Protein Influence on Charge-Asymmetry of the Primary Donor in Photosynthetic Bacterial Reaction Centers Containing a Heterodimer: Effects on Photophysical Properties and Electron Transfer

详细信息    查看全文

• 作者：Michelle A. Harris ; Craig A. Luehr ; Kaitlyn M. Faries ; Marc Wander ; Lucas Kressel ; Dewey Holten ; Deborah K. Hanson ; Philip D. Laible ; Christine Kirmaier
• 刊名：The Journal of Physical Chemistry B
• 出版年：2013
• 出版时间：April 18, 2013
• 年：2013
• 卷：117
• 期：15
• 页码：4028-4041
• 全文大小：523K
• 年卷期：v.117,no.15(April 18, 2013)
• ISSN：1520-5207

文摘

The substantial electronic distinctions between bacteriochlorophyll (BChl) and its Mg-free analogue bacteriopheophytin (BPh) are exploited in two sets of Rhodobacter capsulatus reaction center (RC) mutants that contain a heterodimeric BChl鈥揃Ph primary electron donor (D). The BPh component of the M-heterodimer (Mhd) or L-heterodimer (Lhd) obtains from substituting a Leu for His M200 or for His L173, respectively. Lhd-尾 and Mhd-尾 RCs serve as the initial templates in the two mutant sets, where 尾 denotes that the L-side BPh acceptor (H_L) has been replaced by a BChl (due to substituting His for Leu M212). Three variants each of Lhd-尾 and Mhd-尾 mutants were constructed: (1) a swap (denoted YF) of the native Phe (L181) and Tyr (M208) residues, which flank D and the nearby M- and L-side monomeric BChl cofactors, respectively, giving Tyr (L181) and Phe (M208); (2) addition of a hydrogen bond (denoted L131LH) to the ring V keto group of the L-macrocycle of D, via replacing the native Leu at L131 with His; (3) the combination of 1 and 2. A low yield of electron transfer (ET) to the M-side BPh (H_M) is observed in all four Lhd-containing RCs. Comparison with the yield of ET to 尾 on the L-side shows that electron density on the L-macrocycle of D* favors ET to the M-side cofactors and vice versa. Increasing or decreasing the electronic asymmetry of D* via the YF, L131LH mutations or the combination results in consistent trends in the characteristics of the long-wavelength ground state absorption band of D, the rate constant of internal conversion of D* to the ground state, and the rate constants for ET to both the L- and M-side cofactors. A surprising correlation is that an increase in the charge asymmetry in D* not only increases the D* internal-conversion rate constant, but also the rate constants for ET to both the L- and M-side cofactors, spanning time scales of tens of picoseconds to several nanoseconds. The YF swap has a previously unrecognized effect on the electronic asymmetry of D*, resulting in increased charge asymmetry for the Mhd and decreased charge asymmetry for the Lhd. This result indicates that the native Tyr (M208) and Phe (L181) in the wild-type RC promote an electron distribution in P* that is the reverse of that favorable for ET to the photoactive L-branch. This conclusion reinforces the view that the native configuration of these residues promotes ET to the L branch primarily by poising the free energies of the charge-separated states. Overall, this work addresses the extent to which electronic couplings complement energetics in underpinning the directionality of ET in the bacterial RC.