MOLECULAR-DYNAMICS OF CIS-TRANS ISOMERIZATION IN RHODOPSIN
MOLECULAR-DYNAMICS OF CIS-TRANS ISOMERIZATION IN RHODOPSIN
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DOI：
10.1021/ja00527a008
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发表时间：
1980-01-01
影响因子：
15
通讯作者：
HUBBARD, LM
中科院分区：
文献类型：
--
作者：
BIRGE, RR;HUBBARD, LM
利用INDO-CISD分子轨道理论和半经验分子动力学方法对视色素视紫红质中的发色团进行了光化学性质的研究。分子轨道计算，包括限制性单激发和双激发组态相互作用，预测了质子化Schiff碱生色团顺反异构化的无势垒第一激发单重态势能面。分子动力学计算预测，激发态物质在异构化期间被捕获在具有约1年寿命的活化络合物中。0.5 PS.这种活化的复合物在1.9- 2.3ps内在2种组分之间快速振荡，这2种组分优先衰变形成异构化产物（视紫红质）或未异构化的11-顺式发色团（视紫红质）。这种活化复合物的性质实际上保证了大于0.5的量子产率。预计赖氨酸残基的构象扭曲会扭曲视紫红质中的发色团，阻止其达到平面全反式构象。分子轨道计算表明构象畸变集中在发色团的C7-C10和/或C12-C15区域。由于赖氨酸残基的压缩，预测视紫红质具有比视紫红质高约14 kcal/mol的自由能。在视紫红质中C15 = N16基团附近存在一个抗衡离子将使视紫红质的自由能增加λ。12 kcal/mol，产生aptx的相对视紫红质自由能。26千卡/摩尔以上的视紫红质（忽略其他抗衡离子的影响）。由于生色团的构象畸变和相互作用，使视紫红质相对于视紫红质的最大吸收峰红移。视紫红质中的发色团具有扭曲的全反式几何构型的经典概念显然是视紫红质漂白循环中第一中间体的最现实的模型。
The photochemical properties of the chromophore in the visual pigment rhodopsin are analyzed using INDO-CISD molecular orbital theory and semiempirical molecular dynamics procedures. The molecular orbital calculations, including restricted single and double excitation configuration interaction, predict a barrierless 1st excited singlet state potential surface for cis-trans isomerization of the protonated Schiff base chromophore. The molecular dynamics calculations predict the excited-state species is trapped during isomerization in an activated complex with a lifetime of .apprx. 0.5 ps. This activated complex rapidly oscillates between 2 components which preferentially decay to form isomerized product (bathorhodopsin) or unisomerized 11-cis-chromophore (rhodopsin) within 1.9-2.3 ps. The nature of this activated complex virtually guarantees a quantum yield greater than 0.5. Conformational distortion of the lysine residue is predicted to distort the chromophore in bathorhodopsin, preventing it from reaching a planar all-trans conformation. The molecular orbital calculations imply the conformational distortion is concentrated in the C7-C10 and/or C12-C15 regions of the chromophore. Bathorhodopsin is predicted to have a free energy approximately 14 kcal/mol higher than rhodopsin due to compression of the lysine residue. The presence of a counterion near the C15.dbd.N16 group in rhodopsin will increase the free energy of bathorhodopsin by .apprx. 12 kcal/mol to yield a relative bathorhodopsin free energy of .apprx. 26 kcal/mol above rhodopsin (neglecting the effect of other counterions). The bathochromic shift of the absorption maximum of bathorhodopsin relative to rhodopsin is attributed to the effects of the counterion and conformational distortion of the chromophore. The classical concept that the chromophore in bathorhodopsin has a distorted all-trans geometry apparently is the most realistic model for the 1st intermediate in the bleaching cycle of rhodopsin.