Helix-coil stability constants for amino acids in nonaqueous solvents. Studies of random poly(-benzyl-L-glutamate-co-L-alanine) and poly(-benzyl-L-glutamate-co-L-leucine) in a mixed solvent

The host-guest technique has been applied to the determination of the helix-coil stability constants of two naturally occurring amino acids, L-alanine and L-leucine, in a nonaqueous solvent system. Random copolymers containing L-alanine and L-leucine, respectively, as guest residues and -benzyl-L-glutamate as the host residue were synthesized. The polymers were fractionated and characterized for their amino acid content, molecular weight, and helix-coil transition behavior in a dichloroacetic acid (DCA)-1,2-dichloroethane (DCE) mixture. Two types of helix-coil transitions were carried out on the copolymers: solvent-induced transitions in DCA-DCE mixtures at 25°C and thermally induced transitions in a 82:18 (wt %) DCA-DCE mixture. The thermally induced transitions were analyzed by statistical mechanical methods to determine the Zimm-Bragg parameters, and s, of the guest residues. The experimental data indicate that, in the nonaqueous solvent, the L-alanine residue stabilizes the -helical conformation more than the L-leucine residue does. This is in contrast to their behavior in aqueous solution, where the reverse is true. The implications of this finding for the analysis of helical structures in globular proteins are discussed.

3-Benzyl-6-benzylamino-1-methyl-5-nitro-1,2,3,4-tetrahydropyrimidine

In the title compound, C19H22N4O2, the tetrahydropyrimidine ring adopts an envelope conformation (with the N atom connected to the benzyl group representing the flap). This benzyl group occupies a quasi-axial position. The two benzyl groups lie over the tetrahydropyridimidine ring. The amino group is a hydrogen-bond donor to the nitro group.

Cu(II) complexes of 1-benzyl-2-phenyl-benzimidazole

Copper(II) complexes of 1-benzyl-2-phenylbenzimidazole (BPBI) of the general formula Cu(BPBI)2X2, nH2O [X= Cl-, Br-, NO3 or OAc- (n = O) and X = NO3- or 1 2SO42-(n = 2H2O)] have been prepared. The complexes are found to be nonelectrolytes in nitrobenzene. Conductivity in nonaqueous media, magnetic susceptibilities and i.r. and electronic spectra of the complexes are reported. A tetragonally distorted octahedral structure has been suggested for these complexes.

1-[2,4,6-Trimethyl-3,5-bis(4-oxopiperidin ylmethyl) benzyl ]piperidin- 4-one

In the structure of the title compound, C27H39N3O3, each of the (4-oxopiperidin-1-yl)methyl residues adopts a flattened chair conformation (with the N and carbonyl groups being oriented to either,side of the central C-4 plane) and they occupy positions approximatelym orthogonal to the central benzene ring [C-benzene-C-C-methylene-N torsion angles 103.4 (2), -104.4 (3) and 71.9 (3)degrees]; further, two of these residues are oriented to one side of the central benzene ring with the third to the other side. In the crystal packing, supramolecular layers in the ab plane are sustained by C-H center dotcenter dot center dot O interactions.

Nickel(II) complexes of 1-benzyl-2-phenylbenzimidazole

Nickel(II) complexes of 1-benzyl-2-phenylbenzimidazole (BPBI) of the general formula [Ni(BPBI)2X2](X=Cl-, Br-, NCS- or NO3-) have been prepared and their magnetic moments, i.r. and electronic spectra studied. [Ni(BPBI)2Cl2] has a pseudotetrahedral structure while [Ni(BPBI)2 Br2] exists as square planar and speudotetrahedral isomers. [Ni(BPBI)2I2] and [NI(BPBI)2(NCS)2] have square planar stereochemistry. The nitrato complex [Ni(BPBI)2(NO)3)2] exists in two different octahedral modifications in the solid state.

5-Benzyl-1-(4-fluorophenyl)-2-phenyl-4,5,6,7-tetrahydro-1H-pyrrolo3,2 -c] pyridine

In the title compound, C26H23FN2, the dihedral angle between the 4-fluorophenyl ring and the adjacent phenyl ring is 62.3 (1)degrees. The crystal structure is stabilized by C-H center dot center dot center dot pi interactions.

Directing role of functional groups in selective generation of C-H center dot center dot center dot pi interactions: In situ cryo-crystallographic studies on benzyl derivatives

The in situ cryo-crystallization study of benzyl derivatives reveals that the molecular packing in these compounds is either through methylene (sp(3)) C-H center dot center dot center dot pi or aromatic (sp(2)) C-H center dot center dot center dot pi interactions depending on the level of acidity of the benzyl proton. These studies of low melting compounds bring out the subtle features of such weak interactions and point to the directional preferences depending on the nature (electron withdrawing, polarizability) of the neighbouring functional group.

