Two Distinct Structures of Membrane‐Associated Homodimers of GTP‐ and GDP‐Bound KRAS4B Revealed by Paramagnetic Relaxation Enhancement

KRAS homo‐dimerization has been implicated in the activation of RAF kinases, however, the mechanism and structural basis remain elusive. We developed a system to study KRAS dimerization on nanodiscs using paramagnetic relaxation enhancement (PRE) NMR spectroscopy, and determined distinct structures of membrane‐anchored KRAS dimers in the active GTP‐ and inactive GDP‐loaded states. Both dimerize through an α4–α5 interface, but the relative orientation of the protomers and their contacts differ substantially. Dimerization of KRAS‐GTP, stabilized by electrostatic interactions between R135 and E168, favors an orientation on the membrane that promotes accessibility of the effector‐binding site. Remarkably, “cross”‐dimerization between GTP‐ and GDP‐bound KRAS molecules is unfavorable. These models provide a platform to elucidate the structural basis of RAF activation by RAS and to develop inhibitors that can disrupt the KRAS dimerization. The methodology is applicable to many other farnesylated small GTPases.     

A one-step homogeneous immunoassay for cancer biomarker detection using gold nanoparticle probes coupled with dynamic light scattering

A one-step homogeneous immunoassay for the detection of a prostate cancer biomarker, free-PSA (prostate specific antigen), was developed using gold nanoparticle probes coupled with dynamic light scattering (DLS) measurements. A spherical gold nanoparticle with a core diameter around 37 nm and a gold nanorod with a dimension of 40 by 10 nm were first conjugated with two different primary anti-PSA antibodies and then used as optical probes for the immunoassay. In the presence of antigen f-PSA in solution, the nanoparticles and nanorods aggregate together into pairs and oligomers through the formation of a sandwich type antibody-antigen-antibody linkage. The relative ratio of nanoparticle-nanorod pairs and oligomers versus individual nanoparticles was quantitatively monitored by DLS measurement. A correlation can be established between this relative ratio and the amount of antigen in solution. The light scattering intensity of nanoparticles and nanoparticle oligomers is several orders of magnitude higher than proteins and other typical molecules, making it possible to detect nanoparticle probes in the low picomolar concentration range. f-PSA in the concentration range from 0.1 to 10 ng/mL was detected by this one-step and washing-free homogeneous immunoassay.     

Dinuclear platinum complexes with biological relevance based on the 1,2-diaminocyclohexane carrier ligand

The synthesis of bifunctional dinuclear platinum complexes, [{PtCl(dach)}(2)-mu-Y](n+)Cl(n) (1-3; Y = H(2)N(CH(2))(3)NH(2)(CH(2))(4)NH(2), H(2)N(CH(2))(6)NH(2)(CH(2))(6)NH(2), and H(2)N(CH(2))(6)NH(2)(CH(2))(2)NH(2)(CH(2))(6)NH(2), respectively; Figure 1) is reported. There was no labilization of the polyamine linker groups of the cis-1,2-diaminocyclohexane complexes in the presence of sulfur-containing species at physiological pH, in contrast to previous studies preformed on trans complexes. Metabolism reactions are somewhat dependent on the nature of the polyamine: at physiological pH, the spermidine complex 1 forms an inert (tetraamine)platinum species in which one platinum is chelated by a central and terminal amino group. The stability of cis-geometry complexes may make them viable second-generation polynuclear platinum clinical candidates.     

Self assembly of Janus ellipsoids

We propose a primitive model of Janus ellipsoids that represents particles with an ellipsoidal core and two semisurfaces coded with dissimilar properties, for example, hydrophobicity and hydrophilicity, respectively. We investigate the effects of the aspect ratio on the self-assembly morphology and aggregation processes using Monte Carlo simulations. We also discuss certain differences between our results and those of earlier results for Janus spheres. In particular, we find that the size and structure of the aggregate can be controlled by the aspect ratio.     

