Wang Yajie, Bovee-Geurts Petra H M, Lugtenburg Johan, DeGrip Willem J
Department of Bioorganic Photochemistry, Leiden Institute of Chemistry, Gorlaeus Laboratories, University of Leiden, Post Office Box 9502, 2300 RA Leiden, The Netherlands.
Biochemistry. 2004 Nov 23;43(46):14802-10. doi: 10.1021/bi048404h.
Sterical constraints of the 9-methyl-binding pocket of the rhodopsin chromophore are probed using retinal analogues carrying substituents of increasing size at the 9 position (H, F, Cl, Br, CH(3), and I). The corresponding 11-Z retinals were employed to investigate formation of photosensitive pigment, and the primary photoproduct was identified by Fourier transform infrared difference spectroscopy. In addition, any effects of cumulative strain were studied by introduction of the 9-Z configuration and/or the alpha-retinal ring structure. Our results show that the 9-F analogue still can escape from the 9-methyl-binding pocket and that its photochemistry behaves very similar to the 9-demethyl analogue. The 9-Cl and 9-Br analogues behave very similar to the native 9-methyl pigments, but the 9-I retinal does not fit very well and shows poor pigment formation. This puts an upper limit on the radial dimension of the 9-methyl pocket at 0.45-0.50 nm. Introduction of the alpha-retinal ring constraint in the 11-Z series results in cumulative strain, because the 9-I and 9-Br derivatives cannot bind to generate a photopigment. The 9-Z configuration can partially compensate for the additional alpha-retinal strain. The corresponding 9-Br analogue does form a photopigment, and the other derivatives give increased photopigment yields compared to the corresponding 11-Z derivatives. In fact, 9-Z-alpha-retinal would be an interesting candidate for retinal supplementation studies. Our data provide direct support for the concept that the 9-methyl group is an important determinant in ligand anchoring and activation of the protein and in general agree with a three-point interaction model involving the ring, 9-methyl group, and aldehyde function.
使用在9位带有尺寸不断增加的取代基（H、F、Cl、Br、CH₃和I）的视黄醛类似物，来探究视紫红质发色团9-甲基结合口袋的空间限制。采用相应的11-Z-视黄醛来研究光敏色素的形成，并通过傅里叶变换红外差光谱法鉴定初级光产物。此外，通过引入9-Z构型和/或α-视黄醛环结构，研究了累积应变的任何影响。我们的结果表明，9-F类似物仍能从9-甲基结合口袋中逸出，并且其光化学行为与9-去甲基类似物非常相似。9-Cl和9-Br类似物的行为与天然9-甲基色素非常相似，但9-I视黄醛不太合适，色素形成较差。这为9-甲基口袋的径向尺寸设定了上限，为0.45 - 0.50纳米。在11-Z系列中引入α-视黄醛环限制会导致累积应变，因为9-I和9-Br衍生物无法结合以生成光色素。9-Z构型可以部分补偿额外的α-视黄醛应变。相应的9-Br类似物确实形成了光色素，并且与相应的11-Z衍生物相比，其他衍生物的光色素产率有所提高。实际上，9-Z-α-视黄醛将是视网膜补充研究的一个有趣候选物。我们的数据为9-甲基基团是配体锚定和蛋白质激活的重要决定因素这一概念提供了直接支持，并且总体上与涉及环、9-甲基基团和醛功能的三点相互作用模型一致。
