Fiedor L, Leupold D, Teuchner K, Voigt B, Hunter C N, Scherz A, Scheer H
Botanisches Institut der Universität München, Menzinger Strasse 67, D 80638 München, Germany .
Biochemistry. 2001 Mar 27;40(12):3737-47. doi: 10.1021/bi002257f.
Replacement of the central Mg in chlorophylls by Ni opens an ultrafast (tens of femtoseconds time range) radiationless de-excitation path, while the principal ground-state absorption and coordination properties of the pigment are retained. A method has been developed for substituting the native bacteriochlorophyll a by Ni-bacteriochlorophyll a ([Ni]-BChl) in the light harvesting antenna of the core complex (LH1) from the purple bacterium, Rhodobacter (Rb.) sphaeroides, to investigate its unit size and excited state properties. The components of the complex have been extracted with an organic solvent from freeze-dried membranes of an LH1-only strain of Rb. sphaeroides and transferred into the micelles of n-octyl-beta-glucopyranoside (OG). Reconstitution was achieved by solubilization in 3.4% OG, followed by dilution, yielding a complex nearly identical to the native one, in terms of absorption, fluorescence, and circular dichroism spectra as well as energy transfer efficiency from carotenoid to bacteriochlorophyll. By adding increasing amounts of [Ni]-BChl to the reconstitution mixture, a series of LH1 complexes was obtained that contain increasing levels of this efficient excitation trap. In contrast to the nearly unchanged absorption, the presence of [Ni]-BChl in LH1 markedly affects the emission properties. Incorporation of only 3.2 and 20% [Ni]-BChl reduces the emission by 50% and nearly 100%, respectively. The subnanosecond fluorescence kinetics of the complexes were monoexponential, with the lifetime identical to that of the native complex, and its amplitude decreasing in parallel with the steady-state fluorescence yield. Quantitative analysis of the data, based on a Poisson distribution of the modified pigment in the reconstituted complex, suggests that the presence of a single excitation trap per LH1 unit suffices for efficient emission quenching and that this unit contains 20 +/- 1 BChl molecules.
叶绿素中心的镁被镍取代后，开启了一条超快（几十飞秒时间范围）的无辐射去激发路径，同时保留了色素的主要基态吸收和配位特性。已开发出一种方法，用镍代细菌叶绿素a（[Ni]-BChl）替代紫色细菌球形红杆菌（Rb.）核心复合物（LH1）光捕获天线中的天然细菌叶绿素a，以研究其单元大小和激发态特性。该复合物的组分已用有机溶剂从仅含LH1的球形红杆菌菌株的冻干膜中提取出来，并转移到正辛基-β-D-吡喃葡萄糖苷（OG）胶束中。通过在3.4%的OG中溶解，然后稀释来实现重组，得到的复合物在吸收、荧光、圆二色光谱以及从类胡萝卜素到细菌叶绿素的能量转移效率方面几乎与天然复合物相同。通过向重组混合物中添加越来越多的[Ni]-BChl，获得了一系列含有这种高效激发陷阱水平不断增加的LH1复合物。与几乎不变的吸收相反，LH1中[Ni]-BChl的存在显著影响发射特性。仅掺入3.2%和20%的[Ni]-BChl分别使发射降低50%和近100%。复合物的亚纳秒荧光动力学是单指数的，其寿命与天然复合物相同，并且其幅度与稳态荧光产率平行下降。基于重组复合物中修饰色素的泊松分布对数据进行的定量分析表明，每个LH1单元存在一个单一激发陷阱就足以实现有效的发射猝灭，并且该单元包含20±1个BChl分子。
