C. M. Gramaccioli

Bio: C. M. Gramaccioli is an academic researcher. The author has contributed to research in topic(s): Copper. The author has an hindex of 1, co-authored 1 publication(s) receiving 62 citation(s).

Metal–biomolecule frameworks (MBioFs)

Abstract: Biomolecules are the building blocks of life. Nature has evolved countless biomolecules that show promise for bridging metal ions. These molecules have emerged as an excellent source of biocompatible building blocks that can be used to design Metal–Biomolecule Frameworks (MBioFs). This feature article highlights the advances in the synthesis of this class of MOFs. Special emphasis is provided on the crystal structures of these materials, their miniaturization to the submicron length scale, and their new potential storage, catalytic, and biomedical applications.

Crystal structures of metal-peptide complexes.

Abstract: Publisher Summary This chapter discusses crystal structures of metal–peptide complexes. Most of the crystal-structure analyses of metal–amino acid and metal–peptide complexes have been carried out on the assumption that such complexes act as models for the metal-binding sites on proteins. Crystal-structure analyses show that the geometrical features of metal complexes of amino acids, peptides and imidazole are related in systematic ways to the chemical structure, which the complexes have in the crystalline state. Moreover, the transfer of geometrical information from crystal-structure analyses to species that exist in solution depends on the assumption that the complexes found in crystals are present also in the solutions from which the crystals grow. This chapter illustrates that, the existence of cis-trans isomers coordination isomers, optical isomers, and dimeric species in the crystalline complexes emphasizes the variety of species, which must be considered when equations are written to represent metal–peptide equilibria in solution.

Crystal Structure of the Transition‐Metal Molybdates and Tungstates. II. Diamagnetic Sc2(WO4)3

Abstract: Sc2(WO4)3, diamagnetic above 30°K, crystallizes in the orthorhombic system, Space Group Pnca, with lattice constants a=9.596±0.004, b=13.330±0.003, and c=9.512±0.004 A at 298°K. The complete x‐ray scattering pattern within a reciprocal lattice hemisphere of radius (sinθ)/λ=1.02 A−1 was measured with PEXRAD. The crystal structure was solved by use of three‐dimen sional Patterson and Fourier series and refined by the method of least squares, using 1731 independent structure factors. The final agreement factor R is 0.0622. Scandium atoms occupy slightly distorted octahedra, with average Sc–O=2.063 A and Sc–O distances ranging from 2.026±0.015 to 2.124±0.010 A. Two crystallographically independent W atoms are surrounded by somewhat distorted tetrahedra: the W–O distances vary from 1.695±0.009 to 1.829±0.016 A, the average being 1.761 A. The thermal vibrations are significantly anisotropic. Sc2(WO4)3 forms the structure type for 23 trivalent metal tungstates and molybdates, including the nine smaller rare‐earth tungstates. The larger rare‐earth tungstates, crystallizing in the Eu2(WO4)3 structure type, have 8 coordination about the rare‐earth ion; the smaller have 6 coordination. A simple correlation is found between the variation in radius ratio due to the lanthanide contraction and the change in coordination.

Study of hydrogen bonds in amino acids and peptides.

Abstract: An analysis of the various parameters associated with N-H… O type of hydrogen bonds has been made using data from reported crystal structures of amino acids and simple peptides. The different parameters at the donor and the acceptor ends have been suitably defined and evaluated. In some cases the analysis is done depending upon the chargedness or other characteristics of the donor and acceptor groups. Histograms giving the distribution of these parameters have been drawn and possible conclusions arrived at: 1. The distribution shows a maximum between 2.8 A and 2.9 A for the charged donor group and 2.9 A and 3.0 A for the uncharged donor group and is probably not dependent upon the charge on the acceptor group. 2. The angle between the directions CO and O. N tends to lie between two cones about C O with semi-vertical angles 40 and 70°. The orientation of the directions O. N and O. H with respect to the lone pair orbital directions on the acceptor oxygen atoms are analysed in detail using spherical polar coordinates. The analysis indicates that the group NH has a very strong tendency to point towards the acceptor oxygen atom. A general feature has been found in that the direction N-H tends to be closer to an orbital if the oxygen is an acceptor of two hydrogen bonds, while the direction tends to lie in between the orbitals when the acceptor oxygen is the receipient of only one hydrogen bond. The possible explanation of this on the basis of lone pair interaction is briefly discussed.

The nature of Cu bonding to natural organic matter

Abstract: Copper biogeochemistry is largely controlled by its bonding to natural organic matter (NOM) for reasons not well understood. Using XANES and EXAFS spectroscopy, along with supporting thermodynamic equilibrium calculations and structural and steric considerations, we show evidence at pH 4.5 and 5.5 for a five-membered Cu(malate)2-like ring chelate at 100–300 ppm Cu concentration, and a six-membered Cu(malonate))1–2-like ring chelate at higher concentration. A “structure fingerprint” is defined for the 5.0–7.0 A−1 EXAFS region which is indicative of the ring size and number (i.e., mono- vs. bis-chelate), and the distance and bonding of axial oxygens (Oax) perpendicular to the chelate plane formed by the four equatorial oxygens (Oeq) at 1.94 A. The stronger malate-type chelate is a C4 dicarboxylate, and the weaker malonate-type chelate a C3 dicarboxylate. The malate-type chelate owes its superior binding strength to an –OH for –H substitution on the α carbon, thus offering additional binding possibilities. The two new model structures are consistent with the majority of carboxyl groups being clustered and α-OH substitutions common in NOM, as shown by recent infrared and NMR studies. The high affinity of NOM for Cu(II) is explained by the abundance and geometrical fit of the two types of structures to the size of the equatorial plane of Cu(II). The weaker binding abilities of functionalized aromatic rings also is explained, as malate-type and malonate-type structures are present only on aliphatic chains. For example, salicylate is a monocarboxylate which forms an unfavorable six-membered chelate, because the OH substitution is in the β position. Similarly, phthalate is a dicarboxylate forming a highly strained seven-membered chelate. Five-membered Cu(II) chelates can be anchored by a thiol α-SH substituent instead of an alcohol α-OH, as in thio-carboxylic acids. This type of chelate is seldom present in NOM, but forms rapidly when Cu(II) is photoreduced to Cu(I) at room temperature under the X-ray beam. When the sample is wet, exposure to the beam can reduce Cu(II) to Cu(0). Chelates with an α-amino substituent were not detected, suggesting that malate-like α-OH dicarboxylates are stronger ligands than amino acids at acidic pH, in agreement with the strong electronegativity of the COOH clusters. However, aminocarboxylate Cu(II) chelates may form after saturation of the strongest sites or at circumneutral pH, and could be observed in NOM fractions enriched in proteinaceous material. Overall, our results support the following propositions: (1) The most stable Cu–NOM chelates at acidic pH are formed with closely-spaced carboxyl groups and hydroxyl donors in the α-position; oxalate-type ring chelates are not observed. (2) Cu(II) bonds the four equatorial oxygens to the heuristic distance of 1.94 ± 0.01 A, compared to 1.97 A in water. This shortening increases the ligand field strength, and hence the covalency of the Cu–Oeq bond and stability of the chelate. (3) The chelate is further stabilized by the bonding of axial oxygens with intra- or inter-molecular carboxyl groups. (4) Steric hindrances in NOM are the main reason for the absence of Cu–Cu interactions, which otherwise are common in carboxylate coordination complexes.