DOI: 10.1002/cphc.201000257 UV-Dissipation Mechanisms in the Eumelanin Building Block DHICA** Annemarie Huijser,* [a] Alessandro Pezzella, [b] Jonas K. Hannestad, [c] Lucia Panzella, [b] Alessandra Napolitano, [b] Marco d’Ischia, [b] and Villy Sundstrçm* [a] 1. Introduction 5,6-Dihydroxyindole-2-carboxylic acid (DHICA) and 5,6-dihy- droxyindole (DHI) are key monomer building blocks of eumela- nins, the black to brown pigments found in human skin, hair and eyes. About equal amounts of DHICA- and DHI-derived units have been found to occur in natural eumelanins, al- though in some cases DHICA is prevalent. The primary and secondary structure of basic eumelanin components and their supramolecular aggregation mechanisms are still under debate, though much progress has been made in the past few years. [1, 2] Oligo- and polymerization may occur through differ- ent positions in the molecule, with a prevalence of 2-linked components in DHI oligomers and 4-linked derivatives in DHICA-derived species (see Figure 1 for nomenclature). [2–4] An early widely held view, mostly based on X-ray studies, is that eumelanin fundamental aggregates consist of 4 or 5 p-stacked branched oligomeric units of less than 10 monomers, with a spacing in between the oligomers of 3.4 . [2, 3] Subsequently, AFM images of eumelanin isolated from Sepia officinalis showed three different structures : Filaments, individual parti- cles ranging in size from less than 10 nm to several hundred nanometers, and aggregated structures comprised of individu- al particles. [5] This reflects a hierarchical process with small units assembling into 100 nm structures, which aggregate into the macroscopic pigment. [6] From recent work, based on high- resolution transmission electron microscopy, an improved structural model was proposed in which sheets of protomole- cules stack to form onion-like nanostructures. The inter-sheet spacing between these structures is 3.7–4.0 , consistent with p–p stacking in heteroaromatic systems. [7] An intriguing feature of eumelanin biopolymers is their broad absorption, covering the entire visible range and in- creasing monotonically towards UV wavelengths. This feature- less absorption is rather uncommon for organic chromophores and is attributed to the presence of a large variety of different HOMO–LUMO gaps, owing to the heterogeneity in composi- tion. In addition, the perturbation of conjugated chromophoric The UV-dissipative mechanisms of the eumelanin building block 5,6-dihydroxyindole-2-carboxylic acid (DHICA) and the 4,7-dideutero derivative (DHICA-d 2 ) in buffered H 2 O or D 2 O have been characterized by using ultrafast time-resolved fluo- rescence spectroscopy. Excitation of the carboxylate anion form, the dominating state at neutral pH, leads to dual fluores- cence. The band peaking at l = 378 nm is caused by emission from the excited initial geometry. The second band around l = 450 nm is owed to a complex formed between the mono- anion and specific buffer components. In the absence of com- plex formation, the mono-anion solely decays non-radiatively or by emission with a lifetime of about 2.1 ns. Excitation of the neutral carboxylic acid state, which dominates at acidic pH, leads to a weak emission around l = 427 nm with a short life- time of 240 ps. This emission originates from the zwitterionic state, formed upon excitation of the neutral state by sub-ps excited-state intramolecular proton transfer (ESIPT) between the carboxylic acid group and the indole nitrogen. Future stud- ies will unravel whether this also occurs in larger building blocks and ESIPT is a built-in photoprotective mechanism in epidermal eumelanin. Figure 1. Molecular structures of A) DHICA in H 2 O and B) DHICA-d 2 in D 2 O. [a] Dr. A. Huijser, Prof. V. Sundstrçm Department of Chemical Physics Lund University, Box 124, 22100 Lund (Sweden) Fax: (+ 46) 46 22 24119 E-mail : Annemarie.Huijser@chemphys.lu.se Villy.Sundstrom@chemphys.lu.se [b] Dr. A. Pezzella, Dr. L. Panzella, Prof. A. Napolitano, Prof. M. d’Ischia Department of Organic Chemistry and Biochemistry University of Naples Frederico II Via Cinthia, 80126 Naples (Italy) [c] J. K. Hannestad Department of Physical Chemistry Chalmers University of Technology Kemivägen 10, 41296 Gçteborg (Sweden) [**] DHICA: 5,6-dihydroxyindole-2-carboxylic acid Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201000257. 2424  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2010, 11, 2424 – 2431