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Fucoxanthin is a carotenoid found in brown seaweed. Its potential anti-cancer, anti-obesity, antiinflammatory and antioxidant properties have recently attracted interest. This interest is driven by the growing awareness of natural compounds promoting well-being, ongoing research into their health impacts and their appeal to the food and supplement industries. Brown seaweed is a major source of fucoxanthin and its global cultivation is on the rise. Fucoxanthin's structural characteristics contribute to its bioactivity. Isomerization, influenced by factors like light and heat, can alter its biological activity. Recent studies highlight the importance of the cis/trans isomer ratio in determining fucoxanthin's biological effects. This review uses narrative approaches to explore fucoxanthin's potential applications in the food and pharmaceutical industries, emphasizing its source from brown seaweed and considerations for maximizing bioavailability and stability. The dynamic landscape of fucoxanthin research underscores its promising role in human health, encapsulating its multifaceted benefits within a concise framework. [ABSTRACT FROM AUTHOR]

The synthesis of a new family of pentacoordinated spiro[4.4]phosphoranes stabilized by the ortho -benzamide bidentate ligand (o BA, -C6 H4-2-C(O)NH-) through reaction of Cα ,C ortho -dilithiated phosphazenes with oxiranes is reported. Mixtures of epimers that differ in configuration at the phosphorus atom were obtained with moderate to high yields and diastereoselectivities. C3-Disubstituted derivatives could be separated, which provided access to enantiopure products arising from the reaction with (R)-2-phenyloxirane. Stereomutation was observed by heating the spirophosphoranes at 100 °C. Directed ortho -lithiation of spirophoshoranes followed by electrophilic quench reactions including carbon-based and heteroatom-based electrophiles afforded derivatives functionalized in a remote position with respect to the phosphorus atom. Benzoic acid catalyzed hydrolysis of spirophosphoranes furnished ortho -(γ-hydroxyalkylphosphoryl)benzamides by cleavage of the P–O and P–N bonds with retention of the phosphorus configuration. In contrast, alkaline hydrolysis led to the formation of γ-hydroxyphosphinic acids and benzamide. [ABSTRACT FROM AUTHOR]

The quest for stabilizing planar forms of tetracoordinate carbon started five decades ago and intends to achieve interconversion between [R]- and [S]-stereoisomers without breaking covalent bonds. Several strategies are successful in making the planar tetracoordinate form a minimum on its potential energy surface. However, the first examples of systems where stereomutation is possible were reported only recently. In this study, the possibility of neutral and dications of simple hydrocarbons (cyclopentane, cyclopentene, spiropentane, and spiropentadiene) and their counterparts with the central carbon atom replaced by elements from groups 13, 14, and 15 are explored using ab initio MP2 calculations. The energy difference between the tetrahedral and planar forms decreases from row II to row III or IV substituents. Additionally, aromaticity involving the delocalization of the lone pair on the central atom appears to help in further stabilizing the planar form compared to the tetrahedral form, especially for the row II substituents. We identified 11 systems where the tetrahedral state is a minimum on the potential energy surface, and the planar form is a transition state corresponding to stereomutation. Interestingly, the planar structures of three systems were found to be minimum, and the corresponding tetrahedral states were transition states. The energy profiles corresponding to such transitions involving both planar and tetrahedral states without the breaking of covalent bonds were examined. The systems showcased in this study and research in this direction are expected to realize molecules that experimentally exhibit stereomutation.

The helical stereomutation in the kinetically controlled coassembly of the reported carbonyl‐bridged triarylamines (CBTs) is described. The copolymerization of chiral CBTs (S)‐1 or (R)‐1 with achiral 2 in sergeants‐and‐soldiers (SaS) experiments results in a tunable helicity conditioned by the percentage of the chiral sergeant and by the cooling rate. The dissimilar inherent chirality of the extended monomeric (M) and intramolecularly H‐bonded metastable (M*) species, as well as the different stability of the kinetic trap of the chiral CBTs (S)‐1/(R)‐1 and the achiral CBT 2, condition the chirality transfer and afford J‐aggregates of inverse handedness. [ABSTRACT FROM AUTHOR]

Control over the helical organization of synthetic supramolecular systems is intensively pursued to manifest chirality in a wide range of applications ranging from electron spin filters to artificial enzymes. Typically, switching the helicity of supramolecular assemblies involves external stimuli or kinetic traps. However, efforts to achieve helix reversal under thermodynamic control and to understand the phenomena at a molecular level are scarce. Here we present a unique example of helix reversal (stereomutation) under thermodynamic control in the self-assembly of a coronene bisimide that has a 3,5- dialkoxy substitution on the imide phenyl groups (CBI-35CH), leading to "molecular pockets" in the assembly. The stereomutation was observed only if the CBI monomer possesses molecular pockets. Detailed chiroptical studies performed in alkane solvents with different molecular structures reveal that solvent molecules intercalate or form clathrates within the molecular pockets of CBI-35CH at low temperature (263 K), thereby triggering the stereomutation. The interplay among the helical assembly, molecular pockets, and solvent molecules is further unraveled by explicit solvent molecular dynamics simulations. Our results demonstrate how the molecular design of selfassembling building blocks can orchestrate the organization of surrounding solvent molecules, which in turn dictates the helical organization of the resulting supramolecular assembly. [ABSTRACT FROM AUTHOR]

