Croce R, Zucchelli G, Garlaschi F M, Bassi R, Jennings R C
Dipartimento di Biologia, Università di Milano, Italy.
Biochemistry. 1996 Jul 2;35(26):8572-9. doi: 10.1021/bi960214m.
Photosystem I with its full antenna complement (PSI-LHCI) has been prepared by mild detergent solubilization with octyl beta-D-glucopyranoside from maize thylakoids. A preliminary polypeptide analysis is presented. At room temperature, the steady-state fluorescence derives from an almost perfectly thermalized state, as demonstrated by a Stepanov analysis, in which about 90% of the excited states are associated with the red chlorophyll spectral forms absorbing above 700 nm. Equilibration is temperature-sensitive and is lost at T < 200 K. A careful analysis of fluorescence between 75 and 280 K clearly demonstrates the presence of at least three red chlorophyll spectral forms with emission maxima at 720, 730, and 742 nm, the absorption origin bands of which have been calculated at 714, 725, and 738 nm. On the basis of a minor deviation from thermal equilibration around 695 nm, it is suggested that at least 3-4 antenna chlorophylls, with an average absorption near 695 nm, are strongly coupled to P700. Thermodynamic analysis of absorption and fluorescence spectra indicates that the equilibrium, absorption-weighted excited state population of the P700 dimer is around 0.013 assuming that the low-energy exciton state possesses all the oscillator strength. The average free energy for excitation transfer from antenna to P700 is thus calculated to be -0.26 kT at room temperature. This indicates that P700 is almost isoenergetic with its antenna at room temperature when the red forms are taken fully into account. From the calculated excited state population of P700, we estimate that the primary charge separation rate in PSI is 1-2 ps-1.
通过用来自玉米类囊体的β-D-吡喃葡萄糖苷温和去污剂增溶,制备了具有完整天线补充的光系统I(PSI-LHCI)。本文给出了初步的多肽分析。在室温下,稳态荧光源自几乎完全热化的状态,这通过斯特潘诺夫分析得到证明,其中约90%的激发态与吸收波长在700nm以上的红色叶绿素光谱形式相关。平衡对温度敏感,在T < 200K时消失。对75至280K之间的荧光进行仔细分析清楚地表明,存在至少三种发射最大值分别在720、730和742nm的红色叶绿素光谱形式,其吸收起始带经计算分别在714、725和738nm。基于在695nm附近与热平衡的微小偏差,表明至少3 - 4个平均吸收在695nm附近的天线叶绿素与P700强烈耦合。吸收和荧光光谱的热力学分析表明,假设低能激子态拥有所有振子强度,P700二聚体的平衡、吸收加权激发态布居约为0.013。因此,室温下从天线到P700的激发转移平均自由能计算为-0.26kT。这表明当充分考虑红色形式时,室温下P700与其天线几乎等能。根据计算出的P700激发态布居,我们估计PSI中的初级电荷分离速率为1 - 2 ps-1。