Photoprotection
Encyclopedia
Photoprotection is a group of mechanisms that nature has developed to minimize the damage that the human body suffers when exposed to UV radiation. This damage mostly occurs in the skin, but the rest of the body (especially the testicles) can be affected by the oxidative stress caused by UV light.
Photoprotection of the human skin is achieved by extremely efficient internal conversion
of DNA, proteins and melanin. Internal conversion is a photochemical process that converts the energy of the UV photon into small amounts of heat. This small amount of heat is harmless. If the energy of the UV photon were not transformed into heat, then it would lead to the generation of free radicals or other harmful reactive chemical species (e.g. singlet oxygen, or hydroxyl radical).
In DNA this photoprotective mechanism evolved four billion years ago at the dawn of life. The purpose of this extremely efficient photoprotective mechanism is to prevent direct DNA damage
and indirect DNA damage
. The ultrafast internal conversion of DNA reduces the excited state lifetime of DNA to only a few femtoseconds (10-15s)—this way the excited DNA has not enough time to react with other molecules.
For melanin this mechanism has developed later in the course of evolution. Melanin is such an efficient photoprotective substance that it dissipates more than 99.9% of the absorbed UV radiation as heat.
This means that less than 0.1% of the excited melanin molecules will undergo harmful chemical reactions or produce free radicals.
s do not efficiently dissipate the energy of the UV photon as heat. Instead these substances have a very long excited state lifetime.
In fact, the substances used in sunscreens are often uses as photosensitizer in chemical reactions. (see Benzophenone
).
This discrepancy between melanin and sunscreen ingredients is one of the reasons for the increased melanoma
risk that can be found in sunscreen users compared to non-users. (see sunscreen)
Photoprotection of the human skin is achieved by extremely efficient internal conversion
Internal conversion (chemistry)
Internal conversion is a transition from a higher to a lower electronic state in a molecule or atom. It is sometimes called "radiationless de-excitation", because no photons are emitted...
of DNA, proteins and melanin. Internal conversion is a photochemical process that converts the energy of the UV photon into small amounts of heat. This small amount of heat is harmless. If the energy of the UV photon were not transformed into heat, then it would lead to the generation of free radicals or other harmful reactive chemical species (e.g. singlet oxygen, or hydroxyl radical).
In DNA this photoprotective mechanism evolved four billion years ago at the dawn of life. The purpose of this extremely efficient photoprotective mechanism is to prevent direct DNA damage
Direct DNA damage
Direct DNA damage can occur when DNA directly absorbs the UV-B-photon. UVB light causes thymine base pairs next to each other in genetic sequences to bond together into thymine dimers, a disruption in the strand, which reproductive enzymes cannot copy...
and indirect DNA damage
Indirect DNA damage
Indirect DNA damage occurs when a UV-photon is absorbed in the human skin by a chromophore that does not have the ability to convert the energy into harmless heat very quickly. Molecules that do not have this ability have a long lived excited state. This long lifetime leads to a high probability...
. The ultrafast internal conversion of DNA reduces the excited state lifetime of DNA to only a few femtoseconds (10-15s)—this way the excited DNA has not enough time to react with other molecules.
For melanin this mechanism has developed later in the course of evolution. Melanin is such an efficient photoprotective substance that it dissipates more than 99.9% of the absorbed UV radiation as heat.
This means that less than 0.1% of the excited melanin molecules will undergo harmful chemical reactions or produce free radicals.
Artificial melanin
The cosmetic industry claims that the UV filter acts as a "artificial melanin". But those artificial substances used in sunscreenSunscreen
Sunblock is a lotion, spray, gel or other topical product that absorbs or reflects some of the sun's ultraviolet radiation on the skin exposed to sunlight and thus helps protect against sunburn...
s do not efficiently dissipate the energy of the UV photon as heat. Instead these substances have a very long excited state lifetime.
In fact, the substances used in sunscreens are often uses as photosensitizer in chemical reactions. (see Benzophenone
Benzophenone
Benzophenone is the organic compound with the formula 2CO, generally abbreviated Ph2CO. Benzophenone is a widely used building block in organic chemistry, being the parent diarylketone.-Uses:...
).
This discrepancy between melanin and sunscreen ingredients is one of the reasons for the increased melanoma
Melanoma
Melanoma is a malignant tumor of melanocytes. Melanocytes are cells that produce the dark pigment, melanin, which is responsible for the color of skin. They predominantly occur in skin, but are also found in other parts of the body, including the bowel and the eye...
risk that can be found in sunscreen users compared to non-users. (see sunscreen)
| UV-absorber | other names | percentage of molecules that dissipate the photon energy (quantum yield: Φ ) |
molecules not dissipating the energy quickly |
|---|---|---|---|
| DNA | > 99.9 % | < 0.1 % | |
| natural melanin | > 99.9 % | < 0.1 % | |
| 2-phenylbenzimidazole-5-sulfonic acid | PBSA, Eusolex 232, Parsol HS, | ||
| 2-ethylhexyl 4-dimethylaminobenzoate | Padimate-O, oxtyldimethyl PABA, OD-PABA | 0.1 = 10% | 90% |
| 4-Methylbenzylidene camphor | (4-MBC), (MBC), Parsol 5000, Eusolex 6300 | 0.3 = 30% | 70% |
| 4-tert-butyl-4-methoxydibenzoyl-methane | (BM-DBM), Avobenzone, Parsol 1789, Eusolex 9020 | ||
| Menthyl Anthranilate | (MA), Methyl-2-aminobenzoate, meradimate | 0.6 = 60% | 40% |
| Ethylhexyl methoxycinnamate | (2-EHMC), (EHMC), EMC, Octyl methoxycinnamate, OMC, Eusolex 2292, Parsol | 0.81 = 81% | 19% |