The influence of iron on selected properties of synthetic pheomelanin

Andrzej Zadlo,Kazumasa Wakamatsu,Shosuke Ito,Krystian Mokrzyński,Tadeusz Sarna

doi:10.1007/s12013-020-00918-1

Andrzej Zadlo, Kazumasa Wakamatsu + Show 3 more

Open Access

https://doi.org/10.1007/s12013-020-00918-1

Copy DOI

Abstract

It is believed that while eumelanin plays photoprotective and antioxidant role in pigmented tissues, pheomelanin being more photoreactive could behave as a phototoxic agent. Although the metal ion-sequestering ability of melanin might be protective, transition metal ions present in natural melanins could affect their physicochemical properties. The aim of this research was to study iron binding by pheomelanin and analyze how such a binding affects selected properties of the melanin. Synthetic pheomelanin (CDM), prepared by enzymatic oxidation of DOPA in the presence of cysteine was analyzed by electron paramagnetic resonance (EPR) spectroscopy, spectrophotometry, chemical analysis, and time-resolved measurements of singlet oxygen phosphorescence. Iron broadened EPR signal of melanin and increased its optical absorption. Iron bound to melanin exhibited EPR signal at g = 4.3, typical for high-spin iron (III). Iron bound to melanin significantly altered the kinetics of melanin photodegradation, which in turn modified the accessibility and stability of the melanin–iron complexes as indicated by the release of iron from melanin induced by diethylenetriaminepentaacetic acid and KCN. Although bound to melanin iron little affects initial stages of photodegradation of CDM, the effect of iron becomes more pronounced at later stages of melanin photolysis.

Highlights

Melanin is believed to play a photoprotective and antioxidant role in pigmented tissues [1,2,3]
Ironinduced optical changes were not accompanied with significant changes of electron paramagnetic resonance (EPR) signal of melanin recorded at low pH when iron was released (Fig. 3b)
The binding of iron by CDM is indicated by a strong decrease of the amplitude of the EPR signal of melanin, which coincides with the signal broadening clearly observed at 77 K (Fig. 4a, d)

Summary

Introduction

Melanin is believed to play a photoprotective and antioxidant role in pigmented tissues [1,2,3]. This function is ascribed mainly to eumelanin. Photoinduced oxidative modifications of pheomelanin were shown to increase its potential to generate singlet oxygen and decrease its ability to quench this reactive oxygen species [5]. All natural melanins contain transition metal ions that may significantly modify observable properties of this polymer. It was previously postulated that redox active transition metal ions may be involved in the etiology of skin melanoma [6]

Objectives

Methods

Results

Conclusion