Characterization of Melanin-TiO₂Complexes Using FT-IR and¹³C Solid-state NMR Spectroscopy

The melanin plays key roles to protect internal tissues against the harmful effects of ultraviolet rays in human beings. The most common form of biological melanin is a complex polymer of either or both of 5,6-indolequinone and 5,6-dihydroxyindole carboxylic acid. Micronized TiO 2 has been also widely used as a representative inorganic pigment for the formulation of the UV-care cosmetic products. We synthesized a number of new organic-inorganic nanohybrid compounds to develop sunscreens having stronger UV protecting synergy effects from the sun for the first time. We characterized novel organic-inorganic nanohybrid compounds by 1 H/ 13 C cross-polarization magic angle spinning (CPMAS) solid-state NMR and FT-IR spectroscopy. The complexes A, B, and C, simple mixing of the L-DOPA bearing catechol unit with TiO 2 nanoparticle does not show forming covalently bonded organic-inorganic nanohybrid compounds. On the contrary, the complexes D, E, and F show that the line shape changes and chemical shifts of phenoxy carbons of catechol move downfield upon forming the catecholate with Ti (IV). Relationship between the composition of organic-inorganic hybrid and SPF improvements will be studied more in detail in the near future.