Separation of Neutral Amino Acids with Ligand Exchange Resins

Four neutral amino acids (Gly, Ala,.Val and Leu) were separated with ligand exchange resins. The separation capacity of the ligand exchange resins is compared with that of common ion exchange resins. The effects of eluent, column temperature, and central metal ions of the support on the separation are studied. The relationship between matrix structure of resins and their separation capacity is analysed.     

Boron–Ligand Cooperation: The Concept and Applications

The term boron–ligand cooperation was introduced to describe a specific mode of action by which certain metal-free systems activate chemical bonds. The main characteristic of this mode of action is that one covalently bound substituent at the boron is actively involved in the bond activation process and changes to a datively bound ligand in the course of the bond activation. Within this review, how the term boron–ligand cooperation evolved is reflected on and examples of bond activation by boron–ligand cooperation are discussed. It is furthermore shown that systems that operate via boron–ligand cooperation can complement the reactivity of classic intramolecular frustrated Lewis pairs and applications of this new concept for metal-free catalysis are summarized.     

Ligand Exchange Between Penta-Coordinated Phosphoryl Serine and Histidine Compounds

With the assistance of HPLC-ESI-MS/MS, the self-assembly products of serine and histidine penta-coordinated phosphorus compound were separated and identified. The expectative product was seryl-histidine dipeptide, but it was found that there was almost equimolar amount of histidyl-histidine dipeptide as well as seryl-histidine dipeptide. The mechanism was speculated that there was iigand exchange between penta-coordinated phosphoryl serine and histidine in the reaction process. As a result,two types of dipeptide were produced.     

Calixthiacrown:A New Multidentate Ligand for Platinum Metal

CalixarenesareafamilyofcyclicoligmerspreparedfromfOrmaldehydeandpara-substitutedphenolsviacycliccondensationunderalkalineconditions.Itwassuggestedthatthecalixarenescanbecountedasthethirdhostmoleculeorthethirdgenerationofsupramolecules,aftercrownethersandcyclodextrinst,,,j.TbeycanbeusedaspIatformsorbuildingblockstoconstructaseriesofcompoundsbavingpreorganizedstructures.Whencalixareneswerebridgedbypolyoxyetbylenecbainintra-orintermolecularyly,cal-.ixcrownswereobtained.Someofthelatterexhibitedou…     

A Pyridine-copper Inclusion Complex of a New Tripodal Ligand

1 INTRODUCTION Numerous recent reports have been focused on the construction of supramolecular architectures by using some special tripodal ligands with aromatic ring platforms[1～5]. Mixing this kind of ligands with metal ions in a suitable solution generally yields typical 1D, 2D and 3D coordination polymers or large nanometer-sized cavities supposed to be use- ful for catalysis, molecular selection, ion exchange, specific chemical transformations et al.[6～11]. Now our group is inte…     

Ligand Migration in the Gaseous Insulin-CB7 Complex—A Cautionary Tale About the Use of ECD-MS for Ligand Binding Site Determination

Knowledge of the structure of protein?Cligand complexes can aid in understanding their roles within complex biological processes. Here we use electrospray ionization (ESI) coupled to a Fourier transform ion cyclotron resonance mass spectrometer to investigate the noncovalent binding of the macrocycle cucurbit[7]uril (CB7) to bovine insulin. Recent condensed-phase experiments (Chinai et al., J. Am. Chem. Soc. 133:8810?C8813, 2011) indicate that CB7 binds selectively to the N-terminal phenylalanine of the insulin B-chain. Competition experiments employing ESI mass spectrometry to assess complex formation between CB7 and wild type insulin B-chain vs. a mutant B-chain, confirm that the N-terminal phenylalanine plays in important role in solution-phase binding. However, analysis of fragment ions produced by electron capture dissociation (ECD) of CB7 complexed to intact insulin and to the insulin B-chain suggests a different picture. The apparent gas-phase binding site, as identified by the ECD, lies further along the insulin B-chain. Together, these studies thus indicate that the CB7 ligand migrates in the ESI mass spectrometry analysis. Migration is likely aided by the presence of additional interactions between CB7 and the insulin B-chain, which are not observed in the crystal structure. While this conformational difference may result simply from the removal of solvent and addition of excess protons by the ESI, we propose that the migration may be enhanced by charge reduction during the ECD process itself because ion-dipole interactions are key to CB7 binding. The results of this study caution against using ECD-MS as a stand-alone structural probe for the determination of solution-phase binding sites.     

