Microspectroscopic detection of magnetic field sensitive radical pair processes in biological systems
Lecture 2. Prof. Jonathan Woodward
Title: Microspectroscopic detection of magnetic field sensitive radical pair processes in biological systems
Abstract: In the 1970s, it was established that weak magnetic fields could alter chemical reactions proceeding through the formation of reaction intermediates known as spin-correlated radical pairs. The radical pair mechanism explains this behaviour and reveals how magnetic fields even as weak as the Earth’s can measurably affect chemical reaction rates and yields, despite inducing a change in energy of orders of magnitude less than the thermal energy. The radical pair mechanism currently represents the most widely accepted hypothesis to explain the magnetic compass ability of migratory birds  as well as magnetoreception in many other animals, and represents a unique example of a biological capability enabled exclusively by virtue of a coherent quantum mechanical process. However, direct evidence for radical pairs undergoing magnetic field sensitive reactions in living systems remains sparse.
Photochemically generated radical pairs and magnetic field effects thereon have been extensively studied experimentally in chemical systems. In order to confirm and fully investigate their role in biology, we have developed microspectroscopic methods based on established spectroscopic techniques to monitor photochemical reactions proceeding through radical pair intermediates with sufficient time resolution, spatial resolution and sensitivity in real time to observe species at cellular level concentrations on submicron length scales. Here I present our studies on radical pairs exploiting both transient optical absorption microscopy and fluorescence microscopy for a wide range of radical pair reaction environments from aqueous solution to living cells. While transient optical absorption methods provide direct spectroscopic and time-resolved information about the radical pairs, fluorescence microscopy provides far less detailed information, but with much greater sensitivity. I will present details on our studies of magnetic field effects on cellular autofluorescence  which provide direct evidence for the radical pair mechanism operating in living cells, overview our progress in observing spin effects in single radical pairs , and introduce our most recent technique which exploits the combination of a nanosecond, single colour, laser pump-probe method with a rapidly switched magnetic field, allowing direct measurement of radical pair lifetimes with high sensitivity and time resolution and overcoming the disadvantages of conventional fluorescence microscopy.
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 N. Ikeya and J. R. Woodward, PNAS, 2021, 118 (3), e2018043118.
 N. Ikeya, E. A. Nasibulov, K. L. Ivanov, K. Maeda, J. R. Woodward, Mol. Phys. 2018, 117(19), 2604-2617.