PNAS:新解植物中陽光碟機動的催化反應

用傳統電解技術從水中裂解氫和氧需要大量電能。但是，綠色植物能應用陽光碟機動的催化技術從水中有效地生產氧氣，這個過程是光合作用的一部分，是地球氧氣的主要來源。如果通過人工系統模擬這個光催化過程就能提供豐富的氧及氫。但是，科學家們還沒有完全了解這個複雜過程。

4月2日的Proceedings of the National Academy of Sciences上發表了一篇相關論文，它使科學家們更近一步地了解了氫鍵水網狀系統在已知光系統II在光合作用機制中重要性。喬治亞理工學院 研究人員運用普通波菜中提取的光系統II的傅立葉變換紅外光譜學(FT-IR)，測試了下面這個觀點，即氫鍵水分子網狀系統在生產氧氣過程中起催化作用。

在綠色植物、藻類和藍細菌的葉綠體內，氧氣是由氧氣進化複合物（OEC）結構中光誘導氧化等價物的累積所產生。OEC中含有錳離子和鈣離子，光照使OEC中錳離子氧化，短雷射脈衝可一步完成涉及四個連續光誘導氧化反應的反應循環。氧氣在第四步中產生，然後從OEC中釋放出來。

氫鍵網狀系統，包括結合到錳離子、鈣離子和蛋白質醯胺羰基（C=O）基團上的多個水分子，形成OEC周圍的一個靜電網絡系統。在這種情況下，廣泛的氫鍵網絡系統將作為催化劑的一個組成元件，且此催化劑能分離氧氣。

為研究這個過程，運用FT-IR光譜法來描述該網狀系統對短雷射脈衝如何反應，記錄光系統樣本受雷射脈衝之前與之後的紅外光譜。

FT-IR光譜法的精密敏感性可以測定蛋白質醯胺羰基（C=O）基團鍵強度的變化。這些醯胺羰基基團能量可用作氫鍵強度的標誌。短雷射脈衝使錳離子氧化，導致C=O鍵強度變化，這將指示出氫鍵合水分子的增加。當加入抑制劑氨水時，觀察到C=O基氫鍵減弱。加入改變蛋白質表面水分子排列的海藻糖時，則阻止了氨水的這種效應。

這項研究有助於弄清楚氨水是如何抑制光系統的，這是研究人員多年來一直想知道的。它表明氨水能通過破壞氫鍵網狀系統來抑制反應發生，對於設計實施該反應的人工裝置來說，保持一種類似的氫鍵網狀系統可能很重要。除了在理解光合作用過程具有重要性外，該研究可能導致形成利用太陽光線生產氫和氧的新技術。一種可能就是將在太陽生電的光電系統中添加一個仿生光催化過程。（生物谷bioon.com）

A hydrogen-bonding network plays a catalytic role in photosynthetic oxygen evolution

Brandon C. Polander and Bridgette A. Barry

In photosystem II, oxygen evolution occurs by the accumulation of photo-induced oxidizing equivalents at the oxygen-evolving complex (OEC). The sequentially oxidized states are called the S0-S4 states, and the dark stable state is S1. Hydrogen bonds to water form a network around the OEC; this network is predicted to involve multiple peptide carbonyl groups. In this work, we tested the idea that a network of hydrogen bonded water molecules plays a catalytic role in water oxidation. As probes, we used OEC peptide carbonyl frequencies, the substrate-based inhibitor, ammonia, and the sugar, trehalose. Reaction-induced FT-IR spectroscopy was used to describe the protein dynamics associated with the S1 to S2 transition. A shift in an amide CO vibrational frequency (1664 (S1) to 1653 (S2) cm-1) was observed, consistent with an increase in hydrogen bond strength when the OEC is oxidized. Treatment with ammonia/ammonium altered these CO vibrational frequencies. The ammonia-induced spectral changes are attributed to alterations in hydrogen bonding, when ammonia/ammonium is incorporated into the OEC hydrogen bond network. The ammonia-induced changes in CO frequency were reversed or blocked when trehalose was substituted for sucrose. This trehalose effect is attributed to a displacement of ammonia molecules from the hydrogen bond network. These results imply that ammonia, and by extension water, participate in a catalytically essential hydrogen bond network, which involves OEC peptide CO groups. Comparison to the ammonia transporter, AmtB, reveals structural similarities with the bound water network in the OEC.