Magnetic fields as weak as the Earth's can change the yields of radical pair reactions even though the energies involved are orders of magnitude smaller than k B T at room temperature. Proposed as the source of the light-dependent magnetic compass in migratory birds, the radical pair mechanism is thought to operate in cryptochrome flavoproteins in the retina. Here we demonstrate that the primary magnetic field effect on flavin photoreactions can be chemically amplified by slow radical termination reactions under conditions of continuous photoexcitation. The nature and origin of the amplification are revealed by studies of the intermolecular flavin-tryptophan and flavin-ascorbic acid photocycles and the closely related intramolecular flavin-tryptophan radical pair in cryptochrome. Amplification factors of up to 5.6 have been observed for magnetic fields weaker than 1 mT. Substantial chemical amplification could have a significant impact on the viability of a cryptochrome-based magnetic compass sensor.2 Amongst other directional cues, migratory birds use a light-dependent geomagnetic compass for orientation and navigation [1][2][3] . Although the primary detection mechanism is unclear, the evidence currently points to magnetically sensitive photochemical reactions in cryptochrome proteins located in the retina [4][5][6] . Cryptochromes 7 contain the chromophore flavin adenine dinucleotide (FAD), photoexcitation of which can trigger three consecutive intra-protein electron transfers along a conserved triad of tryptophan (Trp) residues to produce a (FAD  Trp  ) radical pair [8][9][10] . This form of cryptochrome is magnetically sensitive in vitro and possibly also in vivo [11][12][13] . Similar magnetosensitivity is conceivable in other cryptochrome-derived radical pairs in which FAD  or its protonated form, FADH  , is paired with an ascorbic acid radical 14 or, less plausibly, superoxide, 2 O ·- 15,16 . There appear to be different electron transfer pathways in some cryptochromes 17,18 , but no evidence so far that they give rise to magnetic field effects.The radical pair mechanism is well established as the source of magnetic field effects on chemical reactions 19 . Remarkably, the magnetic interactions of electron spins in organic radicals can result in significant changes in reaction kinetics and product yields even though those interactions are many orders of magnitude weaker than the thermal energy, k B T. The sensitivity to applied magnetic fields derives from the coherent spin dynamics of pairs of radicals formed in spincorrelated states. Several conditions need to be satisfied for a radical pair to be suitable as a geomagnetic compass sensor 5 : (a) the electron spin in at least one of the radicals must interact anisotropically with nuclear spins; (b) the mutual interaction of the two electron spins should be small; (c) the radical pair must react spin-selectively; (d) its lifetime should not be too short; and (e) the electron spin relaxation must be relatively slow. Studies of isolated cry...

Lipid rafts are submicron proteolipid domains thought to be responsible for membrane trafficking and signaling. Their small size and transient nature put an understanding of their dynamics beyond the reach of existing techniques, leading to much contention as to their exact role. Here, we exploit the differences in light scattering from lipid bilayer phases to achieve dynamic imaging of nanoscopic lipid domains without any labels. Using phase-separated droplet interface bilayers we resolve the diffusion of domains as small as 50 nm in radius and observe nanodomain formation, destruction, and dynamic coalescence with a domain lifetime of 220 ± 60 ms. Domain dynamics on this timescale suggests an important role in modulating membrane protein function.droplet interface bilayer | iSCAT | lipid nanodomains | label-free imaging | light scattering C ell membranes compartmentalize into lipid domains that enable the selective recruitment of specific proteins (1). These "lipid rafts" have been proposed to control many membrane processes including apical sorting, protein trafficking, and the clustering of proteins during signaling. The dynamic formation and destruction of lipid nanodomains are thought to provide the central mechanism to regulate this wide range of essential processes (2-4). Although many methods now provide strong evidence to support their existence in vivo (5), the combination of nanoscopic size and dynamics on millisecond timescales has placed the direct observation of their behavior beyond the scope of existing techniques. Consequently, although we know they exist, frustratingly little is known regarding their function and dynamics (6).Recent advances in fluorescence nanoscopy provide the only time-dependent information on the behavior of lipid nanodomains (7-9). Stimulated emission depletion-fluorescence correlation spectroscopy has shown cholesterol-mediated hindered nanoscale diffusion of single labeled sphingomyelin lipids that is consistent with the lipid raft hypothesis and transient binding of lipids (9). Superresolution fluorescence microscopy has also revealed protein clusters in cell membranes with 0.5-s temporal resolution (7). All of these experiments, however, are limited in temporal resolution by fluorescence, and must infer lipid nanodomains from the addition of fluorescent labels.Macroscopic phase separation in artificial lipid bilayers has been widely used to help understand the biological implications of domain formation. Different lipid phases can be visualized using fluorescence microscopy with labels that preferentially partition into a specific phase (10-12). This approach is successful for micrometer-sized domains but inevitably fails on the tens to few hundreds of nanometers scale due to limitations in phase specificity, the limited residence time of a label within a specific nanoscopic domain, and the achievable optical resolution (13). The fluorescent probe is itself an additional component that can perturb phase behavior, either directly or through photooxidation (14, 15). As ...

The synthetic biology toolbox lacks extendable and conformationally controllable yet easy-to-synthesize building blocks that are long enough to span membranes. To meet this need, an iterative synthesis of α-aminoisobutyric acid (Aib) oligomers was used to create a library of homologous rigid-rod 310-helical foldamers, which have incrementally increasing lengths and functionalizable N- and C-termini. This library was used to probe the inter-relationship of foldamer length, self-association strength, and ionophoric ability, which is poorly understood. Although foldamer self-association in nonpolar chloroform increased with length, with a ∼14-fold increase in dimerization constant from Aib6 to Aib11, ionophoric activity in bilayers showed a stronger length dependence, with the observed rate constant for Aib11 ∼70-fold greater than that of Aib6. The strongest ionophoric activity was observed for foldamers with >10 Aib residues, which have end-to-end distances greater than the hydrophobic width of the bilayers used (∼2.8 nm); X-ray crystallography showed that Aib11 is 2.93 nm long. These studies suggest that being long enough to span the membrane is more important for good ionophoric activity than strong self-association in the bilayer. Planar bilayer conductance measurements showed that Aib11 and Aib13, but not Aib7, could form pores. This pore-forming behavior is strong evidence that Aibm (m ≥ 10) building blocks can span bilayers.

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