Melanin with Light

Finally superoxide radicals can also be generated photochemically in chloroplasts in the presence of ascorbate or of paraquat. The formation was demonstrated by spin trapping on illumination of spinach chloroplasts in the presence of oxygen and paraquat Superoxide radicals are formed, moreover, in the near-ultraviolet photooxidation of tryptophan, as indicated by the increase of the HjO production in the presence of SOD and on irradiation in aerated solutions of protoporphyrin at 400 nm and of melanin with light of 320—600nmas shown by spintrapping. 

Seagle, B.L. et al., Melanin photoprotection in the human retinal pigment epithelium and its correlation with light-induced cell apoptosis, Proc. Natl. Acad. Sci. USA, 102, 8978, 2005. 

If lighter colors than these are desired it is necessary to decolorize all the melanin in the hair in a preliminary step, and then add color back to the desired depth in a second treatment. This is known as a double-process treatment. The decolorization step consists of treating the hair with an alkaline mixture of persulfate salts and peroxide. The persulfate is added to the peroxide as a dry powder immediately before applying to the hair. Although the persulfate salts alone do not have any bleaching effect, the persulfate—peroxide mixture can remove all the melanin in the hair. Dark brown or darker hair can be lightened to a light blonde shade in about an hour. 

FIGURE 5.6 Calcitonin receptor responses, (a) Real-time melanin dispersion (reduced light transmittance) caused by agonist activation (with human calcitonin) of transfected human calcitonin receptors type II in melanophores. Responses to 0.1 nM (filled circles) and lOnM (open circles) human calcitonin, (c) Dose-response curves to calcitonin in melanophores (open circles) and HEK 293 cells, indicating calcium transient responses (filled circles). 

The reason why we can see the sun when we look up at the sky on a clear day is because it emits photons of visible light. Furthermore, photons of UV light may be seen by the way they cause our skin to acquire a darker colour following irradiation, because a tan forms when UV light reacts with melanin in the skin (see further discussion, later). Again, we notice photons of radiowaves because the radio beside us on the beach plays music and relays the latest news to us. 

There is an embarrassing wealth of material concerning melanin, and the interactions of light on the skin. The Website http //omlc.ogi.edu/spectra/melanin by Steven Jacques of the Oregon Laser Centre introduces the topic well, with a readable and highly informative discussion. 

---