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The de novo design of novel artificial proteins that have predictable structures and functions is the most challenging goal in protein design and mimicry. Creating functional artificial metalloproteins requires a detailed understanding of the protein-folding mechanism as well as of the coordination chemistry of the metal centers. Herein we demonstrate a sliding-helix mechanism for stabilizing a change in the ligand environment of DF1, a model di-MnII protein.

Dinuclear copper(II) complexes with the new ligand 1,6-bis[[bis(1-methyl-2-benzimidazolyl)methyl]amino]-n-hexane (EBA) have been synthesized, and their reactivity as models for tyrosinase has been investigated in comparison with that of previously reported dinuclear complexes containing similar aminobis(benzimidazole) donor groups. The complex [Cu2(EBA)(H2O)4]4+, five-coordinated SPY, with three nitrogen donors from the ligand and two water molecules per copper, can be reversibly converted into the bis(hydroxo) complex [Cu2(EBA)(OH)2]2+ by addition of base (pKa1 = 7.77, pKa2 = 9.01). The latter complex can also be obtained by air oxidation of [Cu2(EBA)]2+ in methanol. The X-ray structural characterization of [Cu2(EBA)(OH)2]2+ shows that a double μ-hydroxo bridge is established between the two Cu(II) centers in this complex. The coordination geometry of the coppers is distorted square planar, with two benzimidazole donors and two hydroxo groups in the equatorial plane, and an additional, lengthened and sev...

A series of organometallic complexes derived by organocobaloximes and organorhodoximes in which either one or both the hydrogen bridges have been replaced by BPh 2 groups, RM(DH)(DBPh 2 ) N -MeIm and RM((DBPh 2 ) 2 N -MeIm, respectively, have been synthesised and characterised, both in solution and in solid state. 1 H NMR spectra show that they assume different interconverting conformations in solution. With increasing steric bulk of R, the axial phenyls of the BPh 2 group tend to face N -MeIm, forcing the latter in an orientation which is quite unusual in organocobaloximes and causing a lengthening of the Co-N bond. Some possible implications on the strength of the trans Co-C bond are discussed.

Crystallization and crystal structure analysis of chlorohydrates of either tri- or tetracationic copper porphyrins, namely copper(5,-10,15-tris(N-methyl-pyridinium-4-yl)-20-pyridine-porphyrinato) (1) and copper(5,10,15,20-tetrakis(N-methyl-pyridinium-4-yl)-porphyrinato), respectively, have been performed. Two crystalline forms, 2 and 3, of the latter have been obtained under different preparation conditions. A novel kind of slipped stack chains of these cationic porphyrins has been detected. The pronounced saddle conformation of the porphyrin reveals π-like interactions between the peripheral pyrrole C b -C b "double bond" and the metal center. DFT calculations on the isolated porphyrins clearly show the HOMO orbitals with the correct topology to yield a bonding interaction among the stacked porphyrin units. To our knowledge, a slipped stack chain of positively charged porphyrins has never been previously reported, if the arrangement of faced units of monocationic metalloporphyrins or phthalocyanins is excluded.

The interactions of the tetra- cationic meso-tetrakis(N-methyl-4-pyri- dyl)porphyrin (H2TMPyP) and its met- allo derivatives (MTMPyP) (where M = copper(ii), zinc(ii), and gold(iii) with the octa-anionic form (at neutral pH) of 5,11,17,23-tetrasulfonato- 25,26,27,28-tetrakis(hydroxycarbonyl- methoxy)calix(4)arene (C4TsTc) lead to a series of complex species whose stoichiometry and porphyrin sequence can be easily tuned. Crystallographic, spectroscopic, and diffusion NMR stud- ies converge towards a common picture in which a central 1:4 porphyrin/calix- arene unit serves as a template for the formation of more complex species. These species arise by successive, step- wise addition of single porphyrin mole- cules above and below the plane of the 1:4 central core to ultimately give a 7:4 complex. Noticeably, the stoichiometry of the various complex species corre- sponds to the actual concentration ratio of porphyrins and calixarenes in solution allowing the stoichiometry of these species to be easily tuned. This behavior and the remarkable stability of these species allow homo-porphyrin and hetero-(metallo)porphyrin species to be formed with control of not only the stoichiometry but also the sequence of the porphyrin array. The flexibility and ease of this approach permit, in principle, the design and synthesis of porphyrin arrays for predetermined purposes. For example, we have shown that it is very easy to design and obtain mixed porphyrin species in which a foreseen photoinduced electron-trans- fer is indeed observed.

