Rienecker Shermiyah B, Mostert A Bernardus, Schenk Gerhard, Hanson Graeme R, Meredith Paul
School of Chemistry and Molecular Biosciences, ‡Centre for Organic Photonics and Electronics, School of Mathematics and Physics, and §Centre for Advanced Imaging, University of Queensland , Brisbane, St Lucia QLD 4072, Australia.
J Phys Chem B. 2015 Dec 3;119(48):14994-5000. doi: 10.1021/acs.jpcb.5b08970. Epub 2015 Nov 18.
Melanins are pigmentary macromolecules found in many locations throughout nature including plants and vertebrate animals. It was recently proposed that the predominant brown-black pigment eumelanin is a mixed ionic-electronic conductor which has led to renewed interest in its basic properties as a model bioelectronic material. This exotic hybrid electrical behavior is strongly dependent upon hydration and is closely related to the free radical content of melanin which is believed to be a mixed population of two species: the semiquinone (SQ) and a carbon-centered radical (CCR). The predominant charge carrier is the proton that is released during the formation of the SQ radical and controlled by a comproportionation equilibrium reaction. In this paper we present a combined solid-state electron paramagnetic resonance (EPR), adsorption, and hydrated conductivity study using D2O as a probe. We make specific predictions as to how the heavy isotope effect, in contrast to H2O, should perturb the comproportionation equilibrium and the related outcome as far as the electrical conductivity is concerned. Our EPR results confirm the proposed two-spin mechanism and clearly demonstrate the power of combining macroscopic measurements with observations from mesoscopic probes for the study of bioelectronic materials.
黑色素是一种存在于自然界许多地方的色素大分子，包括植物和脊椎动物。最近有人提出，主要的棕黑色色素真黑素是一种混合离子 - 电子导体，这引发了人们对其作为一种模型生物电子材料的基本性质的新兴趣。这种奇特的混合电学行为强烈依赖于水合作用，并且与黑色素的自由基含量密切相关，黑色素的自由基含量被认为是两种物质的混合群体：半醌（SQ）和碳中心自由基（CCR）。主要的电荷载体是质子，它在SQ自由基形成过程中释放，并由一个歧化平衡反应控制。在本文中，我们使用D2O作为探针，进行了固态电子顺磁共振（EPR）、吸附和水合电导率的联合研究。我们对重同位素效应（与H2O相比）如何干扰歧化平衡以及就电导率而言的相关结果做出了具体预测。我们的EPR结果证实了所提出的双自旋机制，并清楚地展示了将宏观测量与来自介观探针的观察结果相结合用于研究生物电子材料的力量。