The chiroptical features of supramolecular polymers formed from N-heterotriangulenes 1- 3 have been investigated by circular dichroism (ECD) and vibrational circular dichroism (VCD) techniques. In solution, the CD spectra demonstrate that the helicity of the aggregates depends on only the stereogenic centres located at the peripheral chains. In the gel state, the chiroptical features are conditioned by the point chirality of the stereogenic centres and by the achiral solvent utilised. Sonication of the gels formed in CCl4 reveals both kinetic and thermodynamic phases. These findings reveal the presence of pathway complexity in the gel state triggered by sonication. The described solvent-induced helical stereomutation demonstrates that the gel state can be utilised as an outstanding benchmark for investigating uncommon chiroptical effects and to explore the rules of chirality transmission. [ABSTRACT FROM AUTHOR]

The energy barriers in our recently discovered Walden-type inversion of chlorophosphonium salts are similar to those for Cope rearrangements of caged cyclic hydrocarbons. Therefore, we have designed a molecular system that integrates the two processes, thereby producing the first embodiment of a chemical species that can undergo two entirely different and independent stereomutation mechanisms at the same nominal asymmetric center. Thus, the energy barrier to the rearrangement of 9-phenyl-9-phosphabarbaralane oxide, which is easily prepared by a new high-yielding synthesis, was found to be roughly 11 kcal mol−1. This value is in contrast to the parent barbaralane (7.3 kcal mol−1) but in good agreement with our computational results for the rearrangement barriers. Crucially, in the corresponding chlorophosphonium derivative, two stereomutations occur simultaneously: a fast Cope rearrangement (barrier ≈12 kcal mol−1) and a slow Walden-type inversion of the phosphorus center (barrier ≈21 kcal mol−1). The computational model also revealed a relationship between the Cope rearrangement barrier and the bridgehead distance. The phenomenon of two independent and geometrically orthogonal stereomutations at a single asymmetric center provided important general insights into reaction pathway bifurcation, microscopic reversibility, and dynamic stereochemistry. This first example of coexisting alternative mechanisms that involve covalent bonds may encourage the design of new types of dynamic molecular structures. [ABSTRACT FROM AUTHOR]

Synthetic manifestations of supramolecular chirality have extensively drawn inspiration from naturally occurring systems. Even though in biological systems conformational changes are dissipative, synthetic systems that change conformation under non-equilibrium conditions have still not been established. Herein, we attempt to alleviate this scenario by reporting a synthetic supramolecular system that undergoes temporal changes in its helical conformation as an active system at the expense of a biologically relevant chemical fuel, ATP. Use of enzymes working in tandem provides transient and switchable helices with modular lifetime and stereomutation rates. [ABSTRACT FROM AUTHOR]

Helical vinyl aromatic polymers are emerging as interesting chiral materials due to their dynamic tailorability, synthetic simplicity, and outstanding chemical and physical stabilities. This Personal Account discusses long-range chirality transfer in the radical polymerization of vinylterphenyl monomers and tunable stereomutation of the resultant polymers. It begins with a general introduction to the design, synthesis, and characterization of helical poly{(+)-2,5-bis[4′-(( S)-2-methylbutyloxy)phenyl]styrene}, the first one of this series of polymers. Then, long-range chirality transfer during radical polymerization of terphenyl-based vinyl monomers is explained. After that, the chiroptical property control of the resultant polymers by means of the transition from kinetically controlled conformation to thermodynamically controlled conformation and external stimulus is described. This Personal Account concludes by discussing the advantages and disadvantages of the strategy of using vinylterphenyls to obtain optically active helical polymers and providing a short outlook, especially emphasizing the importance of tacticity on the chiroptical properties of polymers. [ABSTRACT FROM AUTHOR]

