A series of dizinc(II) complexes based on the pyrazolate ligands 3-[(1E)-N-hydroxyethanimidoyl]-4-methyl-1H-pyrazole-5-carboxylic acid (H(3)L(1)), (1E,1'E)-1,1'-(4-methyl-1H-pyrazole-3,5-diyl)diethanone dihydrazone (HL(2)), (E,E)-(4-methyl-1H-pyrazole-3,5-diyl)bis(methylmethanone) dioxime (H(3)L(3)), (E,E)-(4-phenyl-1H-pyrazole-3,5-diyl)bis(phenylmethanone) dioxime (H(3)L(4)), and 1H-pyrazole-3,5-dicarboxylic acid (H(3)L(5)) have been synthesized and investigated as functional models of phosphoesterases, focusing on correlations between the hydrolytic activity and molecular parameters of the bimetallic core. Speciation of the various dizinc complexes in solution has been determined potentiometrically, and the structures in the solid state have been established by X-ray crystallography. The hydrolysis of two phosphoesters, an RNA model 2-hydroxypropyl-p-nitrophenyl phosphate (HPNP) and the pesticide paraoxon-ethyl (POE), promoted by the dinuclear phosphoesterase model complexes has been investigated in DMSO/buffered water (1:1) at 50 degrees C as a function of complex concentration, substrate concentration, and pH. Drastic differences in the hydrolytic activities of [Zn(2)(HL(1))(2)](0), [Zn(2)(L(2))(2)](2+), [Zn(2)(H(2)L(3))(2)](2+), and [Zn(2)(HL(5))(2)](2-) are observed and can be attributed to molecular peculiarities. Pyrazolate-bridged dinuclear zinc(II) complexes seem to provide a sufficient number of coordination sites for both activating the substrate and generating the nucleophile, where the phosphate esters are preferentially bound in a bidentate bridging fashion (in the case of HPNP) and in a monodentate fashion (in the case of POE).

The title pentapeptide, Boc0—Gly1–ΔZPhe2—Gly3–ΔEPhe4—Gly5—OMe, C30H35N5O8·2H2O, adopts the type I β‐turn conformation for the ΔZPhe2—Gly3 residues. It is stabilized by a intramolecular hydrogen bond between the ΔEPhe4 NH and Gly1 CO groups. All the amino acid residues in the pentapeptide sequence are linked trans to each other. The crystal structure is stabilized by intra‐ and intermolecular hydrogen bonds.

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Two new pyrazolate‐based ligands, N′‐[1‐(3‐acetyl‐4‐methyl‐1H‐pyrazol‐5‐yl)ethylidene]‐2‐(hydroxyimino)propanehydrazide (L1) and 5‐[(E)‐1‐(2‐{(E)‐2‐(hydroxyimino}propanoyl}hydrazono)ethyl]‐4‐methyl‐1H‐pyrazole‐3‐carboxylic acid (L2), were synthesized and studied for zinc(II) complexation. A set of pH‐dependent UV/Vis measurements has been performed to determine the complex formation properties of L2. According to the calculations, in solution, L2 forms variously protonated mononuclear (ZnII/L2 = 1:1) and dinuclear (ZnII/L2 = 2:1) complexes. The reaction of the deprotonated ligands with hydrated ZnII salts and slow diffusion of ammonia into the reaction mixtures gave mononuclear [Zn(L1‐2H)(NH3)2]·DMF (1) and trinuclear μ‐pyrazolato‐bridged [Zn3(L2‐3H)2(NH3)5]·4H2O (3). In both complexes, the zinc ions are in the same distorted trigonal‐bipyramidal environment, coordinated to two nitrogen atoms of the ammonia and one oxygen and two nitrogen atoms of the pyrazolate and hydrazide groups. The molecular structures of all of the ligands and complexes have been elucidated by X‐ray crystallography.

Comparison of the crystal structures of two pentadehydropeptides containing ÁPhe residues, namely (Z,Z)-N-(tertbutoxycarbonyl)glycyl-, -phenylalanylglycyl-, -phenylalanylglycine (or Boc 0 -Gly 1 -Á Z Phe 2 -Gly 3 -Á Z Phe 4 -Gly 5 -OH) methanol solvate, C 29 H 33 N 5 O 8 ÁCH 4 O, (I), and (E,E)-N-(tertbutoxycarbonyl)glycyl-, -phenylalanylglycyl-, -phenylalanylglycine (or Boc 0 -Gly 1 -Á E Phe 2 -Gly 3 -Á E Phe 4 -Gly 5 -OH), C 29 H 33 N 5 O 8 , (II), indicates that the Á Z Phe residue is a more effective inducer of folded structures than the Á E Phe residue. The values of the torsion angles ' and show the presence of two type-III 0 -turns at the Á Z Phe residues and one type-II -turn at the Á E Phe residue. All amino acids are linked trans to each other in both peptides. -Turns present in the peptides are stabilized by intramolecular 4!1 hydrogen bonds. Molecules in both structures form two-dimensional hydrogen-bond networks parallel to the (100) plane.