Protein design provides an attractive approach to test the essential features required for folding and function. Previously, we described the design and structural characterization in solution of mimochromes, a series of miniaturized metalloproteins, patterned after the F-helix of the hemoglobin beta-chain. Mimochromes consist of two medium-sized helical peptides, covalently linked to the deuteroporphyrin. CD and NMR characterization of the prototype, mimochrome I, revealed that the overall structure conforms well to the design. However, formation of Delta and Lambda diastereomers was observed. To overcome the problem of diastereomer formation, we re-designed mimochrome I, by engineering intramolecular, interchain interactions. The resulting model was mimochrome IV: the solution structural characterization showed the presence of the Lambda isomer as a unique form. To examine the extent to which the stereochemical stability and uniqueness of mimochrome IV was retained in the solid state, the crystal structure of Co(III)-mimochrome IV was solved by X-ray diffraction, and compared to the solution structure of the same derivative. Co(III)-mimochrome IV structures, both in solution and in the solid state, are characterized by the following common features: a bis-His axial coordination, a Lambda configuration around the metal ion, and a predominant helical conformation of the peptide chains. However, in the crystal structure, intrachain Glu1-Arg9 ion pairs are preferred over the designed, and experimentally found in solution, interchain interactions. This ion pairing switch may be related to strong packing interactions.

University of Napoli Federico II, Department of Chemistry One of the most challenging topics in hemoprotein chemistry is to identify and to rationalise the structural requirements for specific defined functions (dioxygen activation/transport/storage, electron transfer etc.). Recently, mimochrome-IV a synthetic based-peptide model of hemoproteins was designed [Chem.Rev. (2001) 101, 3165]. The molecule is composed by two identical nine-peptide chains covalently linked to the propionic groups of a deuteroporphyrin-IX through the epsilon-amino function of Lys. Each peptide sequence bears a His residue in the central position that may act as axial ligand in metal coordination. With the aim to study the metal environment of this system we have undertaken the X-ray diffraction experiments of the mimochrome-IV cobalt(III) derivative. After several unsuccessful attempts to solve the phase problem by molecular replacement and the direct methods, the presence of a cobalt atom in the structure suggested to perform an MAD experiment. Four MAD data sets, collected at Elettra, were used to solve the XRD structure of the lambda isomer of Co(III)-mimochrome-IV. The structure consists of two antiparallel peptide chains in -helix conformation, sandwiching the porphyrin plane with a pseudo C2 symmetry. This is the first crystal structure of an artificial heme protein of the mimochrome series and represents the starting point to address the future design directions to reproduce in artificial molecules the subtle mechanisms that control the heme functions, thus ensuring the selectivity of the natural systems. Furthermore, at best of our knowledge this result represents the first structure solved using MAD technique on the cobalt absorption edge. Keywords: MINIATURIZED PROTEIN HEMOPROTEIN MAD Acta Cryst. (2002). A58 (Supplement), C277 ENGINEERING GREEN FLUORESCENT PROTEIN TO A REDOX SWITCH A. Henriksen

De novo design of proteins provides an attractive approach to uncover the essential features required for their functions. Previously, we described the design and crystal structure determination of a di-Zn(II) complex of "due-ferri-1" (DF1), a protein patterned after the diiron-dimanganese class of redox-active proteins [Lombardi, A.; Summa, C.; Geremia, S.; Randaccio, L.; Pavone, V.; DeGrado, W. F. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 6298-6305]. The overall structure of DF1, which contains a carboxylate-bridged dinuclear metal site, agrees well with the intended design. However, access to this dimetal site is blocked by a pair of hydrophobic leucine residues (L13 and L13'), which prevent facile entry of metal ions and small molecules. We have now taken the next step in the eventual construction of a catalytically active metalloenzyme by engineering an active site cavity into DF1 through the replacement of these two leucine residues with smaller residues. The crystal structure of the dimanganous form of L13A-DF1 indeed shows a substrate access channel to the dimetal center. In the crystal structure, water molecules and a ligating dimethyl sulfoxide molecule, which forms a monatomic bridge between the metal ions, occupy the cavity. Furthermore, the diferric form of a derivative of L13A-DF1, DF2, is shown to bind azide, acetate, and small aromatic molecules.

This report contains the chemistry of the ruthenium−carbene functionality bonded to the dibenzotetramethyltetraaza[14]annulene dianion, tmtaa. [{Ru(tmtaa)}2(μ-C8H12)], 1, was the most appropriate starting material, as it contains a labile olefin, which can be displaced by a number of ligands to give [Ru(tmtaa)(L)(L‘)] [L = L‘ = Py, 2; L = CO, L‘ = thf, 3; L = ButNC, L‘ = thf, 4; L = L‘ = ButNC, 5]. Reaction of 1 with diazoalkanes led to the displacement of the olefin and gave the corresponding carbene derivatives [Ru(tmtaa)(CRR‘)] [R = R‘ = Ph, 6; R = Ph, R‘ = H, 7; R = Ph, R‘ = COOMe, 8], which have square-pyramidal structures. Complexes 7 and 8 underwent a remarkable labilization of the carbene functionality in the reaction with carbon monoxide, while 6 was only converted to the corresponding carbonyl [Ru(tmtaa)(CPh2)(CO)], 10, by CO. The reaction of 7 with both CO and ButNC led to the migration of the carbene to one of the metalladiimino rings of the ligands in 12 and 13, while the reaction of 8 with b...