Mass Spectrometric and Spectrophotometric Analyses Reveal an Alternative Structure and a New Formation Mechanism for Melanin

Abstract

In this study, we investigated the formation mechanism and chemical structure of melanin that results from the self-assembly of L-3,4-dihydroxyphenylalanine (L-DOPA). Using a combination of "top-down" and "bottom-up" approaches, and on the basis of state-of-the-art electrospray ionization mass spectrometry (ESI-MS) results, we propose a new formation mechanism and an alternative structure for melanin. Specifically, our study of the self-aggregation of L-DOPA based on L-DOPA clusters revealed that melanin is comprised partially of noncovalent supramolecular aggregate that is formed by self-aggregation of L-DOPA and with the individual monomers linked together by a combination of hydrogen bonds, π-π stacking, and ionic bonds. Furthermore, our study showed that unmodified L-DOPA may be part of the building block for melanin in addition to the previously proposed indole derivative based on L-DOPA cyclization. A similar self-aggregation phenomenon was also observed in other structurally related catecholamines, for example, adrenaline.