The distinctive lateral organization of the protein complexes in the thylakoid membrane investigated by Jan Anderson and co-workers is dependent on the balance of various attractive and repulsive ...forces. Modulation of these forces allows critical physiological regulation of photosynthesis that provides efficient light-harvesting in limiting light but dissipation of excess potentially damaging radiation in saturating light. The light-harvesting complexes (LHCII) are central to this regulation, which is achieved by phosphorylation of stromal residues, protonation on the lumen surface and de-epoxidation of bound violaxanthin. The functional flexibility of LHCII derives from a remarkable pigment composition and configuration that not only allow efficient absorption of light and efficient energy transfer either to photosystem II or photosystem I core complexes, but through subtle configurational changes can also exhibit highly efficient dissipative reactions involving chlorophyll–xanthophyll and/or chlorophyll–chlorophyll interactions. These changes in function are determined at a macroscopic level by alterations in protein–protein interactions in the thylakoid membrane. The capacity and dynamics of this regulation are tuned to different physiological scenarios by the exact protein and pigment content of the light-harvesting system. Here, the molecular mechanisms involved will be reviewed, and the optimization of the light-harvesting system in different environmental conditions described.
A sustainable food future Horton, Peter
Royal Society open science,
08/2023, Letnik:
10, Številka:
8
Journal Article
Recenzirano
Odprti dostop
The adverse environmental impacts of food production, the ill-health resulting from excess consumption and malnutrition, and the lack of resilience to the increasing number of threats to food ...availability show that the global system of food provision is not fit for purpose. Here, the causative flaws in the food system are identified and a framework presented for discovering the best ways to eliminate them. This framework is based upon an integrated view of the food system and the socio-economic systems in which it functions. The framework comprises an eight-point plan to describe the structure and functioning of the food system and to discover the optimum ways to bring about the changes needed to deliver a sustainable food future. The plan includes: priorities for research needed to provide options for change; an inclusive analytical methodology that uses the results of this research and incorporates acquisition, sharing and analysis of data; the need for actions at the local and national levels; and the requirements to overcome the barriers to change through education and international cooperation. The prospects for implementation of the plan and the required changes in the outcomes of the food system are discussed.
Herein we report the photocontrol of cucurbit8uril (CB8)-mediated supramolecular polymerization of azobenzene-containing monomers. The CB8 polymers were characterized both in solution and in the ...solid state. These host–guest complexes can be reversibly switched between highly thermostable photostationary states. Moreover, a remarkable stabilization of Z-azobenzene was achieved by CB8 complexation, allowing for structural characterization in the solid state.
Crystal structures described as concomitant triclinic (I) and monoclinic (II) polymorphs of meso‐(E,E)‐1,1′‐1,2‐bis(4‐chlorophenyl)ethane‐1,2‐diylbis(phenyldiazene) Mohamed et al. (2016). Acta Cryst. ...C72, 57–62 have been re‐investigated. The published model for II was distorted due to forcing the symmetry of space group C2/c on an incomplete structure model. It is shown here to be a likely three‐component superposition of S,S and R,R enantiomers with a lesser amount of the meso form. A detailed analysis of how the improbable distortion in the published model aroused suspicion and the subsequent construction of undistorted chemically and crystallographically plausible alternatives having the symmetry of Cc and C2/c is presented. For the sake of completeness, an improved model for the triclinic P structure of the meso isomer I, revised to include a minor disorder component, is also given.
A distorted structure originally described as a monoclinic polymorph of meso‐(E,E)‐1,1′‐1,2‐bis(4‐chlorophenyl)ethane‐1,2‐diylbis(phenyldiazene) is better modeled as a threefold superposition of undistorted S,S and R,R enantiomers with a smaller fraction of the meso isomer. All reasoning behind the reassessment is explained in detail.
The synthesis and characterization of six new substituted guanidium tetrahydroxidohexaoxidopentaborate(1-) salts are reported: C(NH
)
(NHMe)B
O
(OH)
·H
O (
), C(NH
)
(NH{NH
})B
O
(OH)
(
), C(NH
)
...(NMe
)B
O
(OH)
(
), C(NH
)(NMe
)
B
O
(OH)
(
), C(NHMe)(NMe
)
B
O
(OH)
·B(OH)
(
), and TBDHB
O
(OH)
(
) (TBD = 1,5,7-triazabicyclo 4.4.0dec-5-ene). Compounds
-
were prepared as crystalline salts from basic aqueous solution via self-assembly processes from B(OH)
and the appropriate substituted cation. Compounds
-
were characterized by spectroscopic (NMR and IR) and by single-crystal XRD studies. A thermal (TGA) analysis on compounds
-
and
demonstrated that they thermally decomposed via a multistage process to B
O
at >650 °C. The low temperature stage (<250 °C) was endothermic and corresponded to a loss of H
O. Reactant stoichiometry, solid-state packing, and H-bonding interactions are all important in assembling these structures. An analysis of H-bonding motifs in known unsubstituted guanidinium salts C(NH
)
B
O
(OH)
·2H
O, C(NH
)
B
O
(OH)
·H
O, and C(NH
)
B
O
(OH)
and in compounds
-
revealed that two important H-bonding R
(8) motifs competed to stabilize the observed structures. The guanidinium cation formed charge-assisted pincer cation-anion H-bonded rings as a major motif in C(NH
)
B
O
(OH)
·2H
O and C(NH
)
B
O
(OH)
, whereas the anion-anion ring motif was dominant in C(NH
)
B
O
(OH)
·H
O and in compounds
-
. This behaviour was consistent with the stoichiometry of the salt and packing effects also strongly influencing their solid-state structures.
