Congo red (CR) is a commonly used histological amyloid dye and a weak amyloid inhibitor. There is currently no experimentally available structure of CR bound to an amyloid fibril and the binding modes, and the mechanisms governing its inhibitory and optical properties are poorly understood. In this work, we present the first, to our knowledge, atomistically detailed picture of CR binding to protofibrils of the Alzheimer Aβ(9-40) peptide. We identify three major binding modes, with the primary mode residing in the grooves formed by the β-sheets, and observe a restriction of the torsional rotation of the CR molecule upon binding. Our simulations reveal a novel, to our knowledge, electrostatic steering mechanism that plays an important role in the initial recognition and binding of CR to the positively charged surface residues of the fibril. Our simulations provide new, to our knowledge, insights into the striking spectrophotometric and inhibitory properties of CR. In particular, we show that birefringence upon CR binding is due to the anisotropic orientation of the CR dipoles resulting from the spatial ordering of these molecules in the grooves along the fibril axis. The fluorescent enhancement of the bound CR, in turn, is associated with the torsional restriction of this molecule upon binding.

译文

刚果红 (CR) 是一种常用的组织学淀粉样蛋白染料,也是一种弱淀粉样蛋白抑制剂。目前尚无实验上可用的与淀粉样蛋白原纤维结合的CR结构和结合模式,并且对控制其抑制和光学特性的机制知之甚少。在这项工作中,我们首先介绍了CR与Alzheimer a β(9-40) 肽原纤维结合的原子详细图片。我们确定了三种主要的结合模式,其中主要模式存在于由 β-折叠形成的凹槽中,并观察到结合后CR分子扭转旋转的限制。据我们所知,我们的模拟揭示了一种新颖的静电转向机制,该机制在CR与原纤维带正电的表面残基的初始识别和结合中起着重要作用。据我们所知,我们的模拟为CR的惊人分光光度和抑制特性提供了新的见解。特别是,我们表明,CR结合上的双折射是由于CR偶极子的各向异性取向,这是由于这些分子在沿原纤维轴的凹槽中的空间有序而产生的。结合的CR的荧光增强又与该分子结合后的扭转限制有关。

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