How the counterion affects ground- and excited-state properties of the rhodopsin chromophore
How the counterion affects ground- and excited-state properties of the rhodopsin chromophore
复制标题
DOI：
10.1021/jp046147k
复制
发表时间：
2004-12-30
影响因子：
3.3
通讯作者：
Buss, V
中科院分区：
文献类型：
--
作者：
Hufen, J;Sugihara, M;Buss, V
高层次的DFT和从头计算CASSCF/CASPT 2方法已被应用于研究的视网膜生色团的基态和激发态作为不同复杂性的反离子的函数。在最近的2.8埃的X-射线结构的基础上的视紫红质，完整的视网膜发色团的存在下，阴离子Glu 113，Thr 94，和水分子（瓦特2b），它们一起形成一个复杂的counterpart的质子化席夫碱优化。此外，复合物进行了研究，其中的组分的counterparts（Thr 94和/或Wat 2b）被删除，也没有任何counterparts的游离发色团，在质子化和去质子化的形式;为CASSCF/CASPT 2计算减少模型发色团与五个共轭双键。在任何抗衡离子的存在下，在靠近席夫碱氮的区域中，键交替强烈增加，导致类似于质子化席夫碱的结构。从C9到C12的三个键中的键交替明显减少，这在裸露的质子化发色团的结构中已经可见。与其他方法的比较表明，键交替被夸大时，非相关的理论方法，并可能被夸大的实验方法以及。键的交替影响激发态能量只有轻微的。反离子对发色团的强允许S-2态的大的蓝移主要是由于反离子的电荷引起的，激发态的电子密度相对于反离子的电荷发生了移动。反离子对S态的影响很小。这与激发到S-1和S-2时计算的发色团的电偶极矩一致，其在S-1状态下的大小和方向发生强烈变化，但当涉及S-2状态时几乎不受影响。
High-level DFT and ab-initio CASSCF/CASPT2 methodologies have been applied to study the ground and excited states of the retinal chromophore as a function of counterions of different complexities. On the basis of the recent 2.8 Angstrom X-ray structure of rhodopsin, the complete retinal chromophore was optimized in the presence of anionic Glu113, Thr94, and a water molecule (Wat2b), which together form a complex counterion of the protonated Schiff base. In addition, complexes were studied in which components of the counterion (Thr94 and/or Wat2b) were removed and also the free chromophore without any counterion, both in the protonated and deprotonated forms; for the CASSCF/CASPT2 calculations a reduced model chromophore with five conjugated double bonds was employed. In the presence of any counterion, bond alternation increases strongly in the region close to the Schiff base nitrogen, resulting in a structure similar to the protonated Schiff base. There is a conspicuous reduction in bond alternation in the three bonds from C9 to C12, which is already visible in the structure of the bare protonated chromophore. Comparison with other methods shows that bond alternation is exaggerated whenever noncorrelated theoretical methods are employed and may be exaggerated by the experimental methods as well. Bond alternation affects the excited-state energies only slightly. The large blue shift which the counterion exerts on the strongly allowed S-2 state of the chromophore is caused mainly by the charge of the counterion against which the electron density of the excited state is shifted. The effect of the counterion on the S state is small. This agrees with the calculated electric dipole moment of the chromophore upon excitation to S-1 and S-2 which changes strongly in magnitude and direction in the S-1 state but is hardly affected when the S-2 state is involved.