Mechanisms of the Opsin Shift

质子化的视网膜希夫碱与载脂蛋白结合时吸收最大值的移动，即所谓的视蛋白移动，是蛋白质结合袋中发色团与其环境之间特殊相互作用的最明显证据。调整视觉色素对不同光谱区域的敏感性是在不同栖息地生存所必需的；它也是色觉的先决条件。过去提出的解释视蛋白位移起源的机制无法得到适当的测试，因为这些结构在很大程度上是假设的，而且没有可用的光谱定量计算的计算工具。随着近年来视网膜结合蛋白的x射线结构和可靠的量子力学方法的发展，计算包括部分蛋白质结合袋的视网膜发色团的激发态已经成为可能。我们提出了基于CASPT2校正的多构型SCF波函数的视网膜结合蛋白发色团模型激发态的非经验计算。受已知x射线数据的限制，将优化发色团的几何形状。环境将被包括在计算中。在这些计算的基础上，我们将分析opsinshift的不同机制，并评估它们的相对贡献。计算结果将通过紫外/可见光谱支持紫红质和bR光循环中间体的结构解析。

The shift of the absorption maximum of a protonated retinal Schiff base upon binding to the apoprotein, the so-called opsin-shift, is the most conspicuous evidence of the special interaction between the chromophore and its environment in the protein binding pocket. Tuning the sensitivity of the visual pigment to different spectral regions is necessary for survival in different habitats; it is also prerequisite for color vision. Mechanisms which have been proposed in the past to explain the origin of the opsin shift could not be tested properly, since the structures were largely hypothetical and computational tools for quantitative calculation of the spectra were not available. With the recent X-ray structures of retinal binding proteins and the development of reliable quantum mechanical methods the calculation of excited states of the retinal chromophore including parts of the protein binding pocket has become feasible. We propose the non-empirical calculation of the excited states of chromophore models of retinal binding proteins on the basis of CASPT2 corrected multiconfigurational SCF wavefunctions. Chromophore geometries will be optimised subject to the constraints of the known X-ray data. The environment will be included in the calculations. On the basis of these calculations we will analyse different mechanisms of the opsinshift and evaluate their relative contributions. The calculations will support, via the UV/Vis spectra, the structure elucidation of the intermediates of the rhodopsin and the bR photocycles.