本研究以papain對Halobacterium salinarum嗜鹽菌中紫色細胞膜(purple membrane，PM)的細菌視紫質蛋白(bacteriorhodopsin，BR)進行降解，並探討此處理對其所製備生物晶片之光電流響應的影響。隨著降解時間的增加，會使BR所在的細胞膜由紫色變成藍色，而此特性和BR處於低pH值時相同。將以此所製備之光電晶片，在pH 8.5的環境下測量其光電響應，發現papain降解的時間延長至14 h後，其光電流極性會產生反向，此也和BR處於低pH值時之質子泵特性相似。但是以papain降解D96N PM突變種時，可以發現隨著降解時間的增加，晶片光電流的極性並未產生反轉，表示降解雖然會增加BR質子攝取的速率，但還是不足以彌補因Asp96→Asn突變所造成之影響。另外，將降解後的native PM，置於不同pH值的電解液中測量其光電響應，可發現隨著降解時間的增加，其光電流極性反轉時的pH值也隨著增加；而降解D96N PM所製備的晶片，其光電流極性反轉時的pH值卻一直維持在4.0。此說明了降解確實會影響BR的質子泵特性。此外，添加NaN3於電解液中，可發現NaN3也會影響BR的質子泵特性，進而改變其光電流極性。正極同樣塗覆有不同降解時間後之PM的ITO，而負極則由空白ITO分別改為未經降解和經降解18 h後之native PM的ITO，量測其光電流響應時，可得到具加成性的光電流響應。

最後，利用5-(iodoacetamido) fluorescein (IAF)與G241C PM上之cysteine殘基結合後，PM的495 nm吸收光譜會隨著懸浮液之pH值與鹽濃度而改變，進一步可計算出G241C PM的表面電荷為-0.500 e-/BR。而以papain降解過後之G241C PM則無法顯現495 nm吸收值，再次證明降解會移除cysteine所在之胜肽段，而影響PM的表面電荷。

In this study, limited papain-proteolysis of bacteriorhodopsin (BR) residing in the purple membrane (PM) of halophilic Halobacterium salinarum was employed to investigate the digestion effects on the photocurrents of drop-casted PM chips. The PM colors changed from purple to blue with the increase of digestion time, the same phenomenon also observed while the pH of PM suspension solutions was decreased from alkaline to acidic. The photocurrent polarity of native PM-coated chips in a pH 8.5 electrolyte changed upon 14-h prolonged digestion, which was also observed with an undigested native PM chip in acidic electrolytes. On the other hand, the photocurrent polarity of D96N PM-coated chips in a pH 8.5 electrolyte remained the same throughout the whole digestion process, implying the retardation of BR proton-uptake caused by Asp96→Asn mutation could not be completely overcome by the enhancement of the proton-uptake rate with papain digestion. Furthermore, the electrolyte pH where the photocurrent polarity of native PM-coated chips reversed increased with the increase of digestion time, while the value remained at pH 4.0 for the D96N ones, suggesting the effect of papain-digestion on the proton pump of BR. Moreover, the addition of NaN3 in the electrolyte also affected the proton pump and reversed the photocurrent polarity of the digested PM chips back to the original undigested state. The replacement of the counter electrode from blank ITO to ITO coated with undigested and 18-h digested PM decreased and increased the photocurrents, respectively, indicating the photocurrents of PM were addable. Finally, the labeling with 5-(iodoacetamido) fluorescein yielded an increase of absorbance at 495 nm for G241C PM, but not for the digested PM, confirming the removal of the Cys241-containing C-terminal peptide by papain digestion. Tuning the 495-nm absorbance values by changing the pH and salt concentrations of suspension solutions for the labeled G241C PM led to an estimate of its surface charge as -0.5 e- / BR.

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