7.10 MAG. A Novel Host Monoacylglyceride for In Meso (Lipid Cubic Phase) Crystallization of Membrane Proteins.

A novel monoacylglycerol, 7.10 MAG, has been produced for use in the in meso (lipid cubic phase) crystallization of membrane proteins and complexes. 7.10 MAG differs from monoolein, the most extensively used lipid for in meso crystallization, in that it is shorter in chain length by one methylene and its cis olefinic bond is two carbons closer to the glycerol headgroup. These changes in structure alter the phase behavior of the hydrated lipid and the microstructure of the corresponding mesophases formed. Temperature-composition phase diagrams for 7.10 MAG have been constructed using small- and wide-angle X-ray scattering over a range of temperatures and hydration levels that span those used for crystallization. The phase diagrams include lamellar crystalline, fluid isotropic, lamellar liquid-crystalline, cubic-Ia3d, and cubic-Pn3m phases, as observed with monoolein. Conspicuous by its absence is the inverted hexagonal phase which is rationalized on the basis of 7.10 MAG's chemical constitution. The cubic phase prepared with the new lipid facilitates the growth of crystals that were used to generate high-resolution structures of intramembrane ß-barrel and α-helical proteins. Compatibility of fully hydrated 7.10 MAG with cholesterol and phosphatidylcholine means that these two lipids can be used as additives to optimize crystallogenesis in screening trials with 7.10 MAG as the host lipid.

Traceless Thioacid-Mediated Radical Cyclization of 1,6-Dienes.

Five-membered ring systems are ubiquitous throughout natural products and synthetic therapeutics, and thus, efficient methods to access this essential scaffold are required. Herein, we report the thioacid-mediated, 5-exo-trig cyclization of various 1,6-dienes, with high yields of up to 98%. The labile thioester functionality can be exploited to generate a free thiol residue which can be used as a functional handle or removed entirely to provide the traceless cyclized product.

Sulfonium Salts as Acceptors in Electron Donor-Acceptor Complexes.

The photoactivation of electron donor-acceptor complexes has emerged as a sustainable, selective and versatile strategy for the generation of radical species. Electron donor-acceptor (EDA) complexation, however, imposes electronic constraints on the donor and acceptor components and this can limit the range of radicals that can be generated using the approach. New EDA complexation strategies exploiting sulfonium salts allow radicals to be generated from native functionality. For example, aryl sulfonium salts, formed by the activation of arenes, can serve as the acceptor components in EDA complexes due to their electron-deficient nature. This "sulfonium tag" approach relaxes the electronic constraints on the parent substrate and dramatically expands the range of radicals that can be generated using EDA complexation. In this review, these new applications of sulfonium salts will be introduced and the areas of chemical space rendered accessible through this innovation will be highlighted.

A general arene C-H functionalization strategy via electron donor-acceptor complex photoactivation.

The photoactivation of electron donor-acceptor complexes has emerged as a sustainable, selective and versatile strategy for the generation of radical species. However, when it comes to aryl radical formation, this strategy remains hamstrung by the electronic properties of the aromatic radical precursors, and electron-deficient aryl halide acceptors are required. This has prevented the implementation of a general synthetic platform for aryl radical formation. Our study introduces triarylsulfonium salts as acceptors in photoactive electron donor-acceptor complexes, used in combination with catalytic amounts of newly designed amine donors. The sulfonium salt label renders inconsequential the electronic features of the aryl radical precursor and, more importantly, it is installed regioselectively in native aromatic compounds by C-H sulfenylation. Using this general, site-selective aromatic C-H functionalization approach, we developed metal-free protocols for the alkylation and cyanation of arenes, and showcased their application in both the synthesis and the late-stage modification of pharmaceuticals and agrochemicals.

In meso crystallogenesis. Compatibility of the lipid cubic phase with the synthetic digitonin analogue, glyco-diosgenin.

Digitonin has long been used as a mild detergent for extracting proteins from membranes for structure and function studies. As supplied commercially, digitonin is inhomogeneous and requires lengthy pre-treatment for reliable downstream use. Glyco-diosgenin (GDN) is a recently introduced synthetic surfactant with features that mimic digitonin. It is available in homogeneously pure form. GDN is proving to be a useful detergent, particularly in the area of single-particle cryo-electron microscopic studies of membrane integral proteins. With a view to using it as a detergent for crystallization trials by the in meso or lipid cubic phase method, it was important to establish the carrying capacity of the cubic mesophase for GDN. This was quantified in the current study using small-angle X-ray scattering for mesophase identification and phase microstructure characterization as a function of temperature and GDN concentration. The data show that the lipid cubic phase formed by hydrated monoolein tolerates GDN to concentrations orders of magnitude in excess of those used for membrane protein studies. Thus, having GDN in a typical membrane protein preparation should not deter use of the in meso method for crystallogenesis.