensitization. Lambs may lose the tips or all of their ears in severe cases, there may be a general break in the wool, and sheep may appear lame due to inflammation of the coronets.
FAQs on Photosensitization in Chemistry: Types, Causes & Effects
1. What is photosensitization in chemistry?
Photosensitization is a photochemical process where a molecule, known as a photosensitizer, absorbs light energy and then transfers this energy to another molecule (the reactant or substrate). This energy transfer causes the reactant molecule to become excited and undergo a chemical reaction, even though it did not absorb the light directly. The photosensitizer itself is typically regenerated at the end of the reaction, acting like a photocatalyst.
2. How does a photosensitized reaction differ from a direct photochemical reaction?
The primary difference lies in which molecule absorbs the light. In a direct photochemical reaction, the reactant molecule itself absorbs a photon of light, leading to its excitation and subsequent reaction. In a photosensitized reaction, an intermediary molecule, the photosensitizer, absorbs the light. The reactant only gets activated after receiving the absorbed energy from the excited sensitizer. This allows reactions to be triggered by wavelengths of light that the reactant itself cannot absorb.
3. What are the main types of photosensitization mechanisms?
Photosensitization reactions are generally classified into two main types based on the mechanism of energy transfer:
Type I Mechanism: The excited photosensitizer directly interacts with the substrate, usually through an electron or hydrogen atom transfer. This interaction results in the formation of free radicals or radical ions, which then initiate further reactions.
Type II Mechanism: The excited photosensitizer (typically in its triplet state) transfers its energy to ground-state molecular oxygen (3O2). This process generates highly reactive singlet oxygen (1O2), which then oxidizes the substrate.
4. What are the essential properties of an effective photosensitizer?
An effective photosensitizer must possess several key properties to function efficiently:
Strong Absorption: It should have a high molar absorption coefficient in the UV or visible region of the spectrum.
Efficient Intersystem Crossing (ISC): It must efficiently transition from the initial excited singlet state to a longer-lived excited triplet state.
Sufficient Triplet Energy: The energy of its triplet state must be higher than that of the acceptor (reactant) molecule to allow for efficient energy transfer.
Photochemical Stability: It should not degrade or be consumed during the reaction, allowing it to act catalytically.
5. Can you provide a classic example of a photosensitized reaction?
A classic example is the mercury-sensitized decomposition of hydrogen gas (H2). Molecular hydrogen does not absorb light at 253.7 nm. However, mercury vapour (Hg) strongly absorbs light at this wavelength to become excited. The excited mercury atom then collides with a hydrogen molecule and transfers its energy, causing the H-H bond to break and form two hydrogen radicals (H•), initiating a reaction.
Another common example from organic chemistry is the generation of singlet oxygen using dyes like Methylene Blue or Rose Bengal as sensitizers.
6. What is the importance of photosensitization in organic synthesis and other fields?
Photosensitization is extremely important due to its wide range of applications:
Organic Synthesis: It enables specific reactions that are difficult to achieve with heat, such as [2+2] cycloadditions, ene reactions, and specific oxidations using singlet oxygen.
Photodynamic Therapy (PDT): In medicine, photosensitizers are administered to patients and accumulate in cancer cells. When irradiated with light, they produce singlet oxygen (Type II mechanism) that selectively destroys the tumour.
Polymer Chemistry: It is used to initiate polymerization reactions, a process known as photo-initiated polymerization, for creating coatings, adhesives, and 3D-printed materials.
7. Why is the triplet state of a photosensitizer more important than the singlet state for energy transfer?
The triplet state (T1) is more crucial for photosensitization primarily because of its longer lifetime compared to the excited singlet state (S1). Singlet states are very short-lived (nanoseconds), leaving little time for the excited molecule to collide and transfer energy to a substrate. In contrast, triplet states can last for microseconds to seconds, significantly increasing the probability of a successful collision and energy transfer to the reactant molecule before the sensitizer deactivates through other pathways.
