Bindra Jasleen K, Niklas Jens, Jeong Yeonjun, Jasper Ahren W, Utschig Lisa M, Poluektov Oleg G
Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, USA.
Phys Chem Chem Phys. 2025 Jan 16. doi: 10.1039/d4cp03971h.
Photosynthetic reaction center proteins (RCs) provide ideal model systems for studying quantum entanglement between multiple spins, a quantum mechanical phenomenon wherein the properties of the entangled particles become inherently correlated. Following light-generated sequential electron transfer, RCs generate spin-correlated radical pairs (SCRPs), also referred to as entangled spin qubit (radical) pairs (SQPs). Understanding and controlling coherence mechanisms in SCRP/SQPs is important for realizing practical uses of electron spin qubits in quantum sensing applications. The bacterial RC (bRC) provides an experimental system for exploring quantum effects in the SCRP P Q, where P, a special pair of bacteriochlorophylls, is the primary donor, and Q is the primary quinone acceptor. In this study, we focus on understanding how local molecular environments and isotopic substitution, particularly deuteration, influence spin coherence times (). Using high-frequency electron paramagnetic resonance (EPR) spectroscopy, we observed that the local environment surrounding P and Q plays a significant role in determining . Our findings show that while deuteration led to a modest increase in , particularly at low temperatures, but the effect was substantially smaller than predicted by classical nuclear spin diffusion alone. This result is in contrast to our previous study of the photosystem I (PSI) RC, where no increase in was observed upon deuteration. Theoretical modeling identified several methyl groups at key distances from the spin centers of both bRC and PSI, and methyl group tunneling at low temperatures has been previously suggested as a mechanism for enhanced spin decoherence. Additionally, our study revealed a strong dependence of spin coherence on the orientation of the external magnetic field, highlighting the influence of the protein microenvironment on spin dynamics. These results offer new insights for optimizing coherence times in quantum system design for quantum information science and sensing applications.
光合反应中心蛋白（RCs）为研究多个自旋之间的量子纠缠提供了理想的模型系统，量子纠缠是一种量子力学现象，其中纠缠粒子的性质本质上相互关联。在光生顺序电子转移之后，RCs产生自旋相关自由基对（SCRPs），也称为纠缠自旋量子比特（自由基）对（SQPs）。理解和控制SCRP/SQPs中的相干机制对于在量子传感应用中实现电子自旋量子比特的实际应用非常重要。细菌RC（bRC）提供了一个实验系统，用于探索SCRP P Q中的量子效应，其中P是一对特殊的细菌叶绿素，是主要供体，Q是主要醌受体。在本研究中，我们专注于理解局部分子环境和同位素取代，特别是氘代，如何影响自旋相干时间（）。使用高频电子顺磁共振（EPR）光谱，我们观察到P和Q周围的局部环境在决定方面起着重要作用。我们的研究结果表明，虽然氘代导致适度增加，特别是在低温下，但该效应远小于仅由经典核自旋扩散预测的结果。这一结果与我们之前对光系统I（PSI）RC的研究形成对比，在该研究中，氘代后未观察到增加。理论建模确定了bRC和PSI自旋中心关键距离处的几个甲基，并且先前已提出低温下的甲基隧穿是增强自旋退相干的一种机制。此外，我们的研究揭示了自旋相干对外加磁场方向的强烈依赖性，突出了蛋白质微环境对自旋动力学的影响。这些结果为优化量子信息科学和传感应用的量子系统设计中的相干时间提供了新的见解。
