Epigallocatechin-3-gallate remodels apolipoprotein A-I amyloid fibrils into soluble oligomers in the presence of heparin.

Amyloid deposits of WT apolipoprotein A-I (apoA-I), the main protein component of high-density lipoprotein, accumulate in atherosclerotic plaques where they may contribute to coronary artery disease by increasing plaque burden and instability. Using CD analysis, solid-state NMR spectroscopy, and transmission EM, we report here a surprising cooperative effect of heparin and the green tea polyphenol (-)-epigallocatechin-3-gallate (EGCG), a known inhibitor and modulator of amyloid formation, on apoA-I fibrils. We found that heparin, a proxy for glycosaminoglycan (GAG) polysaccharides that co-localize ubiquitously with amyloid in vivo, accelerates the rate of apoA-I formation from monomeric protein and associates with insoluble fibrils. Mature, insoluble apoA-I fibrils bound EGCG (KD = 30 ± 3 µm; Bmax = 40 ± 3 µm), but EGCG did not alter the kinetics of apoA-I amyloid assembly from monomer in the presence or absence of heparin. EGCG selectively increased the mobility of specific backbone and side-chain sites of apoA-I fibrils formed in the absence of heparin, but the fibrils largely retained their original morphology and remained insoluble. By contrast, fibrils formed in the presence of heparin were mobilized extensively by the addition of equimolar EGCG, and the fibrils were remodeled into soluble 20-nm-diameter oligomers with a largely α-helical structure that were nontoxic to human umbilical artery endothelial cells. These results argue for a protective effect of EGCG on apoA-I amyloid associated with atherosclerosis and suggest that EGCG-induced remodeling of amyloid may be tightly regulated by GAGs and other amyloid co-factors in vivo, depending on EGCG bioavailability.

Asgard archaea shed light on the evolutionary origins of the eukaryotic ubiquitin-ESCRT machinery.

The ESCRT machinery, comprising of multiple proteins and subcomplexes, is crucial for membrane remodelling in eukaryotic cells, in processes that include ubiquitin-mediated multivesicular body formation, membrane repair, cytokinetic abscission, and virus exit from host cells. This ESCRT system appears to have simpler, ancient origins, since many archaeal species possess homologues of ESCRT-III and Vps4, the components that execute the final membrane scission reaction, where they have been shown to play roles in cytokinesis, extracellular vesicle formation and viral egress. Remarkably, metagenome assemblies of Asgard archaea, the closest known living relatives of eukaryotes, were recently shown to encode homologues of the entire cascade involved in ubiquitin-mediated membrane remodelling, including ubiquitin itself, components of the ESCRT-I and ESCRT-II subcomplexes, and ESCRT-III and Vps4. Here, we explore the phylogeny, structure, and biochemistry of Asgard homologues of the ESCRT machinery and the associated ubiquitylation system. We provide evidence for the ESCRT-I and ESCRT-II subcomplexes being involved in ubiquitin-directed recruitment of ESCRT-III, as it is in eukaryotes. Taken together, our analyses suggest a pre-eukaryotic origin for the ubiquitin-coupled ESCRT system and a likely path of ESCRT evolution via a series of gene duplication and diversification events.

Time-resolved and two-photon emission imaging microscopy of live cells with inert platinum complexes.

This work explores time-resolved emission imaging microscopy (TREM) for noninvasive imaging and mapping of live cells on a hitherto uncharted microsecond time scale. Simple robust molecules for this purpose have long been sought. We have developed highly emissive, synthetically versatile, and photostable platinum(II) complexes that make TREM a practicable reality. [PtLCl], {HL = 1,3-di(2-pyridyl)benzene and derivatives}, are charge-neutral, small molecules that have low cytotoxicity and accumulate intracellularly within a remarkably short incubation time of 5 min, apparently under diffusion control. Their microsecond lifetimes and emission quantum yields of up to 70% are exceptionally high for transition metal complexes and permit the application of TREM to be demonstrated in a range of live cell types-normal human dermal fibroblast, neoplastic C8161 and CHO cells. [PtLCl] are thus likely to be suitable emission labels for any eukaryotic cell types. The high photostability of [PtLCl] under intense prolonged irradiation has allowed the development of tissue-friendly NIR two-photon excitation (TPE) in conjunction with transition metal complexes in live cells. A combination of confocal one-photon excitation, nonlinear TPE, and microsecond time-resolved imaging has revealed (i) preferential localization of the complexes to intracellular nucleic acid structures, in particular the nucleoli and (ii) the possibility of measuring intracellular emission lifetimes in the microsecond range. The combination of TREM, TPE, and Pt(II) complexes will be a powerful tool for investigating intracellular processes in vivo, because the long lifetimes allow discrimination from autofluorescence and open up the use of commonplace technology.

Localised to intraligand charge-transfer states in cyclometalated platinum complexes: an experimental and theoretical study into the influence of electron-rich pendants and modulation of excited states by ion binding.