3α-Hydroxysteroid dehydrogenase/carbonyl reductase from Comamonas testosteroni reversibly catalyzes the oxidation of androsterone with NAD+ to form androstanedione and NADH. Structurally the substrate-binding loop of the residues, T188-K208, is unresolved, while binding with NAD+ causes the appearance of T188-P191 in the binary complex. This study determines the functional roles of the flexible substrate-binding loop in conformational changes and enzyme catalysis. A stopped-flow study reveals that the rate-limiting step in the reaction is the release of the NADH. The mutation at P185 in the hinge region and T188 in the loop causes a significant increase in the Kd value for NADH by fluorescence titration. A kinetic study of the mutants of P185A, P185G, T188A and T188S shows an increase in kcat, Kandrosterone and KiNAD and equal primary isotope effects of DV and D(V/K). Therefore, these mutants increase the dissociation of the nucleotide cofactor, thereby increasing the rate of release of the product and producing the rate-limiting step in the hydride transfer. Simulated molecular modeling gives results that are consistent with the conformational change in the substrate-binding loop after NAD+ binding. These results indicate that P185, T188 and the flexible substrate-binding loop are involved in binding with the nucleotide cofactor and with androsterone and are also involved in catalysis. [ABSTRACT FROM AUTHOR]

Crystal structure analysis reveals the cocrystallysation of two turnstyle rotamers of a pentacoordinate silicon complex with a chiral backbone derived from 2-amino-1,1,2-triphenylethanol. The structures of the two epimers are well reproduced by DFT calculations. In addition, the coexistence of two rotamers, which readily interconvert in solution, is confirmed by their solid-state NMR spectroscopic data. Possible mechanisms of their interconversion are discussed, and preference is given to a turnstile-like rotation of three ligands at the silicon atom about 120°. [ABSTRACT FROM AUTHOR]

Abstract: The stereomutation at the pyramidal nitrogen atom induced by intense infrared radiation is a prototype, photo-induced chemical reaction. This reaction is addressed here theoretically. We investigate the coherent excitation quantum dynamics as well as of the rearrangement process of atoms with the help of realistic potential energy and dipole moment surfaces. The “Multi-configurational Time-dependent Hartree” (MCTDH) program package is used. We discuss convergence issues related to the choice of basis sets used in this program. A simple control mechanism is proposed to control the rate of the reaction by an appropriate choice of the carrier frequency and duration of the laser pulse. [ABSTRACT FROM AUTHOR]

Abstract: Sterol and alkenone compositions in suspended particle and surface sediment samples collected in the Northwestern Mediterranean Sea during the MEDFLUX program were used to evaluate the relative importance of biotic and abiotic degradation processes on marine organic matter. Alkenone concentrations decreased much more rapidly (~500 fold) between 5 and 800 m than Δ5-sterols (~100-fold) or POC (~100-fold). The diverse functional groups attached to the stable tetracyclic carbon skeleton of Δ5-sterols appeared to be useful for estimating the relative effects of biotic vs. abiotic (photooxidation and autoxidation) degradation. Products of abiotic degradation predominated over products of biotic degradation in suspended particles in the NW Mediterranean. For alkenones, the U 37 K′ index increased from 0.43 to 0.55 with increasing water depth, and a good correlation between variations of U 37 K′ and concentrations of specific Δ5-sterol autoxidation products points to selective autoxidation of alkenones in suspended particles. Stereomutated alkenones (with cis double bonds) were detected in the surface sediment, allowing us to estimate that stereomutation resulted in a +0.05 increase in U 37 K′ . Therefore, abiotic degradation may be another factor effect on alkenone-derived paleothermometry. [Copyright &y& Elsevier]

The quest for stabilizing planar forms of tetracoordinate carbon started five decades ago and intends to achieve interconversion between [R]- and [S]-stereoisomers without breaking covalent bonds. Several strategies are successful in making the planar tetracoordinate form a minimum on its potential energy surface. However, the first examples of systems where stereomutation is possible were reported only recently. In this study, the possibility of neutral and dications of simple hydrocarbons (cyclopentane, cyclopentene, spiropentane, and spiropentadiene) and their counterparts with the central carbon atom replaced by elements from groups 13, 14, and 15 are explored using ab initio MP2 calculations. The energy difference between the tetrahedral and planar forms decreases from row II to row III or IV substituents. Additionally, aromaticity involving the delocalization of the lone pair on the central atom appears to help in further stabilizing the planar form compared to the tetrahedral form, especially for the row II substituents. We identified 11 systems where the tetrahedral state is a minimum on the potential energy surface, and the planar form is a transition state corresponding to stereomutation. Interestingly, the planar structures of three systems were found to be minimum, and the corresponding tetrahedral states were transition states. The energy profiles corresponding to such transitions involving both planar and tetrahedral states without the breaking of covalent bonds were examined. The systems showcased in this study and research in this direction are expected to realize molecules that experimentally exhibit stereomutation. [ABSTRACT FROM AUTHOR]