Molecular self-assembly is the spontaneous association of simple molecules into larger and ordered structures
. It is the basis of several natural processes, such as the formation of colloids, ...crystals, proteins, viruses and double-helical DNA
. Molecular self-assembly has inspired strategies for the rational design of materials with specific chemical and physical properties
, and is one of the most important concepts in supramolecular chemistry. Although molecular self-assembly has been extensively investigated, understanding the rules governing this phenomenon remains challenging. Here we report on a simple hydrochloride salt of fampridine that crystallizes as four different structures, two of which adopt unusual self-assemblies consisting of polyhedral clusters of chloride and pyridinium ions. These two structures represent Frank-Kasper (FK) phases of a small and rigid organic molecule. Although discovered in metal alloys
more than 60 years ago, FK phases have recently been observed in several classes of supramolecular soft matter
and in gold nanocrystal superlattices
and remain the object of recent discoveries
. In these systems, atoms or spherical assemblies of molecules are packed to form polyhedra with coordination numbers 12, 14, 15 or 16. The two FK structures reported here crystallize from a dense liquid phase and show a complexity that is generally not observed in small rigid organic molecules. Investigation of the precursor dense liquid phase by cryogenic electron microscopy reveals the presence of spherical aggregates with sizes ranging between 1.5 and 4.6 nanometres. These structures, together with the experimental procedure used for their preparation, invite interesting speculation about their formation and open different perspectives for the design of organic crystalline materials.
Integrating symmetry-reducing methods into self-assembly methodology is desirable to efficiently realise the full potential of molecular cages as hosts and catalysts. Although techniques have been ...explored for metal organic (coordination) cages, rational strategies to develop low symmetry organic cages remain limited. In this article, we describe rules to program the shape and symmetry of organic cage cavities by designing edge pieces that bias the orientation of the amide linkages. We apply the rules to synthesise cages with well-defined cavities, supported by evidence from crystallography, spectroscopy and modelling. Access to low-symmetry, self-assembled organic cages such as those presented, will widen the current bottleneck preventing study of organic enzyme mimics, and provide synthetic tools for novel functional material design.
The rules underpinning the dynamic low-symmetry conformation of a symmetric organic cage are decoded, and rationally reapplied to access new cages with constrained symmetry. This allows tuning of conformation, height and shape for guest binding.
Amide-based organic cage cavities are, in principle, ideal enzyme active site mimics. Yet, cage-promoted organocatalysis has remained elusive, in large part due to synthetic accessibility of robust ...and functional scaffolds. Herein, we report the acyl transfer catalysis properties of robust, hexaamide cages in organic solvent. Cage structural variation reveals that esterification catalysis with an acyl anhydride acyl carrier occurs only in bifunctional cages featuring internal pyridine motifs and two crucial antipodal carboxylic acid groups. 1H NMR data and X-ray crystallography show that the acyl carrier is rapidly activated inside the cavity as a covalent mixed-anhydride intermediate with an internal hydrogen bond. Michaelis–Menten (saturation) kinetics suggest weak binding (K M = 0.16 M) of the alcohol pronucleophile close to the internal anhydride. Finally, activation and delivery of the alcohol to the internal anhydride by the second carboxylic acid group forms ester product and releases the cage catalyst. Eyring analysis indicates a strong enthalpic stabilization of the transition state (5.5 kcal/mol) corresponding to a rate acceleration of 104 over background acylation, and an ordered, associative rate-determining attack by the alcohol, supported by DFT calculations. We conclude that internal bifunctional organocatalysis specific to the cage structural design is responsible for the enhancement over the background reaction. These results pave the way for organic-phase enzyme mimicry in self-assembled cavities with the potential for cavity elaboration to enact selective acylations.
Two substituted phosphonium tetrahydoxidohexaoxidopentaborate(1-) salts, iPrPPh3B5O6(OH)4·3.5H2O (1) and MePPh3B5O6(OH)4·B(OH)3·0.5H2O (2), were prepared by templated self-assembly processes with ...good yields by crystallization from basic methanolic aqueous solutions primed with B(OH)3 and the appropriate phosphonium cation. Salts 1 and 2 were characterized by spectroscopic (NMR and IR) and thermal (TGA/DSC) analysis. Salts 1 and 2 were thermally decomposed in air at 800 °C to glassy solids via the anhydrous phosphonium polyborates that are formed at lower temperatures (<300 °C). BET analysis of the anhydrous and pyrolysed materials indicated they were non-porous with surface areas of 0.2–2.75 m2/g. Rhe recrystallization of 1 and 2 from aqueous solution afforded crystals suitable for single-crystal XRD analyses. The structure of 1 comprises alternating cationic/anionic layers with the H2O/pentaborate(1-) planes held together by H-bonds. The cationic planes have offset face-to-face (off) and vertex-to-face (vf) aromatic ring interactions with the iPr groups oriented towards the pentaborate(1-)/H2O layers. The anionic lattice in 2 is expanded by the inclusion of B(OH)3 molecules to accommodate the large cations; this results in the formation of a stacked pentaborate(1-)/B(OH)3 structure with channels occupied by the cations. The cations within the channels have vf, ef (edge-to-face), and off phenyl embraces. Both H-bonding and phenyl embrace interactions are important in stabilizing these two solid-state structures.
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