A family of macrocyclic antibiotics with a mixed peptide-peptoid beta-hairpin backbone conformation

Macrocyclic peptidomimetics having a mixed peptide-peptoid backbone have been synthesized and shown to possess antibiotic activity against gram-positive and -negative bacteria with a low hemolytic activity against human erythrocytes; one is shown to adopt a regular beta-hairpin conformation by NMR in aqueous solution.     

Oxidatively robust monophenolate-copper(II) complexes as potential models of galactose oxidase

Cupric complexes of a novel phenanthroline-phenolate ligand have strongly distorted coordination geometries and electrochemical properties conducive to modeling the spectroscopy and reactivity of the enzyme galactose oxidase.     

Increasing the crystallisation temperature to access new spin clusters: conversion of [Ni8(cit)6(OH)2(H2O)2]10- to [Ni8(cit)6(OH)2]10-

The role of temperature in the formation of high nuclearity nickel(II) citrate spin clusters is explored, revealing how changes in structure and hence magnetic properties can be triggered through desolvation and ligand reorganisation.     

Synthetic and structural studies on 1,2,4-dithiazolidine-3,5-dione derivatives

Methods have been developed for the N-alkylation of 1,2,4-dithiazolidine-3,5-dione 2 and the subsequent conversion of the N-alkylated derivatives into isocyanates 5. An extension of this methodology onto a solid-support is also reported. X-ray crystallographic analysis has been carried out on potassium 1,2,4-dithiazolidine-3,5-dione 3 for structural comparison with the parent heterocycle 2.     

Syntheses and solid state structures of cyclic diynes with two chalcogen centres--a competition between weak interactions

In this paper we report the synthesis of cyclic diynes of the general formula mX2n. The letters m and n indicate the lengths of chains between two C triple bond C-X units where X indicates S, Se, or Te. The lengths of the bridges vary between m=4-8 and n=2-6. Structural investigations by means of X-ray diffraction reveal for most systems a chair-like conformation in the solid state. For 5S(2)2, 5S(2)3, 7S(2)5, 5Se(2)2 and 5Te(2)3 tubular structures were encountered. These structures can be traced back to weak XX or weak C-Hpi interactions.     

Unusual aryl-porphyrin rotational barriers in peripherally crowded porphyrins

Previous studies of 5,10,15,20-tetraarylporphyrins have shown that the barrier for meso aryl-porphyrin rotation (DeltaG++(ROT)) varies as a function of the core substituent M and is lower for a small metal (M = Ni) compared to a large metal (M = Zn) and for a dication (M = 4H(2+)) versus a free base porphyrin (M = 2H). This has been attributed to changes in the nonplanar distortion of the porphyrin ring and the deformability of the macrocycle caused by the core substituent. In the present work, X-ray crystallography, molecular mechanics (MM) calculations, and variable temperature (VT) (1)H NMR spectroscopy are used to examine the relationship between the aryl-porphyrin rotational barrier and the core substituent M in some novel 2,3,5,7,8,10,12,13,15,17,18,20-dodecaarylporphyrins (DArPs), and specifically in some 5,10,15,20-tetraaryl-2,3,7,8,12,13,17,18-octaphenylporphyrins (TArOPPs), where steric crowding of the peripheral groups always results in a very nonplanar macrocycle. X-ray structures of DArPs indicate differences in the nonplanar conformation of the macrocycle as a function of M, with saddle conformations being observed for M = Zn, 2H or M = 4H(2+) and saddle and/or ruffle conformations for M = Ni. VT NMR studies show that the effect of protonation in the TArOPPs is to increase DeltaG++(ROT), which is the opposite of the effect seen for the TArPs, and MM calculations also predict a strikingly high barrier for the TArOPPs when M = 4H(2+). These and other findings suggest that the aryl-porphyrin rotational barriers in the DArPs are closely linked to the deformability of the macrocycle along a nonplanar distortion mode which moves the substituent being rotated out of the porphyrin plane.     

Intramolecular charge transfer and biomimetic reaction kinetics in galactose oxidase model complexes

One-electron oxidation of two structurally similar CuII-diphenolate complexes, 1 and 2, creates EPR-silent CuII-phenoxyl complexes [1]+ and [2]+ that mimic the oxidized form of the enzyme galactose oxidase (GOase). Both model complexes display novel NIR absorptions assigned to phenolate-phenoxyl charge transfer that resemble a tyrosinate-tyrosyl charge-transfer band observed in the enzymatic system. [1]+ and [2]+ react with benzyl alcohol to form 0.5 equivs of benzaldehyde per complex; biomimetic reduction to CuI-phenol complexes is not observed, but such species may exist transiently. Initial kinetic studies show that [2]+ reacts faster with benzyl alcohol than does [1]+, despite being a significantly weaker oxidant (DeltaE degrees = 370 mV). This acceleration is ascribed to mechanistic differences: [2]+ appears to bind substrate prior to the rate-determining step. Large, nonclassical kinetic isotope effects confirm C-H bond cleavage as the rate-determining step in the reactions of both [1]+ and [2]+ with benzyl alcohol, as is the case for GOase.     

Application of isomorphous replacement in the structure determination of a cubic liquid crystal phase and location of counterions

A second generation monodendron with dodecyl end-groups based on the AB(3) monomer 3,4,5-trihydroxy benzoate has previously been shown to form a thermotropic cubic phase with Pmthremacr;n symmetry (Balagurusamy et al., J. Am. Chem. Soc. 1997, 119, 1539). A structure consisting of spherical "micelles" was proposed originally, but an alternative choice of structure factor phases, giving a structure of interlocked squashed columns, could not be ruled out by diffraction data on the original material alone. We have therefore synthesized two selectively fluorinated equivalent compounds, the carboxylic acid and its Rb salt, to be able to apply a variant of the isomorphous replacement crystallographic technique. On the basis of the electron density maps of the new labeled compounds, reconstructed using small-angle X-ray diffraction intensities, the interlocking columns model is unequivocally rejected and the spheres model is upheld. Furthermore, the location of the metal cation in the center of the "micelles" is directly confirmed. Micellar diameter was shown to decrease on fluorination of the dodecyl chain ends, and increase significantly on introduction of Rb. This is interpreted in terms of changes in the number of wedge-shaped dendrons fitting into a spherical micelle due to their changing taper angle. It was found that the Rb-rich regions at the centers of six out of eight "micelles" in the unit cell are elongated in the direction of their closest packing. This adds support to the suggestion of a partial "column-like" character of stacked rows of such micelles, consistent with the position of the Pmthremacr;n phase next to the columnar phase in the phase sequence of most taper-shape compounds. The results illustrate the potential of isomorphous replacement, used a great deal in protein crystallography, in structure investigation of liquid crystals and supramolecular soft matter.