The neopentyl ester of 1,3-di(2-pyridyl)benzene-5-boronic acid (dpy-B) is a useful intermediate in the divergent synthesis of N;C;N-coordinating, 1,3-di(2-pyridyl)benzene ligands, HL(n), that carry aryl substituents at the 5-position of the central ring. The platinum(ii) complexes, PtL(n)Cl, of several such ligands have been prepared, incorporating pendant anisoles, arylamines, an oxacrown, and an azacrown, all of which are strongly luminescent in solution at 298 K. The emission of the complexes is partially quenched by oxygen, and all of the compounds are very efficient sensitisers of singlet oxygen. The quantum yields of (1)O(2) formation have been measured on the basis of the intensity of the O(2)(1)Delta(g) emission at 1270 nm, and are in the range 0.25-0.65. Density functional theory (DFT) calculations have been carried out that include the effect of the solvent, on the unsubstituted complex PtL(1)Cl and on the derivatives incorporating p-dimethylaminophenyl and phenyl-15-mono-N-azacrown-5 pendants (PtL(9)Cl and PtL(12)Cl respectively). Absorption spectra have been simulated on the basis of the calculated singlet excitations: they closely resemble the experimental spectra. In particular, the DFT successfully accounts for the appearance of low-energy absorption bands that accompany the introduction of the aryl pendants, indicating the participation of the aryl group in the HOMO but not significantly in the LUMO. The calculated lowest energy triplet excitation of PtL(1)Cl is close to the observed 0-0 emission maximum of this complex in solution. Taking together data for this series of complexes and related compounds previously studied, the energies of the lowest-energy spin-allowed absorption bands are shown to correlate approximately linearly with the oxidation peak potential. The emission energies show a similar correlation in toluene, but in CH2Cl2 the value for PtL(9)Cl is anomalously low. The differing emission properties of this complex in the two solvents suggest a switch to a TICT-like state in CH2Cl2 (TICT = twisted intramolecular charge transfer), stabilised in the more polar environment. Transient DC photoconductivity measurements confirm that the dipole moment of the triplet excited state is larger in CH2Cl2 than in toluene. The azacrown PtL(12)Cl displays similar behaviour. Binding of metal ions such as Ca2+ to the azacrown unit of this complex leads to a pronounced blue shift in the emission, which can be readily understood in terms of the large increase in the TICT energy that will accompany the binding of the metal ion to the lone pair of the azacrown nitrogen atom.

A novel luminescence-based colorimetric oxygen sensor with a "traffic light" response.

We report a novel dual-lumophore oxygen sensor incorporating two lumophores: (1) the platinum(II) cyclometalated complex of the N;C;N-coordinating ligand 1,3,5-tri-(2-pyridyl)benzene and (2) Platinum octaethylporphyrin, which differ in both their emission colours and oxygen sensitivities. Sensor response is given by a dramatic change in emission colour, from red to green at different oxygen concentrations, due to the complete or partial luminescence quenching of one or both of the sensing elements. This approach enables the simple and rapid evaluation of the O2 concentration present, making it applicable to commercial technologies such as modified atmosphere packaging.

Controlling emission energy, self-quenching, and excimer formation in highly luminescent N--C--N-coordinated platinum(II) complexes.

A series of cyclometalated platinum(II) complexes have been prepared, [PtL(n)Cl], containing N--C--N-coordinating, terdentate ligands based on 1,3-dipyridylbenzene (HL(1)), incorporating aryl substituents at the central 5 position of the ligand. All of the new complexes are intensely luminescent in a degassed solution at 298 K (phi = 0.46-0.65 in CH(2)Cl(2)) with lifetimes in the microsecond range (7.9-20.5 micros). The introduction of the aryl substituents leads to a red shift in the lowest-energy, intense charge-transfer absorption band compared to [PtL(1)Cl] (401 nm in CH(2)Cl(2)), in the order H < mesityl < 2-pyridyl < 4-tolyl < 4-biphenylyl < 2-thienyl < 4-(dimethylamino)phenyl (431 nm in CH(2)Cl(2)), which correlates with the decreasing order of oxidation potentials. A similar order is also observed in the emission maxima, ranging from 491 nm for [PtL(1)Cl] to 588 nm for the 4-(dimethylamino)phenyl-substituted complex. The emission spectra of all of the complexes, except for the amino-substituted compound, are highly structured in a dilute solution in CH(2)Cl(2), and the emission is assigned to excited states of primarily (3)LC (ligand-centered) character. At higher concentrations, self-quenching accompanied by structureless excimer emission centered at 700 nm is observed, but the aryl groups attenuate the self-quenching compared to the parent compound [PtL(1)Cl], particularly for the most sterically hindered mesityl complex. The introduction of the strongly electron-donating 4-dimethylamino substituent leads to a switch in the nature of the lowest-energy excited state from (3)LC to one of primarily intraligand charge-transfer (ILCT) character in CH(2)Cl(2): this complex displays a structureless and much broader emission band than the other compounds and a high degree of positive solvatochromism. No excimer emission is observed in CH(2)Cl(2), and self-quenching is an order of magnitude lower than that for the other complexes. However, in nonpolar solvents such as CCl(4), the ILCT state is destabilized, such that the (3)LC remains the lowest-energy excited state. Reversible switching between the ILCT and (3)LC states can also be achieved in a CH(2)Cl(2) solution by protonation of the amine, with an accompanying large change in the emission maxima of >100 nm. The X-ray structures of the biphenylyl- and methyl-substituted complexes are reported, together with those of the 2-pyridyl- and mesityl-substituted ligands and the key synthetic intermediate 1-bromo-3,5-di(2-pyridyl)benzene.