Characterisation of electrosynthetic L-dopa- melanin ®lms by electrochemical and spectroelectrochemical techniques Gemma M. Robinson, Emmanuel I. Iwuoha and Malcolm R. Smyth* School of Chemical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland (Received 30 October 1997; in revised form 20 January 1998) AbstractÐMelanin was electrochemically synthesised by the oxidation of L-dopa in aqueous medium. The intermediates of the oxidation process were characterised by cyclic voltammetry and spectroelectrochemis- try. The ®nal product of melanin oxidation exhibited properties of a heterogeneous system containing a mixture of quinones and other oxidation intermediates of L-dopa. The charge transport properties of the redox polymeric compound was studied by chronocoulometric techniques using phosphate, perchlorate and sulphonate as electrolytes. The melanin electron transport coecient (D e 1/2 C es ) estimated for melanin in the various electrolytes are in good agreement with what is reported for other redox polymers. UV spectroelec- trochemical reactivities of the melanogenesis process was used to identify some of the intermediate pro- ducts. The excellent charge transport characteristics of melanin and its redox characteristics suggest that it could be used as an ecient immobilisation matrix for biomolecules in constructing reagentless ampero- metric biosensors. # 1998 Elsevier Science Ltd. All rights reserved Key words: electrosynthetic polymer, melanin, L-dopa spectroelectrochemistry, charge transport in redox polymers. INTRODUCTION Recently, there has been intense interest in studies aimed at understanding the structure, synthesis and reactivities of melanin due mainly to its role in the pigmentation of human skin and hair. The structure of melanin remains uncertain. However, the current opinion [1] is that melanin is a heterogeneous poly- mer containing a mixture of 5,6-dihydroxyindole units of dierent oxidation levels, and to a lesser extent of pyrrole units, linked randomly by carbon± carbon bonds. In other words melanin contains dihydroxyindole-based polymer and L-dopa at var- ious stages of oxidation [2, 3]. ESR studies [3] have shown that melanin contains quinones and hydro- quinone groups from L-dopa and from 5,6-dihy- droxyindole derivatives in equilibrium with their free radicals (Q + QH 2 t2Q . +2H + ). This poly- mer, or mixture of polymers, precipitates as a black amorphous, highly hygroscopic powder, the elemen- tal composition of which varies, depending on their isomeric precursors. According to the Raper± Masons's [4±6] model, the following steps are involved in melanin biosynthetics: the hydroxylation of L-tyrosine to L-3,4-dihydroxyphenylalanine (L- dopa); oxidation of L-dopa to dopaquinone; cycliza- tion to leucodopachrome; oxidation to dopachrome; decarboxylation to 5,6-dihydroxyindole or re- arrangement to 5,6-dihydroxyindole-2-carboxylic acid; and oxidation to melanin (see Fig. 1). Under physiological conditions, the ®rst two steps are cat- alysed by tyrosinase enzyme. Melanin is a highly reactive substance due to the presence of quinol and quinone units that undergo facile oxidation and reduction [3]. This makes it possible for melanin to exchange electrons with both electron donors and electron acceptors, including the reduction of ferri- cyanide, the oxidation of ascorbic acid, the oxi- dation of NAD(P)H and the reduction of arenediazonium cations [3]. We have previously reported the simultaneous determination of products and intermediates of L-dopa oxidation using capillary electrophoresis with diode-array Electrochimica Acta, Vol. 43, No. 23, pp. 3489±3496, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0013±4686/98 $19.00 + 0.00 PII: S0013-4686(98)00096-6 *Author to whom correspondence should be addressed. Tel.: +353 1 704 5308; Fax: +353 1 704 5503. 3489