Metal–ligand Lability and Ligand Mobility Enables Framework Transformation via Ligand Release in a Family of Crystalline 2D Coordination Polymers

A family of seven silver(I)-perfluorocarboxylate-quinoxaline coordination polymers, [Ag₄(O₂CR_F)₄(quin)₄] 1 – 5 (R_F=(CF₂)_n-1CF₃)₄, n=1 to 5); [Ag₄(O₂C(CF₂)₂CO₂)₂(quin)₄] 6 ; [Ag₄(O₂CC₆F₅)₄(quin)₄] 7 (quin=quinoxaline), denoted by composition as 4 : 4 : 4 phases, was synthesised from reaction of the corresponding silver(I) perfluorocarboxylate with excess quinoxaline. Compounds 1 – 7 adopt a common 2D layered structure in which 1D silver-perfluorcarboxylate chains are crosslinked by ditopic quinoxaline ligands. Solid-state reaction upon heating, involving loss of one equivalent of quinoxaline, yielding new crystalline 4 : 4 : 3 phases [Ag₄(O₂C(CF₂)_n-1CF₃)₄(quin)₃]_n ( 8 – 10 , n=1 to 3), was followed in situ by PXRD and TGA studies. Crystal structures were confirmed by direct syntheses and structure determination. The solid-state reaction converting 4 : 4 : 4 to 4 : 4 : 3 phase materials involves cleavage and formation of Ag−N and Ag−O bonds to enable the structural rearrangement. One of the 4 : 4 : 3 phase coordination polymers ( 10 ) shows the remarkably high dielectric constant in the low electric field frequency range.     

Synthesis of Imidazole Cyclophane as Ligand of Binuclear Metalloprotein Models

Theelucidationofthemechanismbywhichthedicoppermetalloproteinhemocyanin1bindsandtransportsdioxygencontmuestobeatopicofgreatmterest.Modelcompounds~idrigtheactionofhemOCyatriandrelatedmetalloenZymehavebeendesignedandsyntheslzedbyseveralpeups.2-4Mo8tofthemusedbis(Pyridine),5bis(Pyrazole),6bts(benzforidazole)7andbistwine)8asllgands,butmodelswithbiologicanYrelevantimidazoleareveryseldom.Sincetwoorthreehistidinere8iduesbindtoeacheopperintheaCtivesitesofdicoppermetalloproteins,thebinudearllgandscon…     

Is Cyanide Really a Strong‐Field Ligand?

Iron man or weakling? Ligand‐field strengths are conveniently expressed by the empirical spectrochemical series. Although cyanide has been deeply entrenched as a strong‐field ligand, a couple of recent examples cast doubt toward the position of this ligand, namely the high‐spin (S=2) states of [Cr^II(CN)₅]^3? and [Fe^II(tpp)(CN)]^?. tpp=meso‐tetraphenylporphinate.

     

Thin-Layer Chromatographic Separation of Amino Acid Enantiomers using Ligand Exchange

ManyopticaIantiPOdescanexertverydifferenteffects0nIivingorganisms.ThedevelopentofnewtechniquesfortheresoIution0fracemicmixtureshasbeenreceivingmuchattention[1-6],eSPeciallyinbiologicalandpharrmcoIogicaIfields,Inthispeper.anewmethodoffigandexchang6thin-layerchreretogr8phyfortheresoIutionofaminoacidenantiarersispresented..ThesilicageIplatestr0atedwithL-arginlneandcopP6ra0etatecombinedwiththesoIventsystemo+MeOH/AocTHF/H,0=8O/8-2/'5.8/6wereusedfortheseParationoftenpeirsofaminoacidenantim…     

