Photoprotection

The term refers to the photoprotection mechanisms use nature, to minimize the harmful effects of ultraviolet radiation on human skin.

Mechanism of action

After the absorption of a UV photon is the respective molecule in an excited state and is therefore very reactive. To make damaging reactions with molecules from the environment as unlikely as possible, convert some molecules the excitation energy very quickly to heat. This prevents the formation of free radicals that can damage DNA. The photochemical process that converts the energy of the radiation into heat, is called internal conversion ( engl.: internal conversion). By this photo- protective mechanism of the excited state of the absorbing molecule is very short lived. The ultrafast internal conversion prevents the formation of free radicals and reactive oxygen species. This indirect DNA damage is minimized.

Both the DNA and proteins, and have the property of melanin ultrafast internal conversion. The photo- protective mechanism of DNA has evolved evolutionarily millions of years ago. At this time, the atmosphere containing no oxygen, and there was no protective ozone layer, so that the intensity of the UV radiation was 100 to 1000 times stronger than today. This selection process has also determined, from which blocks all life is built up on the earth. For DNA, these blocks adenosine, guanine, cytosine and thymine. Ultrafast inner transformation drastically shortened the life of the reactive excited state of adenosine, guanine, cytosine and thymine. In DNA, this life is only a few femtoseconds ( 10 - 15s ). This is several orders of magnitude shorter than in the filters that are used in sunscreen.

The excited state of the melanin is also very short-lived, so it provides a good photoprotection. Melanin converts more than 99.9 % of the absorbed radiant energy to heat.

Artificial melanin

The cosmetic industry claims sometimes that the UV filters in sunscreen, such as synthetic melanin function. The chemicals that are used for sunscreen, but have not the ability radiant energy as quickly and efficiently convert it to heat, as nature does. Instead, these chemicals have a much longer lifetime of the excited state. Indeed, several of the substances which are used in sunscreens of photochemistry known as a photosensitizer (see benzophenone).

The bad photochemical properties of the UV filters lead to higher concentrations of free radicals. The form in the skin after sun cream is absorbed into the skin, about 20 minutes after application.