Journal Article

PreJuSER-22273

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Eumelanin Buildup on the Nanoscale: Aggregate Growth/Assembly and Visible Absorption Development in Biomimetic 5,6-Dihydroxyindole Polymerization

Arzillo, M. ; Mangiapia, G. ; Pezzella, A. ; Heenan, R. K. ; Radulescu, A.FZJ* ; Paduano, L. ; d\\'Ischia, M.

2012
American Chemical Soc. Columbus, Ohio

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Please use a persistent id in citations: doi:10.1021/bm3006159

Abstract: Establishing structure-property relationships in the black insoluble eumelanins, the key determinants of human pigmentation and skin photoprotective system, is a considerable conceptual and experimental challenge in the current drive for elucidation of the biological roles of these biopolymers and their application as advanced materials for organoelectronics. Herein, we report a new breakthrough toward this goal by the first detailed investigation on the nanoscale level of the oxidative polymerization of 5,6-dihydroxyindole (DHI), a model process of eumelanin synthesis. On the basis of a combined use of spectrophotometry, dynamic light scattering (DLS), and small-angle neutron scattering (SANS) investigations, it was possible to unveil the dynamics of the aggregation process before precipitation, the key relationships with visible light absorption and the shape of fundamental aggregates. The results indicated a polymerization mechanism of the type: Polymer(n) + DHI(x) = Polymer(n+x), where DHI(x) indicates monomer, dimer, or low oligomers (x ≤ 5). During polymerization, visible absorption increases rapidly, reaching a plateau. Particle growth proceeds slowly, with formation of 2-D structures ~55 nm thick, until precipitation occurs, that is, when large aggregates with a maximum hydrodynamic radius (R(h)) of ~1200 nm are formed. Notably, markedly smaller R(h) values, up to ~110 nm, were determined in the presence of poly(vinyl alcohol) (PVA) that was shown to be an efficient aggregation-preventing agent for polymerizing DHI ensuring water solubilization. Finally, it is shown that DHI monomer can be efficiently and partially irreversibly depleted from aqueous solutions by the addition of eumelanin suspensions. This behavior is suggested to reflect oxidant-independent competing pathways of polymer synthesis and buildup via monomer conversion on the active aggregate surface contributing to particle growth. Besides filling crucial gaps in DHI polymerization, these results support the attractive hypothesis that eumelanins may behave as a peculiar example of living biopolymers. The potential of PVA as a powerful tool for solution chemistry-based investigations of eumelanin supramolecular organization and for technological manipulation purposes is underscored.

Keyword(s): Health and Life (1st) ; Health and Life (1st) ; Biology (2nd) ; J

Note: This work was carried out in the frame of the EuMelaNet project (http://www.esper.org/eumelanet/). Financial support by MIUR, PRIN 2008 project, is gratefully acknowledged. We thank the Julich Centre for Neutron Science and the Rutherford Appleton Laboratory for provision of beam time. SANS experiments were supported by the European Commission, NMI3 contract RII3-CT-2003-505925.

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Contributing Institute(s):

1. Neutronenstreuung (ICS-1)
2. JCNS-FRM-II (JCNS (München) ; Jülich Centre for Neutron Science JCNS (München) ; JCNS-FRM-II)
3. Neutronenstreuung (JCNS-1)

Research Program(s):

1. BioSoft: Makromolekulare Systeme und biologische Informationsverarbeitung (FUEK505) (FUEK505)
2. 544 - In-house Research with PNI (POF2-544) (POF2-544)

Experiment(s):

1. KWS-2: Small angle scattering diffractometer (NL3ao)

Appears in the scientific report 2012

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Database coverage:
; BIOSIS Previews ; Current Contents - Life Sciences ; Current Contents - Physical, Chemical and Earth Sciences ; JCR ; NCBI Molecular Biology Database ; SCOPUS ; Science Citation Index ; Science Citation Index Expanded ; Thomson Reuters Master Journal List ; Web of Science Core Collection

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