F-cluster: Reaction-induced spin correlation in multi-radical systems

Abstract

We provide a theoretical analysis of spin-selective recombination processes in clusters of n ≥ 3 radicals. Specifically, we discuss how spin correlation can ensue from random encounters of n radicals, i.e., "F-clusters" as a generalization of radical F-pairs, acting as precursors of spin-driven magnetic field effects. Survival probabilities and the spin correlation of the surviving radical population, as well as transients, are evaluated by expanding the spin density operator in an operator basis that is closed under application of the Haberkorn recombination operator and singlet-triplet dephasing. For the primary spin cluster, the steady-state density operator is found to be independent of the details of the recombination network, provided that it is irreducible; pairs of surviving radicals are triplet-polarized independent of whether they are actually reacting with each other. The steady state is independent of the singlet-triplet dephasing, but the kinetics and the population of sister clusters of smaller size can depend on the degree of dephasing. We also analyze reaction-induced singlet-triplet interconversion in radical pairs due to radical scavenging by initially uncorrelated radicals ("chemical Zeno effect"). We generalize previous treatments for radical triads by discussing the effect of spin-selective recombination in the original pair and extending the analysis to four radicals, i.e., radical pairs interacting with two radical scavengers.

Anisotropic magnetic field effects in the re-oxidation of cryptochrome in the presence of scavenger radicals

“…Encounters of initially uncorrelated radicals (F-pairs/radical cluster) give rise to qualitatively comparable MFEs. 39 The quantum yield of the signaling state, once all radicals have reacted, is given by…”

Radical pairs and superoxide amplification can explain magnetic field effects on planarian regeneration

“…F-pair state is a completely mixed state and is given as , where Î 4 M is an identity matrix of dimension 4 M . Such RPs behave similarly to triplet-born pairs in the presence of an effective singlet channel (i.e., k S > k T ) and behave similarly to singlet-born pairs in the presence of effective triplet channel (i.e., k S < k T ) [47]. It should also be pointed out that for allowed regions, the size of WMF effects is similar in strength to that of triplet-born RPs in the corresponding regions (See the supporting information).…”

Radical triads, not pairs, may explain effects of hypomagnetic fields on neurogenesis

“…The primary -pair will thus be born in the triplet state; in combination with the A •− , the system will be a mixed state of total electronic spin S = 1/2 and 3/2. Alternatively, the radical triad could result from the random encounter of the radicals [ 15 ]. The flavin radical could be protein bound (e.g., formed in NADPH oxidases, as suggested by Rishabh et al [ 4 ]) or free.…”

“…Alternatively, could result from random encounters of the two constitutive radicals forming a so-called F-pair. However, in the presence of an effective singlet recombination channel, the spin dynamics of F-pairs again resemble those of triplet-born pairs [ 15 ]. The only viable pathway to a singlet-born radical pair thus appears to be the reversion of a peroxide, e.g., the C4a-hydroperoxyflavin, which however is deemed the product of the -recombination, again precluding it as initial state.…”