Comparison of the miscibility strategies in poly(benzyl methacrylate)-polystyrene blends

Exothermic interactions like hydrogen bonding, ionic and charge transfer, etc., and ''copolymer effect'' are commonly used to induce miscibility in immiscible blends. The efficacy of these methods in promoting miscibility in poly(benzyl methacrylate) (PBMA)-polystyrene (PS) immiscible blends has been studied by suitably modifying the structure of the component polymers. It has been found that hydrogen bonding approach is most advantageous among these approaches as it involves the need for minimum interacting sites. It has also been shown that these results can be extended to the blends of poly(acrylate)s or poly(methacrylate)s with PS. (C) 1996 John Wiley & Sons, Inc.

Modification of photochemical reactivity on formation of inclusion complexes: photorearrangement of benzyl phenyl ethers and methyl phenoxyacetates

The photorearrangement of benzyl phenyl ethers and methyl phenoxyacetates was investigated in methanol and in complexes with cyclodextrin in both the solid state and aqueous solutions. Irradiation in cyclodextrin media leads to a large change in product distribution with a very significant ortho selectivity different from that found in methanol where the reaction is non-selective. For meta-substituted ethers and phenoxyacetates, an impressive regioselectivity between the two ortho-rearranged isomers is observed and this is significantly enhanced by increasing the substituent chain length which acts as a spacer to induce a tight fit between the host and the guest. The observed results are rationalized on the basis of specific orientations of the unsubstituted and meta-substituted ethers and phenoxyacetates in the cyclodextrin cavity.

New triquinane-based host molecules : binding with diamines

Synthesis of several shape-specific hosts through heteroaromatic annulation on cis,syn,cis-triquinanedione 1 and X-ray crystal structure determination of one of them, 4a, is reported. Preliminary results of complexation between cleft 5a and diamines are reported.

Short-bite chiral diphosphazanes derived from (S)-?-methyl benzyl amine and their Pd, Pt and Rh metal complexes

New chiral diphosphazane ligands of the type Ph(2)PN(S-*CHMePh)PYY' {YY'= Ph(2) (2), O2C6H4 (3); Y= Ph, Y'= Cl {4a (SS), 4b (SR)}, N(2)C(3)HMe(2)-3,5 {5a (SR), 5b (SS)} are synthesised starting from a chiral aminophosphine, Ph(2)PNH(S-*CHMePh) (1). The structure of one of the diastereomer 5a has been confirmed by single crystal X-ray diffraction {Orthorhombic system, P2(1)2(1)2(1); a=10.456 (4), b=15.362 (7), c=17.379 (6) Angstrom, Z=4}. Transition metal mononuclear complexes [Rh{eta(2)-(Ph(2)P)(2)N- (S-*CHMePh)}(2)](+)(BF4)(-) (6), [PdCl2{eta(2)-(Ph(2)P)(2)N(S-*CHMePh)}] (7) and [PtCl2{eta(2)-(Ph(2)P)(2)N- (S-*CHMePh)}] (8) have also been synthesised. The structure of the palladium complex 7 is solved by X-ray crystallography {Orthorhombic system, P2(1)2(1)2(1); a=8.746 (2), b=18.086 (2), c=20.811 (3) Angstrom, Z=4}. All these compounds are characterised by micro analyses, IR and NMR spectroscopic data.

Sulfates of organic diamines: hydrogen-bonded structures and properties

In order to investigate the supramolecular hydrogen-bonded networks and other structural features exhibited by compounds containing an organic cation and an inorganic anion, sulfates of the organic diamines, ethylenediamine (I), 1,3-diaminopropane (II), piperazine (III), and 1,4-diazabicyclo[2.2.2]octane (DABCO) (IV) have been prepared investigated by X-ray crystallography. While II, III, and IV crystallize in the centrosymmetric space group, Pbca, P2(1)/n, Pbcn, respectively, I crystallizes in the non-centrosymmetric space group, P4(1) exhibiting chirality and weak NLO properties. I-IV exhibit different types of supramolecular H-bonded networks involving the organic cation and the SO42- anion. The nature and strength of the H-bonding network vary from one compound to another, with the strongest network found in piperazinium sulfate, III, and the weakest in II. While in III, water molecules form part of the H-bonded network, they are present as guest molecules in the channels of IV. Thermal stability of the compounds as well as the infrared spectra reflect the stabilities of these H-bonded solids. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

Role of Nonplanar Peptide Unit in Regular Polypeptide Helices - New Model for Pply-Beta-Benzyl-L-Aspartate

The effect of non-planarity of the peptide unit on helical structures stabilized by intrachain hydrogen bonds is discussed. While the present calculations generally agree with those already reported in the literature for right-handed helical structures, it is found that the most stable left-handed structure is a novel helix, called the delta-helix. Its helical parameters are close to these reported for poly-beta-benzyl-L -aspartate. Conformational energy calculations show that poly-beta-benzyl-L -aspartate with the delta-helical structure is considerably more stable than the structure it is generally believed to take up (the omega-helix) by about 15 kcal/mol-residue.