Abstract: Hydrogen peroxide (H–O–O–H) is among the simplest prototype molecules showing a chiral equilibrium geometry with the possibility of fast quantum stereomutation in the low barrier limit. We report full dimensional quantum dynamical tunneling calculations on a semi-global fully six-dimensional empirically adjusted potential hypersurface for H2O2, which is realistically close to spectroscopic and thermochemical accuracy (B. Kuhn, T.R. Rizzo, D. Luckhaus, M. Quack, M.A. Suhm, J. Chem. Phys. 111 (1999) 2565). Solutions of the time independent Schrödinger equation lead to levels of well defined parity (but undefined chirality), which compare well with available spectroscopic results and provide numerous predictions. Solutions of the time dependent Schrödinger equation with initial conditions of well defined chirality for P and M enantiomers show the time dependent wavepacket motion and periodic change of chirality for time scales between picoseconds and hundreds of picoseconds. Complete six-dimensional dynamics and adiabatic separation of the torsional mode from the high-frequency modes leads to essentially identical results for the stereomutation dynamics in terms of the relevant time dependence. Mode selective inhibition and catalysis of stereomutation by exciting various vibrationally excited levels are reported and discussed in relation to the concept of quasiadiabatic above barrier tunneling. The transition to relaxation behaviour and racemisation is demonstrated with quasithermal wavepackets and analysed in terms of a simple statistical model. At 3000K the racemisation relaxation time is calculated to be 30fs. We also discuss the results in the context of recent results on hydrogen peroxide as a prototype system for parity violation in chiral molecules. [Copyright &y& Elsevier]

Abstract: The stereochemical aspects and photochemistry of a series of 2,4,4,6-tetrasubstituted-4H-thiopyran-1-oxides are described. Substitution-induced changes to the sulfinyl group stereomutation and the ring conformation are investigated. The sulfoxides were configurationally stable at a wide thermal range, but underwent photocatalysed stereomutation at sulfur without concomitant isomerization at C-4. In almost all cases the trans isomer predominated at equilibrium. The theoretical studies are in close agreement with experimental results. [Copyright &y& Elsevier]

We investigate the stereomutation tunneling processes in the axially chiral prototype ion HOClH+ and in H2Te2 isotopomers in their relation to parity violation using quantum chemical calculations including our recently developed MC-LR approach to electroweak quantum chemistry and the quasiadiabatic channel reaction path Hamiltonian (RPH) approach. All the molecules dealt with here exhibit intermediate barriers to stereomutation (in the range from 0.1 to 0.3 eV depending on the molecule and cis- or trans-type of transition structure considered). Whereas tunneling dominates the quantum dynamics of stereomutation in all isotopomers of HOClH+, the ground-state torsional tunneling splittings for hydrogen ditelluride isotopomers D2Te2 and T2Te2 are calculated to be much smaller than the parity violating energy differences ΔEpv between the enantiomers of these molecules. We present a systematic investigation of the dependence of tunneling splittings upon the excitation of various vibrational modes and we identify some strongly promoting and some weakly inhibiting modes as well as essentially inactive modes. A comparison of the new results for HOClH+ with our previous results for the isoelectronic HSOH shows some similarities but also some striking differences. HOClH+ is predicted to have sufficient kinetic stability for a spectroscopic observation, as a barrier of more than 1 eV separates it from the more stable isomer H2OCl+. We also provide a summary comparing the whole series of axially chiral HXYH(+) isotopomers with X, Y = O, S, Se, Te, Cl and discuss the outlook for experiments on molecular parity violation in this series of molecular and ionic species. For the hydrogenic compounds D2Te2 is the only non-radioactive compound, in which parity violation is predicted to dominate over tunneling, similar to the chlorinated species Cl2S2, which we had investigated earlier. [Copyright &y& Elsevier]

Carotenoids are versatile isoprenoids of great importance in food science and nutrition. These compounds are natural colorants and key to combat vitamin A deficiency, a major global nutritional problem. In addition, carotenoids have health-promoting biological actions that can contribute to reduce the risk of serious diseases, including cancer, cardiovascular disease, skin, bone and eye conditions and metabolic disorders. Beneficial roles of carotenoids in cognition and fetal development are also attracting increased attention. The presence of double bonds cause carotenoids to be prone to geometrical isomerization. The distinction among geometrical isomers of carotenoids is important as there can be important differences in terms of bioavailability, stability during processing or even biological actions. This chapter summarizes strategies to separate and tentatively identify geometrical isomers of major dietary carotenoids (neoxanthin, violaxanthin, lutein, zeaxanthin, β-cryptoxanthin, α-carotene, β-carotene, lycopene, phytoene and phytofluene)., (Copyright © 2022 Elsevier Inc. All rights reserved.)

Four ‐ faced Janus, the god of transitions, now permits interconversion of two mirror images of a P ‐ phenyl ‐ phosphabarbaralane via two orthogonal mechanisms at the same asymmetric center; a Walden ‐ type inversion of a phosphonium salt and a Cope rearrangement of its caged cyclic hydrocarbon skeleton. More information can be found in the Full Paper by K. Nikitin and D. G. Gilheany et al. on page 2332 ff. [ABSTRACT FROM AUTHOR]