Synthesis and Crystal Structure of a New N-alkylated Azacrown Ligand

1mTRODUCTIONAzamacrocyclicligandsandtheirmacrocyclicderivativeswithincorporatingpendantarmsgroupshavereceivedmuchattentionfortheirusesiniontransportandasmodelsinbiomimicresearchc1-51.Recentlymuchattentionisespeciallyfocusedonstudyingtransitionmetalcomplexesofmacrocycles"-".Conversely,onlyafewre-Portsareaboutcomplexesofmixedoxygen-nitrogendonormacrocyclesincorporatingtwoPendantarms,mostofwhicharecarboxylicacidgroups.Theymaybeappliedinionidentification,iontransportandionseparation"-"'aswell…     

Hydrocarbyl Ligand Transformation on the Tungsten–Triosmium Cluster Framework

A comprehensive review of the hydrocarbon derivative chemistry of WOs₃ mixed-metal cluster compounds including synthesis, reactivity, ligand transformation, and solution dynamics is presented.     

Quantifying labile protein—Ligand interactions using electrospray ionization mass spectrometry

A new electrospray ionization mass spectrometry (ES-MS) approach for quantifying protein—ligand complexes that are prone to in-source (gas-phase) dissociation is described. The method, referred to here as the reference ligand ES-MS method, is based on the direct ES-MS assay and competitive ligand binding. A reference ligand (L_ref), which binds specifically to the protein (P), at the same binding site as the ligand (L) of interest, with known affinity and forms a stable protein—ligand complex in the gas phase, is added to the solution. The fraction of P bound to L_ref, which is determined directly from the ES mass spectrum, is sensitive to the fraction of P bound to L in solution and enables the affinity of P for L to be determined. A mathematical framework for the implementation of the method in cases where P has one or two specific ligand binding sites is given. Affinities of two carbohydrate-binding proteins, a single chain fragment of a monoclonal antibody and the lectin concanavalin A, for monosaccharide ligands are reported and the results are shown to agree with values obtained using isothermal titration calorimetry.     

“Ligand Buffer”: A concept useful in the theoretical consideration of equilibria involving chelating agents : Precipitation and solvent extraction in Ligand Buffers

The concept “ligand buffer” is proposed for a system containing a ligand and an excess of metal. The general properties of the ligand buffer are discussed in connexion with the results of pA calculations under various conditions. The “ligand buffer” concept and pA calculations are useful in the consideration of various equilibria involved in solvent extraction, precipitation, etc., in the presence of masking agents.     

Fabrication of Self-Expanding Metal–Organic Cages Using a Ring-Openable Ligand

Metal–organic cages (MOCs), which are formed via coordination-driven assembly, are being extensively developed for various applications owing to the utility of their accessible molecular-sized cavity. While MOC structures are uniquely and precisely predetermined by the metal coordination number and ligand configuration, tailoring MOCs to further modulate the size, shape, and chemical environment of the cavities has become intensively studied for a more efficient and adaptive molecular binding. Herein, we report self-expanding MOCs that exhibit remarkable structural variations in cage size and flexibility while maintaining their topology. A cyclic ligand with an oligomeric chain tethering the two benzene rings of stilbene was designed and mixed with Rh^II ions to obtain the parent MOCs. These MOCs were successfully transformed into expanded MOCs via the selective cleavage of the double bond in stilbene. The expanded MOCs could effectively trap multidentate N-donor molecules in their enlarged cavity, in contrast to the original MOCs with a narrow cavity. As the direct synthesis of expanded MOCs is impractical because of the entropically disfavored structures, self-expansion using ring-openable ligands is a promising approach that allows precision engineering and the production of functional MOCs that would otherwise be inaccessible.     

Highly Enantioselective Phenylacetylene Addition to Benzaldehyde Catalyzed by Chiral Tridentate Ligand

Development of new or improved methods for the asymmetric preparation of chiral propargylic alcohols has gained considerable significance during the past years because they are useful building blocks for the synthesis of many biologically active compounds and natural products.[1] A series of chiral tridentate ligands were conveniently synthesized from amino acids with good yields (Scheme 1).[2] A preliminary study of the enantioselective alkynylation of benzaldehyde catalyzed by this chiral tridentate ligand was carried out and up to 83% ee of chiral propargyl alcohols was obtained (Table 1 ). A further investigation of the tridentate ligand is currently underway.