Free radicals from eumelanins: quantum yields and wavelength dependence

Area of Science:

• Biophysics
• Photochemistry
• Materials Science

Background:

• Melanin, a biopolymer, plays crucial roles in photoprotection and light absorption.
• Understanding melanin's photochemical properties is essential for various biological and technological applications.

Purpose of the Study:

• To investigate the formation of light-induced free radicals in natural and synthetic melanins.
• To determine the action spectra and quantum yields of free radical production.
• To compare the radical-generating efficiency of natural versus synthetic melanin.

Main Methods:

• Electron spin resonance (ESR) spectroscopy was employed to detect and quantify free radicals.
• Action spectra for radical formation were measured across a wide UV-Vis range (600-230 nm).
• Optical absorbance spectra were compared with action spectra.

Main Results:

• Action spectra for natural and synthetic melanins were similar and protein-independent.
• The chromophore responsible for radical production exhibited stronger wavelength dependence than the main visible light absorber.
• Natural eumelanin showed approximately three times greater radical production efficiency than synthetic melanin.
• Radical production was oxygen-independent, but correlated with oxygen consumption.

Conclusions:

• The primary chromophore for radical generation in melanin is distinct from the main visible light-absorbing chromophore.
• Natural eumelanin is more efficient at generating free radicals upon light exposure than synthetic melanin.
• Melanin's photochemical activity, particularly free radical formation, has implications for its biological functions and potential applications.