Laboratory for Atomistic and Molecular Mechanics (LAMM), Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Room 1-235A&B, Cambridge, Massachusetts 02139, USA.
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
Nat Commun. 2014 May 22;5:3859. doi: 10.1038/ncomms4859.
Eumelanin is a ubiquitous biological pigment, and the origin of its broadband absorption spectrum has long been a topic of scientific debate. Here, we report a first-principles computational investigation to explain its broadband absorption feature. These computations are complemented by experimental results showing a broadening of the absorption spectra of dopamine solutions upon their oxidation. We consider a variety of eumelanin molecular structures supported by experiments or theoretical studies, and calculate the absorption spectra with proper account of the excitonic couplings based on the Frenkel exciton model. The interplay of geometric order and disorder of eumelanin aggregate structures broadens the absorption spectrum and gives rise to a relative enhancement of absorption intensity at the higher-energy end, proportional to the cube of absorption energy. These findings show that the geometric disorder model is as able as the chemical disorder model, and complements this model, to describe the optical properties of eumelanin.
真黑素是一种普遍存在的生物色素，其宽带吸收光谱的起源长期以来一直是科学争论的话题。在这里，我们报告了一项第一性原理计算研究，以解释其宽带吸收特性。这些计算得到了实验结果的补充，实验表明多巴胺溶液在氧化过程中吸收光谱变宽。我们考虑了多种实验或理论研究支持的真黑素分子结构，并根据福井激子模型，在适当考虑激子耦合的情况下，计算了吸收光谱。真黑素聚集体结构的几何有序和无序的相互作用拓宽了吸收光谱，并在高能端引起吸收强度的相对增强，与吸收能量的立方成正比。这些发现表明，几何无序模型与化学无序模型一样，能够描述真黑素的光学性质，并补充了该模型。
