Supramolecular platinum complexes for cancer therapy

The rise of supramolecular chemistry offers new tools to design therapeutics and delivery platforms for biomedical applications. This review aims to highlight the recent developments that harness host-guest interactions and self-assembly to design novel supramolecular Pt complexes as anticancer agents and drug delivery systems. These complexes range from small host-guest structures to large metallosupramolecules and nanoparticles. These supramolecular complexes integrate the biological properties of Pt compounds and novel supramolecular structures, which inspires new designs of anticancer approaches that overcome problems in conventional Pt drugs. Based on the differences in Pt cores and supramolecular structures, this review focuses on five different types of supramolecular Pt complexes, and they include host-guest complexes of the FDA-approved Pt(II) drugs, supramolecular complexes of nonclassical Pt(II) metallodrugs, supramolecular complexes of fatty acid-like Pt(IV) prodrugs, self-assembled nanotherapeutics of Pt(IV) prodrugs, and self-assembled Pt-based metallosupramolecules.     

On the relative stabilities of the alkali cations 222 cryptates in the gas phase and in water-methanol solution

The relative stabilities of the alkali [M ⊂ 222]⁺ cryptates (M = Na, K, Rb and Cs) in the gas phase and in solution (80:20 v/v methanol:water mixture) at 298 K, are computed using a combination of ab initio quantum-chemical calculations (HF/6-31G and MP2/6-31+G*//HF/6-31+G*) and explicit-solvent Monte Carlo free-energy simulations. The results suggest that the relative stabilities of the cryptates in solution are due to a combination of steric effects (compression of large ions within the cryptand cavity), electronic effects (delocalization of the ionic charge onto the cryptand atoms) and solvent effects (dominantly the ionic dessolvation penalty). Thus, the relative stabilities in solution cannot be rationalized solely on the basis of a simple match or mismatch between the ionic radius and the cryptand cavity size as has been suggested previously. For example, although the [K ⊂ 222]⁺ cryptate is found to be the most stable in solution, in agreement with experimental data, it is the [Na ⊂ 222]⁺ cryptate that is the most stable in the gas phase. The present results provide further support to the notion that the solvent in which supramolecules are dissolved plays a key role in modulating molecular recognition processes. Figure Alkali cryptates [M ⊂ 222]+ (M = Na, K, Rb and Cs) relative stabilities in gas and methanol:water solution: solvent effects and molecular recognition

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Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Small-angle neutron and X-ray scattering analysis of the supramolecular organization of rhodopsin in photoreceptor membrane

The supramolecular organization of the visual pigment rhodopsin in the photoreceptor membrane remains contentious. Specifically, whether this G protein-coupled receptor functions as a monomer or dimer remains unknown, as does the presence or absence of ordered packing of rhodopsin molecules in the photoreceptor membrane. Completely opposite opinions have been expressed on both issues. Herein, using small-angle neutron and X-ray scattering approaches, we performed a comparative analysis of the structural characteristics of the photoreceptor membrane samples in buffer, both in the outer segment of photoreceptor cells, and in the free photoreceptor disks. The average distance between the centers of two neighboring rhodopsin molecules was found to be ~5.8 nm in both cases. The results indicate an unusually high packing density of rhodopsin molecules in the photoreceptor membrane, but molecules appear to be randomly distributed in the membrane without any regular ordering.     

Use of continuous-elution gel electrophoresis as a preparative tool for blot overlay analysis

Blot overlay techniques have long been used to directly visualize protein-protein interactions within membrane complexes. However, this approach is often hampered by the limited quantities of purified membrane proteins available for conjugation with marker molecules. Here we applied continuous-elution gel electrophoresis as a preparative alternative to isolate sufficient amounts of a homogeneous protein sample to be used as a peroxidase-labeled probe in blot overlays. Microsomal muscle proteins ranging from approximately 20 to 600 kDa were electrophoretically separated and various marker proteins present in eluted fractions were identified by immunoblotting. Since the supramolecular structure of calsequestrin has recently been determined, this terminal cisternae protein was isolated as a model protein for studying protein-protein interactions. In blot overlay assays, peroxidase-conjugated calsequestrin specifically bound to the ryanodine receptor, triadin, calsequestrin itself, and junctin, illustrating that the biological binding affinities are retained in electrophoretically prepared muscle proteins. Potential applications for differential blot overlay approaches and for analyzing pathophysiological preparations from dystrophic muscle were evaluated. Since continuous-elution gel electrophoresis can separate a wide range of differently sized proteins from subcellular fractions, our report indicates that this technique can be utilized for the rapid identification of protein-protein interactions in future high-throughput analyses of subproteomes.     

