K.Motovilov Signatures of H5O2 + cation formation in vibrational spectra of pigment melanin call for reinterpretation of previously published DC conductivity, EPR and SR da a

Signatures of H5O2 + Cation Formation in Vibrational Spectra of Pigment Melanin Call for Reinterpretation of Previously Published DC Conductivity, EPR and µSR data

Zakhar V. Bedran1, Sergei S. Zhukov1, Pavel A. Abramov1, Ilya O. Tyurenkov1, Boris P. Gorshunov1 , A.Bernard Mostert2, Konstantin A. Motovilov1 1Moscow Institute of Physics and Technology, Dolgoprudny, Moscow Region, 141701 Russia 2Swansea University, Department of Chemistry, Singleton Park, Wales, UK

Abstract— Water fundamentally affects properties of all biomaterials. However, in the case of pigment melanin, we encounter one of the most complex and impressive hydrationdependent behavior. Being natural polyradical material promising for bioelectronics, melanin was long believed to be an amorphous semiconductor. Later, it was clearly demonstrated that proton component is the main contributor to melanin electric conductivity. The increase of proton contribution was conjugated with the synthesis of semiquinone anion in course of hydration by means of comproportionation reaction model. It explained the general increase of conductivity caused by water adsorption. However, the model was unable to interpret the slight decrease of conductivity at intermediate values of hydration. We performed the study of hydration-dependent permittivity in terahertz (THz) and mid-infrared ranges and revealed signatures of an amusing interplay between water-based proton species in the material. Introduction of H5O2 + cation allowed us to explain the local decrease of proton mobility and several other related observations that were previously unapprehended.

I. INTRODUCTION elanin is widely used as a model biomaterial for development of organic electrochemical transistors [1], M edible batteries [2], supercapacitors [3], sensors etc. Most of these applications rely on melanin’s ability to be an effective proton conductor in the solid hydrated state. Therefore, the increase of mobile proton species concentration is the key necessity for melanin-based technologies [4]. Water content came into the regular focus of melanin studies only in 1996 [5-9]. The experiments with thorough hydration control demonstrated paradoxical behavior of DC conductivity [8] and µSR components [7], namely, consequent fast increase, plateau (or decrease) and increase again. Chemistry of water was limited in those studies by formal reaction of hydronium H3O+ formation, i.e., it was more or less absent.

Terahertz spectroscopy is a perfect tool for the study of collective dynamics in various phases including biomaterials. In combination with mid-infrared (MIR) spectroscopy, it gives us an opportunity to track behavior of water-based species and the water-induced transformations of monomers in melanin.

II. RESULTS

We have performed combined THz-MIR spectroscopic investigation of melanin films at several levels of hydration (Fig. 1). Besides the features related to transformation of melanin monomer units, in accordance with the comproportionation reaction model, we observed the distinctive signatures related to transition of dominating proton species from H3O+ to H5O2 + , in both THz and MIR ranges. Those were the decrease of dielectric strength (Δε) of resonance absorption upon continuous hydration corresponding to the well-known Debye feature associated with collective water dynamics [10] and the sharp growth of 3474 cm-1 absorption line together with the flattening of intensity for 3608 cm-1 peak which we associate with formation of H5O2 + ion [11] (fig. 1). Introduction of H5O2 + synthesis from H3O+ and H2O in melanin at the definite hydration levels allows to explain the discussed peculiarities of DC conductivity [8] and µSR components behavior [7].

Proton mobility is known to be strongly slowed down by H5O2 + formation in solid acids [12], which is rather stable ion [13]. At the same time, H5O2 + cation can dominate among other proton species only within the definite range of ratios between water and acidic groups for particular system. Further hydration increases proton mobility [12], also in accordance with the previously published results [7, 8].

The work was supported by the RSF grant 19-73-10154. References

Fig. 1. Absorbance of melanin film under various conditions of relative humidity at 299 K.

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