THE ORIGIN OF THE RED-SHIFTED ABSORPTION MAXIMUM OF THE M412 INTERMEDIATE IN THE BACTERIORHODOPSIN PHOTOCYCLE

The factors that red shift the absorption maximum of the retinal Schiff base chromophore in the M₄₁₂ intermediate of bacteriorhodopsin photocycle relative to absorption in solution were investigated using a series of artificial pigments and studies of model compounds in solution. The artificial pigments derived from retinal analogs that perturb chromophore-protein interactions in the vicinity of the ring moiety indicate that a considerable part of the red shift may originate from interactions in the vicinity of the Schiff base linkage. Studies with model compounds revealed that hydrogen bonding to the Schiff base moiety can significantly red shift the absorption maximum. Furthermore, it was demonstrated that although s-trans ring-chain planarity prevails in the M₄₁₂ intermediate it does not contribute significantly (only ca 750 cm⁻¹) to the opsin shift observed in M₄₁₂. It is suggested that in M₄₁₂, the Schiff base linkage is hydrogen bonded to bound water and/or protein residues inducing a considerable red shift in the absorption maximum of the retinal chromophore.     

人参二醇类皂苷的生物转化动态及人参稀有皂苷C-K或F2的制备

为了低成本有效制备人参稀有皂苷C-K或F₂, 将A. niger g.848菌酶用于转化含有人参皂苷(质量分数)分别为49.6% Rb₁, 25.9% Rd, 19.3% Rc和5.23% Rb₂的西洋参二醇混合皂苷. 霉菌发酵时, 采用人参二醇皂苷诱导物比人参提取液诱导物的产酶总活力提高10%~15%. 所产的2种诱导酶均能水解人参二醇皂苷的3-O-和20-O-多种糖基, 均为人参皂苷酶Ⅰ型; 但是人参二醇皂苷诱导物所产酶几乎全部转化人参二醇皂苷为C-K, 而人参提取液诱导物所产酶则残留中间产物. 使用黑曲霉人参二醇皂苷诱导所产酶, 在转化西洋参二醇皂苷的动态研究中发现, 酶反应1.5~2.5 h, 主要为产物F₂; 酶反应12 h后, 主要产物为C-K皂苷. 基于此, 40 g人参二醇类皂苷在45 ℃粗酶反应24 h, 经处理得到含C-K质量分数为87%的23 g酶反应产物, C-K转化率达85%(摩尔分数). 用40 g西洋参二醇皂苷在45 ℃粗酶反应2.5 h, 经处理得到含有质量分数为58%的F₂和27%的C-K的26 g酶反应产物, F₂转化率为50.4%, C-K转化率为29.5%. 通过人参二醇皂苷诱导的黑曲霉粗酶转化人参二醇类皂苷动态研究, 建立了C-K转化率为85%, F₂转化率为50%的制备方法, 为大批量制备提供了基础依据.