NMR diffusion as a novel tool for measuring the association constant between cyclodextrin and guest molecules

In this paper we introduce the use of diffusion measurements by nuclear magnetic resonance (NMR) spectroscopy for determining association constants of weak and very weak interactions between cyclodextrin and guest molecules, as long as both the free and complexed guest molecules are soluble to an extent that allows good sensitivity in the NMR experiment. The experimental setup and data analysis is discussed for three different guest molecules: L-phenylalanine, L-leucine and L-valine, representing different strengths of interaction. The underlying assumptions are discussed and the scope of the method (range of K(a) values, requirements to the guest molecule) are discussed. The method's main advantage is its general applicability independent of chromogenic or electrochemical properties of the guest molecule. Whereas calorimetric methods that exhibit a similar generality, are applicable mainly to strong interactions, NMR diffusion measurements are applicable to weaker interactions down to the theoretical limit of 1 M(-1), the upper limit for K(a) values to be determined by it is approximately 200. A further advantage of the method is the low amount of sample needed. The method is in principle applicable to any case of molecular recognition between a host and guest molecule leading to weak interactions.     

Light harvesting, energy transfer and electron cycling of a native photosynthetic membrane adsorbed onto a gold surface

Photosynthetic membranes comprise a network of light harvesting and reaction center pigment-protein complexes responsible for the primary photoconversion reactions: light absorption, energy transfer and electron cycling. The structural organization of membranes of the purple bacterial species Rb. sphaeroides has been elucidated in most detail by means of polarized light spectroscopy and atomic force microscopy. Here we report a functional characterization of native and untreated membranes of the same species adsorbed onto a gold surface. Employing fluorescence confocal spectroscopy and light-induced electrochemistry we show that adsorbed membranes maintain their energy and electron transferring functionality. Gold-adsorbed membranes are shown to generate a steady high photocurrent of 10 μA/cm² for several minutes and to maintain activity for up to three days while continuously illuminated. The surface-adsorbed membranes exhibit a remarkable functionality under aerobic conditions, even when exposed to light intensities well above that of direct solar irradiation. The component at the interface of light harvesting and electron cycling, the LH1 complex, displays exceptional stability, likely contributing to the robustness of the membranes. Peripheral light harvesting LH2 complexes show a light intensity dependent decoupling from photoconversion. LH2 can act as a reversible switch at low-light, an increased emitter at medium light and photobleaches at high light.     

Energy transfer pathways in pyridylporphyrin Re(I) adducts

Dimeric and pentameric adducts between a meso-4′ pyridylporphyrin core and either one or four rhenium(I) bipyridine tricarbonyl units, fac-[Re(CO)₃(bipy)(4′MPyP)][CF₃SO₃] and fac-[{Re(CO)₃(bipy)}₄(4′TPyP)][CF₃SO₃]₄(4′MPyP = 4′-monopyridylporphyrin, 4′TPyP = 4′-tetrapyridylporphyrin), respectively, were synthesized and their photophysical behaviors were investigated by emission and absorption time resolved experiments. The adducts exhibit distinctive supramolecular features, different from those of the molecular components. Upon excitation of the core, the typical porphyrin fluorescence is quenched. This effect is attributed to enhanced intersystem crossing in the porphyrin unit, owing to the heavy atom effect provided by the attached rhenium unit(s). Following excitation of rhenium fragments the typical rhenium MLCT emission is not observed while partial sensitization of the porphyrin fluorescence occurs, indicating that fast intercomponent energy and/or electron transfer processes take place in competition with the intersystem crossing in the rhenium unit.     

