細菌視紫質為嗜鹽古生菌Halobacterium salinarum在低氧或是缺氧的環境時，於細胞膜上合成的深紫色蛋白質。它的結構主要由七個α螺旋及一個視黃醛組成，當受到光激發時，視黃醛會進行光異構化反應，進而引發光迴圈反應進行質子幫浦，讓生物體合成生長所需的能量ATP。過去研究中指出，視黃醛周圍的電荷分佈變化會影響光異構化的速率。而將細菌視紫質包覆於脂質奈米碟中，雖可提供優異的水相分散力及模擬細胞膜中脂雙層的環境，但是奈米碟的環境對於光異構化速率的影響仍是未知的。因此在本實驗中吾人使用飛秒瞬態吸收技術，以波長550 nm和580 nm雷射光分別激發樣品之trans及cis結構，探討不同pH值(7.0和5.8)及不同尺寸之奈米碟(nbR_DMPG_cΔH5和nbR_DMPG_cE3D1)對細菌視紫質光異構化行為的影響，並與原生狀態的紫膜(PM)以及微胞環境下(Triton X-100)的單體細菌視紫質(mbR)比較。吾人發現改變pH值(7.0和5.8)並不會影響光異構化的速率，是因為改變pH值對於視黃醛周圍的電荷分佈並無影響，改變奈米碟之尺寸大小，對周圍電荷分佈影響一樣。而在不同結構中， PM有最快的光異構化速率(0.56~0.59 ps)-1，mbR次之(0.84~0.94 ps)-1，nbR最慢(0.9~1.1 ps)-1。吾人認為，當由PM的結構變成mbR及nbR時，Asp85會靠近視黃醛，讓視黃醛周圍的電荷分佈變得較負，使得光異構化位能面之能障昇高，進而使得光異構化速率變慢。

Bacteriorhodopsin is a dark purple protein synthesized by Halobacterium salinarum on the cell membrane in hypoxic environment. Its structure is composed of seven α-helices and one retinal. When bacteriorhodopsin is excited by light, the retinal undergoes isomerization reaction, then initiates a photocycle reaction, allowing the organism to synthesize the energy ATP. Embedding bacteriorhodopsin in lipid nanodisc provides excellent aqueous dispersibility and native-mimic lipid bilayer environment, however, the influence of the nanodisc environment on the photoisomerization rate is unclear. It has been pointed out in previous studies that changes in the charge distribution around retinal affect the rate of photoisomerization. Therefore, in the present work, we used femtosecond transient absorption to detect the first isomerization process of retinal in the photocycle in pH 7.0 and 5.8, and embedded in two sizes of nanodiscs nbR_DMPG_cΔH5 and nbR_DMPG_cE3D1. For comparison, the natural state purple membrane (PM) and the monomeric bacteriorhodopsin (mbR) in micelle (Triton X-100) were also studied. We found that changing the size of the nanodiscs and the pH (7.0 and 5.8) has no effect on the rate of photoisomerization because there is no variation in the charge distribution around the retinal. PM has the fastest photoisomerization rate (0.56~0.59 ps)1, followed by mbR (0.84~0.94 ps)1, and nbR(0.9~1.1 ps)1. In mbR and nbR, Asp85 becomes closer to the terminal N of retinal resulting in raising the energy of the S1 state, greater isomerization barrier in the S1 surface, and slower rate of isomerization.

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