Singlet–triplet dephasing in radical pairs in avian cryptochromes leads to time-dependent magnetic field effects
Abstract
Cryptochrome 4a (Cry4a) has been proposed as the sensor at the heart of the magnetic compass in migratory songbirds. Blue-light excitation of this protein produces magnetically sensitive flavin–tryptophan radical pairs whose properties suggest that Cry4a could indeed be suitable as a magnetoreceptor. Here, we use cavity ring-down spectroscopy to measure magnetic field effects on the kinetics of these radical pairs in modified Cry4a proteins from the migratory European robin and from nonmigratory pigeon and chicken. B1/2, a parameter that characterizes the magnetic field-dependence of the reactions, was found to be larger than expected on the basis of hyperfine interactions and to increase with the delay between pump and probe laser pulses. Semiclassical spin dynamics simulations show that this behavior is consistent with a singlet–triplet dephasing (STD) relaxation mechanism. Analysis of the experimental data gives dephasing rate constants, rSTD, in the range 3–6 × 107 s−1. A simple “toy” model due to Maeda, Miura, and Arai [Mol. Phys. 104, 1779–1788 (2006)] is used to shed light on the origin of the time-dependence and the nature of the STD mechanism. Under the conditions of the experiments, STD results in an exponential approach to spin equilibrium at a rate considerably slower than rSTD. We attribute the loss of singlet–triplet coherence to electron hopping between the second and third tryptophans of the electron transfer chain and comment on whether this process could explain differences in the magnetic sensitivity of robin, chicken, and pigeon Cry4a’s.
Document Details
- Document Type
- Pub Defense Publication
- Publication Date
- Sep 11, 2023
- Source ID
- 10.1063/5.0166675
Entities
People
- Christiane R. Timmel
- Dan Selby
- Henrik Mouritsen
- Jessica Schmidt
- Jiate Luo
- Jingjing Xu
- Kevin B. Henbest
- Lauren E. Jarocha
- Matthew J. Golesworthy
- Olivier Paré-labrosse
- Peter Hore
- Rabea Bartölke
- Stuart R. Mackenzie
- Tilo Zollitsch
Organizations
- Biotechnology and Biological Sciences Research Council
- Furman University
- German Research Foundation
- Office of Naval Research Global
- University of Oldenburg
- University of Oxford