Photoprotection
The term photoprotection describes the mechanisms that nature uses to minimize the harmful effects of UV radiation on human skin .
Mechanism of action
After the absorption of a UV photon , the respective molecule is in an excited state and is therefore very reactive. In order to make damaging reactions with molecules from the environment as unlikely as possible, some molecules convert the excitation energy very quickly into heat. This prevents free radicals from forming, which can damage DNA . The photochemical process that transforms the energy of the radiation to heat, is called internal conversion ( engl. : Internal conversion ). Because of this photo-protection mechanism, the excited state of the absorbing molecule is very short-lived. The ultra-fast internal transformation prevents the formation of free radicals and reactive oxygen species . This minimizes indirect DNA damage .
Both DNA, proteins and melanin have the property of ultrafast internal transformation. The photo-protection mechanism of DNA developed evolutionarily many millions of years ago . At that time the atmosphere did not contain any oxygen and there was no protective ozone layer, so the intensity of UV radiation was 100 to 1000 times stronger than it is today. This selection process helped determine the building blocks from which all life on earth is built. In DNA, these building blocks are adenosine , guanine , cytosine and thymine . The ultra-fast internal conversion drastically shortens the life of the reactive excited state of adenosine, guanine, cytosine and thymine. In DNA, this lifetime is only a few femtoseconds (10 −15 s). This is several orders of magnitude shorter than the filters used in sunscreen .
| UV filter | other names | Proportion of molecules that convert the energy into heat ( quantum yield : Φ) |
Molecules that do not convert energy into heat quickly enough |
|---|---|---|---|
| Deoxyribonucleic acid (DNA) | more than 99.9% | less than 0.1% | |
| Melanin | more than 99.9% | less than 0.1% | |
| 2-phenylbenzimidazole-5-sulfonic acid | PBSA, Eusolex 232, Parsol HS, | ||
| 2-ethylhexyl-4-dimethylaminobenzoate | Padimate-O, oxtyldimethyl PABA, OD-PABA, EHDAB | 0.1 = 10% | 90% |
| 3- (4'-methyl) benzylidenebornan-2-one | (4-MBC), (MBC), Parsol 5000, Eusolex 6300 | 0.3 = 30% | 70% |
| 4- tert- butyl-4'-methoxydibenzoylmethane | (BM-DBM), Avobenzone, Parsol 1789, Eusolex 9020 butyl-methyoxydibenzoylmethane | ||
| Anthranilic acid methyl ester | (MA), methyl anthranilate, methyl 2-aminobenzoate | 0.6 = 60% | 40% |
| 2-ethylhexyl 4-methoxycinnamate | (2-EHMC), (EHMC), EMC, Octyl-methoxycinnamate , OMC, Eusolex 2292, Parsol | 0.81 = 81% | 19% |
The excited state of melanin is also very short-lived, which is why it offers good photo protection. Melanin converts more than 99.9% of the absorbed radiation energy into heat.
Artificial melanin
The cosmetic industry sometimes claims that the UV filters in sunscreen work like artificial melanin. The chemicals used in sunscreen, however, do not have the ability to convert radiant energy into heat as quickly and efficiently as nature does. Instead, these chemicals have a much longer excited state life.
In fact, several of the substances used in sunscreens are known from photochemistry as photosensitizers .
The poor photochemical properties of the UV filters lead to higher concentrations of free radicals. These form in the skin after the sunscreen is absorbed into the skin, about 20 minutes after application.
literature
- R. Dummer among others: Physical therapy measures in dermatology. , Verlag Steinkopff, 2006, pp. 56-60. ISBN 978-3-7985-1625-0 . doi : 10.1007 / 978-3-7985-1626-7
- W. Stahl and others: Systemic photoprotection by carotenoids. In: Der Hautarzt , 57/2006, pp. 281–285. doi : 10.1007 / s00105-006-1095-x
Individual evidence
- ↑ ultrafast internal conversion of DNA . Archived from the original on June 5, 2008. Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved February 13, 2008.
- ↑ a b Cantrell, Ann; McGarvey, David J ;: 3 (Sun Protection in Man) . In: Comprehensive Series in Photosciences . 495, 2001, pp. 497-519. CAN 137: 43484.
- ↑ Meredith, Paul; Riesz, Jennifer: Radiative Relaxation Quantum Yields for Synthetic Eumelanin . In: Photochemistry and photobiology . 79, No. 2, 2004, pp. 211-216. doi : 10.1111 / j.1751-1097.2004.tb00012.x .
- ↑ Hanson Kerry M .; Gratton Enrico; Bardeen Christopher J .: Sunscreen enhancement of UV-induced reactive oxygen species in the skin . In: Free Radical Biology and Medicine . 41, No. 8, 2006, pp. 1205-1212. doi : 10.1016 / j.freeradbiomed.2006.06.011 .
