Pulsed Gradient Spin Echo (PGSE) Diffusion Measurements as a Tool for the Elucidation of a New Type of Hydrogen-Bonded Bicapsular Aggregate

Self-assembly approach toward chiral bimetallic catalysts: bis-urea-functionalized (salen)cobalt complexes for the hydrolytic kinetic resolution of epoxides

A series of novel bis-urea-functionalized (salen)Co complexes has been developed. The complexes were designed to form self-assembled structures in solution through intermolecular urea-urea hydrogen-bonding interactions. These bis-urea (salen)Co catalysts resulted in rate acceleration (up to 13 times) in the hydrolytic kinetic resolution (HKR) of rac-epichlorohydrin in THF by facilitating cooperative activation, compared to the monomeric catalyst. In addition, one of the bis-urea (salen)Co(III) catalyst efficiently resolves various terminal epoxides even under solvent-free conditions by requiring much shorter reaction time at low catalyst loading (0.03-0.05 mol %). A series of kinetic/mechanistic studies demonstrated that the self-association of two (salen)Co units through urea-urea hydrogen bonds was responsible for the observed rate acceleration. The self-assembly study with the bis-urea (salen)Co by FTIR spectroscopy and with the corresponding (salen)Ni complex by (1)H NMR spectroscopy showed that intermolecular hydrogen-bonding interactions exist between the bis-urea scaffolds in THF. This result demonstrates that self-assembly approach by using non-covalent interactions can be an alternative and useful strategy toward the efficient HKR catalysis.

NMR spectroscopy and ion pairing: Measuring and understanding how ions interact

Pulsed gradient spin-echo (PGSE) diffusion and Overhauser NMR data together with density functional theory (DFT) calculations afford a qualitative estimation of the amount of ion pairing, as well as insight into the structures of a variety of inorganic, organic, and organometallic salts.

PGSE NMR diffusion overhauser studies on [Ru(Cp*)(eta6-arene)][PF6], plus a variety of transition-metal, inorganic, and organic salts: an overview of ion pairing in dichloromethane

PGSE diffusion, 19F, 1H HOESY and 13C NMR studies for a series of [Ru(Cp*)(eta6-arene)][PF6] (1) salts are presented. The solid-state structure of [Ru(Cp*)(eta6-fluorobenzene)][PF6] (1 c) is reported. The extent of the ion pairing and the relative positions of the ions are shown to depend on the arene. For the solvent dichloromethane, new and literature PGSE data for PF6(-) salts of transition-metal, inorganic, and organic salts are compared. Taken together, these new results show that the charge distribution and the ability of the anion to approach the positively charged positions (steric effects due to molecular shape) are the determining factors in deciding the amount of ion pairing. DFT calculations of the charges in four salts of type 1, as well as in a variety of other salts, using a natural population analysis (NPA), support this view. This represents the first attempt, using experimental data, to understand, correlate, and partially explain the various degrees of ion pairing in a widely different collection of salts.

Model peptides based on the binding loop of the copper metallochaperone Atx1: selectivity of the consensus sequence MxCxxC for metal ions Hg(II), Cu(I), Cd(II), Pb(II), and Zn(II)

The amino acid sequence MxCxxC is conserved in many soft-metal transporters that are involved in the control of the intracellular concentration of ions such as Cu(I), Hg(II), Zn(II), Cd(II), and Pb(II). A relevant task is thus the selectivity of the motif MxCxxC for these different metal ions. To analyze the coordination properties and the selectivity of this consensus sequence, we have designed two model peptides that mimic the binding loop of the copper chaperone Atx1: the cyclic peptide P(C) c(GMTCSGCSRP) and its linear analogue P(L) (Ac-MTCSGCSRPG-NH2). By using complementary analytical and spectroscopic methods, we have demonstrated that 1:1 complexes are obtained with Cu(I) and Hg(II), whereas 1:1 and 1:2 (M:P) species are successively formed with Zn(II), Cd(II), and Pb(II). The complexation properties of the cyclic and linear peptides are very close, but the cyclic compound provides systematically higher affinity constants than its unstructured analogue. The introduction of a xPGx motif that forms a type II beta turn in P(C) induces a preorganization of the binding loop of the peptide that enhances the stabilities of the complexes (up to 2 orders of magnitude difference for the Hg complexes). The affinity constants were measured in the absence of any reducing agent that would compete with the peptides and range in the order Hg(II) > Cu(I) >> Cd(II) > Pb(II) > Zn(II). This sequence is thus highly selective for Cu(I) compared to the essential ion Zn(II) that could compete in vivo or compared to the toxic ions Cd(II) and Pb(II). Only Hg(II) may be an efficient competitor of Cu(I) for binding to the MxCxxC motif in metalloproteins.

Structure, Stability and Guest Affinity of Tris(3-ureidobenzyl)amine Capsules in Solution

In non-competitive solvents, the tris(3-ureidobenzyl)amines 1 a-c form dimeric assemblies in which guests such as CH(3)CN, CH(3)NO(2), CH(2)Cl(2), CH(3)I, CH(2)BrCl, CH(2)Br(2), CHCl(3) and C(6)H(6) can be encapsulated. Variable temperature (1)H and (1)H,(1)H-ROESY NMR spectroscopy, as well as pulsed-gradient spin-echo (PGSE) diffusion measurements were used to investigate the encapsulation within 1 a1 a (1 a: tris{3-[N'-(4-butylphenyl)ureido]benzyl}amine). Kinetic parameters for the encapsulation of CH(3)NO(2), CH(2)Cl(2) and CH(3)I, both in CDCl(3) and in [D(8)]toluene have been obtained by using magnetisation transfer methods. These data are discussed together with the thermodynamic parameters. The affinity between guest and capsule seems to be dictated mainly by the electronic, size and shape complementarity between cavity and guest. A gating mechanism for guest exchange is proposed.