Thermodynamic study of polyethylene-glycols in micellar solutions water+sodium dodecylsulfate

Interactions of a series of polyethyleneglycols (PEG) in aqueous micellar solutions of sodium dodecylsulfate (SDS) were investigated through thermodynamic properties. Volumes, heat capacities and enthalpies of dilution were measured at 25°C. The resulting transfer properties of PEG are reflecting hydrophobic interactions which increase with the polymer length and the hydrophilic interactions occurring in the aqueous polar layer of the micelles. Typically heat capacities clearly evidence various structural changes taking place in the micellar solutions.     

Direct synthesis of 4-arylpiperidines via palladium/copper(I)-cocatalyzed Negishi coupling of a 4-piperidylzinc iodide with aromatic halides and triflates

A general procedure for the synthesis of 4-arylpiperidines via the coupling of 4-(N-BOC-piperidyl)zinc iodide with aryl halides and triflates is presented. The reaction requires cocatalysis with both Cl(2)Pd(dppf) and a copper(I) species. An improved, safer procedure for the activation of zinc dust is also presented.     

Thermal and volumetric properties of chloroform + triethylamine mixtures and the ideal associated solution model of complex formation

In the ideal associated solution model, activity coefficients of all species (labelled A, B, and AB here) are taken to be unity at all mole fractions and all temperatures, with several derivative consequences that have not previously been investigated. We have applied this model to an analysis of the thermodynamic properties (vapor pressures, excess volumes, excess enthalpies, partial molar enthalpies of solution, and excess heat capacities) of the chloroform + triethylamine system in terms of K, V, H, and C _p for the equilibrium represented by A+B=AB. It is shown that there is good consistency between the relatively simple chemical ideal associated solution model and all available thermodynamic data except heat capacities, for which the consistency between model and data is less good. Some limitation of the ideal associated solution model are discussed, along with the relationship of this model to hydrogen bonding in the AB complex and to spectroscopic investigations. New thermal (partial molar enthalpies of solution and excess heat capacities) and volumetric data are presented.     

Apparent molar heat capacities and volumes,van der Waals volumes and accessible surface areas of alkylated derivatives of cytosine and uracil in aqueous solutions at 25°C.

Densities and specific heat capacities of aqueous solutions: 1,3,5,6-tetramethyluracil, 1,6-dimethyl-3-ethyluracil, 1,6-dimethyl-3-propyluracil, 1,6-dimethyl-3-butyluracil, 1,N⁴-trimethylcytosine, 1,N⁴-dimethyl-5-ethylcytosine, 1,N⁴ dimethyl-5-propylcytosine, 1,N⁴-dimethyl-5-butylcytosine were determined using flow calorimetry and flow densimetry at 25°C. Apparent molar volumes and heat capacities, van der Waals volumes and accessible surface areas were determined. It was stated that for alkylcytosines and alkyluracils partial molar volumes and heat capacities correlate linearly with the number of substituted methylene groups-CH₂-as well as with the van der Waals volumes and accessible surface areas of the compounds studied; for cyclooligouracils the cyclization effect was discussed.     

Apparent molar heat capacities and volumes of electrolytes and ions in acetonitrile-water mixtures

Apparent molar volumes V and heat capacities C_p, of NaCl, KCl, KNO₃, AgNO₃, KI, NaBPh₄ and Ph₄PCl have been measured in acetonitrile (AN)-water mixtures up to x_AN=0.25 by flow densitometry and flow microcalorimetry. Limited data have also been obtained for NaF, LiCl and KBr up to x _AN =0.15. Single ion volumes and heat capacities of transfer were obtained using the assumption _tX(PH₄P⁺) = _tX(BPh₄-) where X=V or C _p and _tX is the change in X for a species on transfer from H₂O to AN-H₂O mixtures. Volumes and heat capacities for simple salts show relatively little dependence on solvent composition. However, _tX for simple ions show more pronounced variations, exhibiting at least one extremum. These extrema are similar to but much less pronounced than those derived previously for ions in t-butanol-water mixtures. Surprisingly little correlation is found between the present data and other thermodynamic transfer functions. This is attributed to the predominance of ion-solvent over solvent-solvent interactions in AN-H₂O solutions. _tV and _tC_p, for the silver ion differ markedly from those of the alkali metal ions as a result of the well-known specific interaction between Ag⁺ and AN.