Sutter Jens-Uwe, Birch David J S
Department of Physics, Photophysics Group, SUPA, University of Strathclyde, Glasgow G4 0NG, UK.
Methods Appl Fluoresc. 2014 Apr 10;2(2):024005. doi: 10.1088/2050-6120/2/2/024005.
Melanin has long been thought to have an unworkably weak and complex fluorescence, but here we study its intrinsic fluorescence in order to demonstrate how metal ions can be used to control the rate of formation, constituents and structure of eumelanin formed from the well-known laboratory auto-oxidation of 3,4-dihydroxy-L-phenylalanine (L-DOPA). The effect on eumelanin absorption and fluorescence of a range of solvated metal ions is reported including Cu, Zn, Ni, Na and K. Monovalent cations and Zn have little effect, but the effect of transition metal cations can be considerable. For example, at pH 10, copper ions are shown to accelerate the onset of eumelanin formation, but not the rate of formation once it commences, and simplify the usual complex structure and intrinsic fluorescence of eumelanin in a way that is consistent with an increased abundance of 5,5-dihydroxyindole-2-carboxylic acid (DHICA). The presence of a dominant 6 ns fluorescence decay time at 480 nm, when excited at 450 nm describes a distinct photophysical species, which we tentatively assign to small oligomers. Copper is well-known to normally quench fluorescence, but increasing amounts of copper surprisingly leads to an increase in the fluorescence decay time of eumelanin, while reducing the fluorescence intensity, suggesting copper modification of the excited state. Such results have bearing on diverse areas. The most accepted morphology for melanin is that of a graphite-like sheet structure, and one which readily binds metal ions, an interaction that is thought to have an important, though as yet unclear bearing on several areas of medicine including neurology. There is also increasing interest in bio-mimicry by preparing and labelling sheet structures with metal ions for new electronic and photonic materials.
长期以来，人们一直认为黑色素具有微弱且复杂、难以起作用的荧光，但在此我们研究其固有荧光，以证明金属离子如何用于控制由著名的实验室3,4 - 二羟基 - L - 苯丙氨酸（L - DOPA）自氧化形成的真黑素的形成速率、成分和结构。报道了一系列溶剂化金属离子对真黑素吸收和荧光的影响，包括铜、锌、镍、钠和钾。单价阳离子和锌的影响很小，但过渡金属阳离子的影响可能相当大。例如，在pH值为10时，铜离子被证明可加速真黑素形成的起始，但一旦开始形成，并不会加快形成速率，并且以一种与5,5 - 二羟基吲哚 - 2 - 羧酸（DHICA）丰度增加相一致的方式简化了真黑素通常复杂的结构和固有荧光。当在450nm激发时，在480nm处存在占主导地位的6ns荧光衰减时间，描述了一种独特的光物理物种，我们初步将其归为小寡聚物。众所周知，铜通常会淬灭荧光，但令人惊讶的是，铜含量的增加会导致真黑素荧光衰减时间增加，同时降低荧光强度，这表明铜对激发态进行了修饰。这些结果与多个领域相关。黑色素最被认可的形态是类似石墨的片状结构，并且它很容易结合金属离子，这种相互作用被认为在包括神经学在内的多个医学领域具有重要影响，尽管目前尚不清楚。通过用金属离子制备和标记片状结构以用于新型电子和光子材料，对生物模拟的兴趣也在不断增加。