Structural models of the supramolecular organization of AQP0 and connexons in junctional microdomains

Membrane proteins perform many essential cellular functions. Over the last years, substantial advances have been made in our understanding of the structure and function of isolated membrane proteins. However, like soluble proteins, many membrane proteins assemble into supramolecular complexes that perform specific functions in specialized membrane domains. Since supramolecular complexes of membrane proteins are difficult to study by conventional approaches, little is known about their composition, organization and assembly. The high signal-to-noise ratio of the images that can be obtained with an atomic force microscope (AFM) makes this instrument a powerful tool to image membrane protein complexes within native membranes. Recently, we have reported high-resolution topographs of junctional microdomains in native eye lens membranes containing two-dimensional (2D) arrays of aquaporin-0 (AQP0) surrounded by connexons. While both proteins are involved in cell adhesion, AQP0 is a specific water channel whereas connexons form cell–cell communication channels with broad substrate specificity. Here, we have performed a detailed analysis of the supramolecular organization of AQP0 tetramers and connexon hexamers in junctional microdomains in the native lens membrane. We present first structural models of these junctional microdomains, which we generated by docking atomic models of AQP0 and connexons into the AFM topographs. The AQP0 2D arrays in the native membrane show the same molecular packing of tetramers seen in highly ordered double-layered 2D crystals obtained through reconstitution of purified AQP0. In contrast, the connexons that surround the AQP0 arrays are only loosely packed. Based on our AFM observations, we propose a mechanism that may explain the supramolecular organization of AQP0 and connexons in junctional domains in native lens membranes.     

Competing Lipid-Protein and Protein-Protein Interactions Determine Clustering and Gating Patterns in the Potassium Channel from Streptomyces lividans (KcsA)

There is increasing evidence to support the notion that membrane proteins, instead of being isolated components floating in a fluid lipid environment, can be assembled into supramolecular complexes that take part in a variety of cooperative cellular functions. The interplay between lipid-protein and protein-protein interactions is expected to be a determinant factor in the assembly and dynamics of such membrane complexes. Here we report on a role of anionic phospholipids in determining the extent of clustering of KcsA, a model potassium channel. Assembly/disassembly of channel clusters occurs, at least partly, as a consequence of competing lipid-protein and protein-protein interactions at nonannular lipid binding sites on the channel surface and brings about profound changes in the gating properties of the channel. Our results suggest that these latter effects of anionic lipids are mediated via the Trp⁶⁷–Glu⁷¹–Asp⁸⁰ inactivation triad within the channel structure and its bearing on the selectivity filter.     

Solid state and solution structures of 2-halophenolate complexes of lithium

Solvent-free 2-halophenolate complexes of lithium, [LiOC₆H₄-2-X)]_n (X = F, Cl, Br), were synthesized by treating the corresponding 2-halophenols with n-butyllithium. ortho-Lithiation was avoided by cooling the reaction mixture to −78 °C and using n-hexane as solvent. Recrystallization from THF/n-hexane lead to tetrameric [Li(THF)(μ₃-OC₆H₄-2-X)]₄ (X = F, 1; X = Cl, 2; X = Br, 3), which was verified by X-ray crystallography for 1 and 2 and derived for 3 by the strong similarities in the ¹H, ¹³C and ⁷Li NMR data for 1, 2 and 3. Cubane structures were revealed for the Li₄O₄ core of 1 and 2. While complex salt 2 possesses only slight distortions within its cubane core, complex salt 1 has an unusual C_2v distortion towards a butterfly shape. Additionally, dimeric [Li(H₂O)₂(μ-OC₆H₄-2-Br)]₂ was obtained containing an extensive network of hydrogen bonding between water molecules. Upfield shifts in the ¹H NMR spectra for the coordinated THF molecules as well as ¹³C and ⁷Li NMR spectra are interpreted as indicating the tetrameric form observed in the X-ray crystal structure is preserved in solution.