Bacteriophytochrome is a photoreceptor protein that contains the biliverdin (BV) chromophore as its active component. The spectra of BV upon mutation remain remarkably unchanged, as far as spectral positions are concerned. This points toward the minimal effect of electrostatic effects on the electronic structure of the chromophore. However, the relative intensities of the Q and Soret bands of the chromophore change dramatically upon mutation. In this work, we delve into the molecular origin of this unusual intensity modulation. Using extensive classical MD and QM/MM calculations, we show that due to mutation, the conformational population of the chromophore changes significantly. The noncovalent interactions, especially the stacking interactions, lead to extra stabilization of the cyclic form in the D207H mutated species as opposed to the open form in the wild-type BV. Thus, unlike the commonly observed direct electrostatic effect on the spectral shift, in the case of BV the difference observed is in varying intensities, and this in turn is driven by a conformational shift due to enhanced stacking interaction.

中文翻译：

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构象对光敏色素吸收光谱的影响

菌素色素是一种光感受器蛋白，含有胆绿素 (BV) 发色团作为其活性成分。就光谱位置而言，突变后的 BV 光谱保持显着不变。这表明静电效应对发色团电子结构的影响最小。然而，发色团的 Q 和 Soret 带的相对强度在突变时会发生显着变化。在这项工作中，我们深入研究了这种不寻常的强度调制的分子起源。通过广泛的经典 MD 和 QM/MM 计算，我们发现由于突变，发色团的构象群发生了显着变化。非共价相互作用，尤其是堆积相互作用，导致 D207H 突变物种中的环状形式比野生型 BV 中的开放形式更加稳定。因此，与通常观察到的对光谱偏移的直接静电效应不同，在 BV 的情况下，观察到的差异是不同强度的，而这又是由增强的堆积相互作用引起的构象偏移驱动的。