jp9b06636_si_001.pdf (544.46 kB)
Spin-Correlated Radical Pairs as Quantum Sensors of Bidirectional ET Mechanisms in Photosystem I
journal contribution
posted on 2019-08-23, 13:47 authored by Oleg G. Poluektov, Jens Niklas, Lisa M. UtschigFollowing
light-generated electron transfer reactions in photosynthetic
reaction center proteins, an entangled spin qubit (radical) pair is
created. The exceptional sensitivity of entangled quantum spin states
to weak magnetic interactions, structure, and local environments was
used to monitor the directionality of electron transfer in Photosystem
I (PSI). Electron paramagnetic resonance (EPR) spectra of radical
pairs formed via each symmetric branch of cofactors, A or B, exhibit
distinctive line shapes. By photochemical reduction and biochemical
modification of PSI we created samples where the radical pair(s) from
(1) only A branch, (2) only B branch, or (3) both A and B branches
are detectable. These PSI samples were used to analyze the asymmetry
of electron transfer as a function of temperature, freezing condition,
and temperature cycling. The temperature dependency agrees with a
dynamic model in which the conformational states of the protein regulate
the directionality of electron transfer. High spectral resolution
afforded by high-frequency (130 GHz) EPR, combined with extra resolution
afforded by deuterated proteins, provides new mechanistic insight
via structural and environmental sensitivity of the entangled electron
spins of photogenerated radical pairs.