Insight into the Binding of Divalent Cations to Sepia Eumelanin from IR Absorption Spectroscopy

Abstract

IR absorption spectroscopy is used to examine the binding of the divalent cations Mg(II), Ca(II), Zn(II) and Cu(II) to melanin granules isolated from the ink sacs of Sepia officinalis. The functional groups of the melanin granules interacting with the bound metal ions are deduced by examining the effect of metal concentration on transition frequencies associated with the COOH, NH and OH moieties of the pigment. The coordinating groups vary with metal ion and with concentration. For the experimental conditions used (initial solution pH of 4, ionic strength of 100 mM and a melanin concentration of 1 mg mL(-1)) Mg(II), Ca(II) and Zn(II) bind to carboxylate groups and Cu(II) binds predominantly to phenolic (catechol) groups However, at a concentration of 10 mM Cu(II) also shows evidence of binding to carboxylate and amine groups, reflecting a secondary binding site that becomes populated as the catechol sites are depleted.

Direct chemical evidence for eumelanin pigment from the Jurassic period

“…The FTIR spectrum of S. officinalis melanin consists of three major bands centered at 3;400 cm −1 , 1;605 cm −1 , and 1;371 cm −1 (28)(29)(30). The band at 3;400 cm −1 is dominated by absorption due to the stretching mode of the OH bond, while the band at 1;605 cm −1 is attributed to the carbonyl stretch in indole quinone.…”

Current Understanding of the Binding Sites, Capacity, Affinity, and Biological Significance of Metals in Melanin

“…40 The IR spectrum of Ca-loaded granules reveals that the COOH stretching vibration at 1710 cm −1 decreases with increasing metal concentration, indicating loss of the proton in the presence of metal. The pH of the solution also decreases with increasing Ca concentration, indicating that Ca binds to carboxylate groups.…”

“…However, these two studies report different concentrations of bound metals for the same biological system: the total metal content (mostly Ca, Mg, Na, and K) determined by Liu et al (1.6 mmol/g), 7 was on the order of four times greater than that reported by Sarzanini et al 39 While one might expect some variation in the distribution among these metals from sample to sample, the total metal content of Ca, Mg, Na, and K is expected to be the same, vide infra, because these metals share a common binding site and so the sum of their concentrations reflect the total concentration of available binding sites in Sepia eumelanin. 34,40 This finding highlights the importance of the methods of isolation and preparation of the melanin samples, the difficulty in obtaining reproducible samples, and the lack of an accepted set of standardized procedures to obtain in vitro samples that preserve, as best as can be achieved, the inherent properties of the natural in vivo organelles. Specifically, the variation in metal abundance in Sepia melanin obtained by Sarzanini et al and Liu et al can be attributed to the procedures employed to purify the melanin granules.…”

Natural melanin pigments and their interfaces with metal ions and oxides: emerging concepts and technologies

“…Nevertheless, promising results have been obtained with synthetic melanin-like polymers, e.g., polydopamine. [10,103] L-dopa [43,55] Natural (Sepia, [48,56] bovine eye [41] ) 5,6-Dihydroxyindole (DHI)-melanin (QI) [57] Mg 2+ Natural (Sepia [58] ) Natural (bovine eye, [41,45] Sepia, [48,56,59] squid [60] ), dopa-melanin [45] Ca 2+ Natural (bovine eye, [41,45] Sepia, [48,56,59,61] squid [60] ), dopa-melanin [45] Fe 3+ Natural (Sepia, [56,59,62] human hair, [63] black rabbit hair [64] ), neuromelanin, [65][66][67] dopa, [68][69][70] catechols, [71] polydopamine, [72][73][74] DHI-melanin, [63] melanin from cys-dopamine, [75] diethylaminedopamine-melanin [74] Neuromelanin, [76] dopa-melanin, [74] natural (Sepia), [74] diethylamine-dopaminemelanin [74] Natural (Sepia), …”

“…[47] Ion binding includes coulombic electrostatic interactions as well as coordination, possibly through chelation (multidentate binding, from Greek χηλή, chelè, meaning "claw"). The amine and catechol hydroxyl groups of DHI and DHICA as well as the carboxylic group of DHICA can work as primary or secondary binding sites for metal cations [11,48] ; factors determining the binding site for each cation are mainly the type of eumelanin (natural or synthetic) and the pH (Table I). Crucial features of melanogenesis, such as the rate of formation of eumelanin [49] and the relative percentage of the two indolic building blocks present (DHI versus DHICA), [50,51] are influenced by the presence of metal ions.…”