Spectral responses of melanin to ultraviolet A irradiation.
The purpose of this investigation was to study the ultraviolet A-induced effects on melanin pigmentation both in an in vitro model system and in vivo. Ultraviolet-Vis absorbance spectra of L-3,4-dihydroxyphenylalanine-melanin solutions at different concentrations were measured before and after ultraviolet A exposure (10-120 J per cm2). The difference spectra reveal that following ultraviolet A exposure the absorbance increases exponentially from 800 nm to 450 nm accompanied by a prominent decrease of absorbance in the ultraviolet A range. This change of spectral features depends on both ultraviolet A doses and melanin concentrations. The photo-bleaching effect observed in the ultraviolet A range also depends on oxygen. Human subjects were irradiated with ultraviolet A (40-80 J per cm2) on their back and diffuse reflectance spectra were collected at both irradiated and untreated sites. The absorption spectra of ultraviolet A-induced pigment were calculated as the difference of the two. The ultraviolet A-induced pigment in vivo has similar spectral characteristics and dose dependency as the in vitro system. Photo-oxidation of pheomelanin solutions presents distinctively different spectral and dose-response characteristics from eumelanin. After ultraviolet A irradiation pheomelanin absorbance decreases both in the visible and the ultraviolet A range. We conclude that irradiation with ultraviolet A induces significant photochemical alterations in the skin witnessed by increased photoprotection in the visible spectral range and reduced protection in the ultraviolet A range. We suggest that soluble melanin plays an important part in ultraviolet A-induced pigment in skin and two distinct absorption mechanisms of melanin may be involved in ultraviolet A photo-oxidation. We also propose that eumelanin and pheomelanin could be differentiated according to their spectral responses to ultraviolet A irradiation.
Ou-Yang
Hao
H
Models and Methods Development, Johnson and Johnson Consumer and Personal Products Worldwide, Skillman, New Jersey 08558, USA.
Stamatas
Georgios
G
Kollias
Nikiforos
N
D004307:Dose-Response Relationship, Radiation; D004817:Epidermis / Q000528:radiation effects*; D006801:Humans; D066298:In Vitro Techniques; D008543:Melanins / Q000032:analysis / Q000737:chemistry*; D010777:Photochemistry; D012880:Skin Pigmentation / Q000528:radiation effects; D013057:Spectrum Analysis; D014466:Ultraviolet Rays
D008543:Melanins; C018362:pheomelanin; C041877:eumelanin