Figure 1
From: Observations about utilitarian coherence in the avian compass
(a) Structure of an avian cryptochrome (PDB identifier: 6PU0, Columba livia), including the central electron transfer chain comprising four tryptophans (W) labelled A (W395), B (W372), C (W318) and D (W369) and ending in the surface exposed tyrosine Y319. Photo-excitation of FAD in cryptochrome initiates consecutive electron transfer reactions of adjacent donors/acceptor pairs (red arrows), producing sequential radical pairs of the form [\(\hbox {FAD}^{\cdot -}\) / \(\hbox {W}^{\cdot +}\)] and possibly [\(\hbox {FAD}^{\cdot -}\) / \(\hbox {Y}^{\cdot }\)] (see e.g. ref. 21). The well separated radical pairs involving \(\hbox {W}_{{C}}\) and \(\hbox {W}_{{D}}\) have been implicated with magnetoreception. Alternative radical pair models have been discussed, e.g., in the context of dark-state reoxidation22,23,24. (b) Generic radical-pair reaction scheme. The radical pair is born in the singlet state (spin multiplicity indicated by superscript labels, total electronic spin: \(S = 0\)) which, via coherent interconversion, can interconvert to a triplet state (\(S = 1\)). Here, \(\hbox {A}^{\cdot }\) is assumed to be FAD and \(\hbox {B}^{\cdot }\) is a tryptophan (W) or tyrosine (Y) residue. The radical pair may form singlet and triplet products with rate constants \(k_{S}\) and \(k_{T}\), respectively. The singlet channel typically comprises radical pair recombination and spin-independent protein structural rearrangements; the latter also contributes to the triplet channel. (c) Graphical representation of hyperfine interactions in \(\hbox {FAD}^{\cdot -}\) (top), \(\hbox {Y}^{\cdot }\) (bottom left) and \(\hbox {W}^{\cdot +}\) (bottom right). Here, in the direction given by the unit vector \({\mathbf {d}}\), the plotted surfaces are drawn at distance \(\vert \vert \mathbf {Ad} \vert \vert\) from the location of the nucleus in question, whereby \(1\)Å corresponds to \(17\) MHz. Surfaces are coloured according to the sign of the projection, with blue and red corresponding to positive and negative signs, respectively. The molecules are shown in their respective standard orientations.
