In many biological membranes, the major lipids are ''non-bilayer lipids,'' which in purified form cannot be arranged in a lamellar structure. The structural and functional roles of these lipids are poorly understood. This work demonstrates that the in vitro association of the two main components of a membrane, the non-bilayer lipid monogalactosyldiacylglycerol (MGDG) and the chlorophyll-a͞b light-harvesting antenna protein of photosystem II (LHCII) of pea thylakoids, leads to the formation of large, ordered lamellar structures: (i) thin-section electron microscopy and circular dichroism spectroscopy reveal that the addition of MGDG induces the transformation of isolated, disordered macroaggregates of LHCII into stacked lamellar aggregates with a long-range chiral order of the complexes; (ii) small-angle x-ray scattering discloses that LHCII perturbs the structure of the pure lipid and destroys the inverted hexagonal phase; and (iii) an analysis of electron micrographs of negatively stained 2D crystals indicates that in MGDG-LHCII the complexes are found in an ordered macroarray. It is proposed that, by limiting the space available for MGDG in the macroaggregate, LHCII inhibits formation of the inverted hexagonal phase of lipids; in thylakoids, a spatial limitation is likely to be imposed by the high concentration of membrane-associated proteins.circular dichroism ͉ chloroplast thylakoid membranes ͉ electron microscopy ͉ light-harvesting complex ͉ lipid-protein interactions T he lamellar organization of biological membranes provides a structural matrix for various proteins and controls the permeability of organic molecules, water, and ions; it also prevents nonspecific protein-protein aggregation, whereas it allows protein diffusion and conformational changes in the membrane. However, biomembranes usually contain substantial amounts of non-bilayer lipids, which in purified form assume nonlamellar structures. In fact, in many membranes, e.g., thylakoid membranes of chloroplasts, membranes of Escherichia coli, rhodopsin, and mitochondria, non-bilayer lipids constitute about half or more of the total lipid content. It is well established that the physical and functional properties of these membranes depend to a large extent on protein-lipid interactions (1, 2). There are a few examples showing that lipid polymorphism can be modulated by proteins, and, in some cases, small unilamellar vesicles can be reconstituted from non-bilayer lipids and membrane proteins (e.g., refs. 3 and 4). However, the structural role of large amounts of non-bilayer lipids has remained enigmatic, and the assembly of extended bilayer lamellae from proteins and predominantly non-bilayer lipids is poorly understood (1, 2, 5). In this work, we use a simple system, the two main constituents of pea thylakoid membranes, purified non-bilayer lipids and isolated protein complexes, to demonstrate that the formation of a large, ordered lamellar structure is possible even in the presence of large amounts of lipids.In chloroplast thylakoid membranes of g...

Synchrotron radiation circular dichroism (SRCD) is a well established technique in structural biology. The first UV-VIS beamline, dedicated to circular dichroism, at Diamond Light Source Ltd, a third-generation synchrotron facility in south Oxfordshire, UK, has recently become operational and it is now available for the user community. Herein the main characteristics of the B23 SRCD beamline, the ancillary facilities available for users, and some of the recent advances achieved are summarized.

The B23 Circular Dichroism beamline at Diamond Light Source has been operational since 2009 and has seen visits from more than 200 user groups, who have generated large amounts of data. Based on the experience of overseeing the users' progress at B23, four key areas requiring the most assistance are identified: planning of experiments and note-keeping; designing titration experiments; processing and analysis of the collected data; and production of experimental reports. To streamline these processes an integrated software package has been developed and made available for the users. The subsequent article summarizes the main features of the software.

We report two crystal structures of a synthetic porphyrin molecule which was programmed for self-assembly. The same groups which ensure that bacteriochlorophylls c, d, and e can self-assemble into the chlorosomal nanorods, the photosynthetic antenna system of some green bacteria, have been engineered into desired positions of the tetrapyrrolic macrocycle. In the case of the 5,15-meso-substituted anchoring groups, depending upon the concentration, by using the same crystallization solvents, either a tetragonal or a layered structure of porphyrin stacks were encountered. Surprisingly, pi-pi interactions combined with extensive dispersive interactions, which also encompass cyclohexane, one of the crystallization solvents, win over putative hydrogen bonding. We are aware that our compounds differ considerably from the natural bacteriochlorophylls, but based upon our findings, we now question the hydrogen-bonding network, previously proposed to organize stacks of bacteriochlorophylls. Transmission electron microscopy (TEM), atomic force microscopy (AFM), and small-angle X-ray scattering (SAXS) on various isomeric compounds support our challenge of current models for the chlorosomal antenna as these show structures, astonishingly similar to those of chlorosomes.

Polymer thin films that emit and absorb circularly polarised light have been demonstrated with the promise of achieving important technological advances; from efficient, high-performance displays, to 3D imaging and all-organic spintronic devices. However, the origin of the large chiroptical effects in such films has, until now, remained elusive. We investigate the emergence of such phenomena in achiral polymers blended with a chiral small-molecule additive (1-aza[6]helicene) and intrinsically chiral-sidechain polymers using a combination of spectroscopic methods and structural probes. We show that – under conditions relevant for device fabrication – the large chiroptical effects are caused by magneto-electric coupling (natural optical activity), not structural chirality as previously assumed, and may occur because of local order in a cylinder blue phase-type organisation. This disruptive mechanistic insight into chiral polymer thin films will offer new approaches towards chiroptical materials development after almost three decades of research in this area.

A detailed investigation of the circular dichroism imaging (CDi) technique on thin films of a chiral 1,4-dialkoxyphenylene-based oligothiophene with outstanding chiroptical features revealed the primary role of local supramolecular structures.

Synchrotron radiation circular dichroism (SRCD) is a well-established method in structural biology. The first UV-VIS beamline dedicated to circular dichroism at Diamond Light Source, a third generation synchrotron facility in South Oxfordshire, has recently become operational and it is now available for the user community. Herein we present an important application of SRCD: the CD measurement of protein solutions in fused silica rectangular capillary cells. This was achieved without the use of any lens between the photoelastic modulator and the photomultiplier tube detectors by exploiting the high photon flux of the collimated beam that can be as little as half a millimeter squared. Measures to minimize or eliminate vacuum-UV protein denaturation effects are discussed. The CD spectra measured in capillaries is a proof of principle to address CD measurements in microdevice systems using the new B23 SRCD beamline.

We introduce the concept of circular dichroism (CD) imaging CDi, which is made possible by the use of the highly collimated light beam of synchrotron radiation (SR) only available at Diamond Light Source B23 beamline for surface scanning of solid samples. At the moment, the space resolution is about 1 mm2, but there are prospects to increase it to at least 0.1 mm2 in the near future. We apply SR-CDi to thin films of conjugated polymers, which give rise to large chiral aggregates. Electronic CD is able to reveal and characterize these supramolecular structures; thus, SR-CDi provides the ultimate complement to microscopy for investigating local domains with different molecular order and to fill a gap between a purely morphological knowledge and an intimate insight into the molecular level. As a corollary of this proof-of-concept work, we showed that such a new technique may provide rational and successful protocols to obtain homogeneous active layers for the fabrication of optoelectronic devices