Reflections on the calculation and publication of potentiometrically-determined formation constants

Calculation and reporting procedures are described, which aim to improve the quality and comparability of published formation constant values obtained by glass-electrode potentiometry. Ways in which the processing of data by computer optimization programs can be standardized are the main concern, particularly in respect of improving the usefulness of equilibrium data through incorporation into large publically available databases. These recommendations may be particularly valuable to researchers beginning in this field as well as to those who determine formation constants only occasionally.     

Spirolactams as conformationally restricted pseudopeptides: synthesis and conformational analysis

The synthesis of 1-(tert-butoxycarbonyl)-7-[1-(tert-butoxycarbonyl)-3-methylbutyl]-6-oxo-1,7-diazaspiro[4.5]decanes (S,S)-1a and (S,R)-1b is described. Derivatives 17a,b and 19a are prepared for use in peptide synthesis as constrained surrogates of the Pro-Leu and Gly-Leu dipeptides. The Ac-[Gly-Leu]-Met-NH(2) derivatives (S,S,S)-2a and (S,R,S)-2b, with the tripeptidic C-terminal region present in tachykinins, are also synthesized. Conformational analyses of these tripetide analogues by NMR experiments and molecular modeling calculations show that both (S,S,S)-2a and (S,R,S)-2b epimers are gamma-turn/distorted type II beta-turn mimetics.     

Practical synthesis of a dithiane-protected 3',5'-dialkoxybenzoin photolabile safety-catch linker for solid-phase organic synthesis.

The solution-phase preparation of the 3',5'-dialkoxybenzoin photolabile safety-catch linker 16 is described. Pivotal to this convenient synthesis is the selection of appropriate orthogonal protecting groups for the alkoxy functionalities present. The new linker can be readily loaded onto any standard amine-terminating resin under peptide-coupling conditions, without the need to protect the secondary alcohol functionality, and subsequently loaded with substrate. Alternatively, the loading efficiency of sterically hindered substrates can be enhanced by preloading the semiprotected linker variant 10 in solution prior to immobilization onto the resin. This second generation of benzoin photolabile safety-catch linkers provides greater control of both linker loading and resin attachment and should prove to be a more versatile and convenient form of the linker.     

Volatile compounds of red and white wines by headspace--solid-phase microextraction using different fibers

The behavior of four fibers [polydimethylsiloxane (PDMS), PDMS-divinylbenzene (DVB), carboxen (CAR)-PDMS, PDMS-DVB-CAR), is tested for the analysis of volatile compounds of white and red wine. The PDMS-DVB-CAR fiber is the most appropriate to obtain the most wide volatile profile of wines. The better extraction conditions are 40 min at 35 degrees C. Satisfactory data about the reproducibility and uptake are obtained for more than 40 volatile compounds of red and white wine.     

Strategy for the determination of mixtures of alpha and beta emitters in water samples with a combination of rapid methods

Journal of Radioanalytical and Nuclear Chemistry - Quantification of several alpha and beta emitters in mixtures of radionuclides arouses great interest in many fields such as surveillance around...     

The application of pyranoside phosphite-pyridine ligands to enantioselective Ir-catalyzed hydrogenations of highly unfunctionalized olefins

Eight (biaryl)phosphite/pyridine ligands 1–2a–d have been prepared by the modular functionalization of positions C-2 and C-3 of two d-glucopyranoside backbones. The chiral ligands were examined in the iridium-catalyzed asymmetric hydrogenation of poorly functionalized alkenes, as a function of the relative position of the coordinating groups and the geometric properties of the biaryl phosphite moieties. Enantiomeric excesses of up to 90% were achieved in the hydrogenation of E-2-(4-methoxyphenyl)-2-butene by using 1a and 1c, which seemingly combine the beneficial effect of the phosphite at the 2-position with the matching (R_ax)-configuration of their encumbered biaryl substituents. The results of the hydrogenation of more challenging substrates, such as Z-trisubstituted alkenes, alkenes with a neighboring polar group or demanding 1,1-di-substituted alkenes, generally confirmed this trend, and in some significant cases, the chiral hydrogenated products were isolated with ees of 65–79%.     

A Capped Octahedral MHC6 Compound of a Platinum Group Metal

A MHC₆ complex of a platinum group metal with a capped octahedral arrangement of donor atoms around the metal center has been characterized. This osmium compound OsH{κ²‐C,C‐(PhBIm‐C₆H₄)}₃, which reacts with HBF₄ to afford the 14 e^? species [Os{κ²‐C,C‐(PhBIm‐C₆H₄)}(Ph₂BIm)₂]BF₄ stabilized by two agostic interactions, has been obtained by reaction of OsH₆(PiPr₃)₂ with N,N′‐diphenylbenzimidazolium chloride ([Ph₂BImH]Cl) in the presence of NEt₃. Its formation takes place through the C,C,C‐pincer compound OsH₂{κ³‐C,C,C‐(C₆H₄‐BIm‐C₆H₄)}(PiPr₃)₂, the dihydrogen derivative OsCl{κ²‐C,C‐(PhBIm‐C₆H₄)}(η²‐H₂)(PiPr₃)₂, and the five‐coordinate osmium(II) species OsCl{κ²‐C,C‐(PhBIm‐C₆H₄)}(PiPr₃)₂.     

Using expert’s rules as background knowledge in the ClusDM methodology

In complex domains it is usually quite difficult to introduce context information. However, sometimes that information should be taken into account to make decisions, because it provides some relevant knowledge that cannot be expressed using an attribute-value representation. This is the case of the determination of risk of contamination of soils. In this paper, we propose to use conjunctive rules to introduce additional background knowledge to a MCDM sorting method called ClusDM. ClusDM is based on the aggregation of the data with unsupervised clustering techniques. The paper presents a new algorithm to incorporate rules to guide the clustering process in a semi-supervised way. The paper also describes how it works in the case sorting a set of possible contaminated soils, and compares the results obtained by ClusDM when rules are used or not.     

X-ray powder structure of a new two-dimensional nickel(II) coordination polymer with pyrazine-2,3,5,6-tetracarboxylic acid

The new nickel(II) coordination polymer poly­[di­aqua­nickel(II)-μ-(pyrazine-2,3,5,6-tetra­carboxyl­ato)-tetra­aqua­nickel(II)], {[{Ni(C₈N₂O₈)(H₂O)₂}Ni(H₂O)₄]}_n, has been synthesized and characterized both spectroscopically and crystallographically, by X-ray powder diffraction analysis. In this two-dimensional coordination polymer, Ni^II ions are bridged by pyrazine-2,3,5,6-tetra­carboxyl­ic acid, coordinating in a bis-bidentate manner, so forming one-dimensional polymeric chains. The chains are linked by a second Ni^II ion, via an O atom of the coordinated carboxyl­ate group, resulting in the formation of a two-dimensional layer-like polymer. The remaining coordination sites of the two independent octahedral Ni^II ions are occupied by water mol­ecules. The layers are connected via hydrogen bonds involving all six coordinated water mol­ecules.