Metal binding to ligands with the potential of existing in different tautomeric structures can dramatically alter the tautomeric equilibrium by stabilizing a particular, frequently minor, tautomer. The assumption that metal complexation of a minor tautomer is chemically irrelevant because of its very low abundance is misleading and in many cases wrong. In fact, from available X-ray structural data on metal–nucleobase complexes it is evident that metal binding to rare, as opposed to preferred tautomers, is anything but an exception. This “promotion of rare tautomers” through metal coordination is of particular biological relevance in the case of nucleobases because any deviation from Watson–Crick base pairing is potentially mutagenic. In recent years models of “metal-stabilized rare nucleobase tautomers” have been characterized for all common DNA nucleobases, including by X-ray crystallography. Though metal binding causes relatively minor structural changes in the nucleobases, electronic changes as expressed by acid–base properties, for example, can be substantial. In this perspective article the biological consequences of the occupation of nucleobase sites by a metal entity and the altered acid–base chemistry of the nucleobase with regard to base mismatch formation, prevention of base pairing, and acid–base catalysis in nucleic acids are examined. Although not relevant to biology, the behaviour of the unsubstituted parent nucleobases is illuminating in this respect and therefore included.

Promotion of rare nucleobase tautomers by metal binding.

Abstract In this paper, guanidine copper compounds are reviewed with an emphasis on structural characteristics and their application in bioinorganic chemistry and catalysis. The literature survey includes the copper coordination chemistry of biological guanidine derivatives, peralkylated guanidines including bicyclic ones and of further nitrogen-rich guanidine-type systems such as azoimidazoles, triazolopyrimidines and triaminoguanidines. From a sporadic interest dating back to the 1960s, research on this ligand class and its use in copper coordination chemistry has gained new impetus since 2000. With the synthesis of examples with sophisticated substitution at the characteristic CN3 framework, complex problems can be addressed in several fields of chemistry. This paper analyses the different types of guanidines for their special donor properties and highlights the specific advantages of guanidines as neutral donor ligands in copper coordination chemistry where a great variety of coordination modes was found. These compounds offer the ability to distribute the formal positive charge of the metal throughout the guanidine unit and represent more than simple σ-donating ligands.

(Guanidine)copper complexes: structural variety and application in bioinorganic chemistry and catalysis

Adsorption of cytosine, thymine, guanine and adenine on Cu(1 1 0) studied by infrared reflection absorption spectroscopy

We have established a possibility to obtain the photoinduced optical second harmonic generation in the initially centrosymmetric single crystals of tris(thiourea)copper(I) perchlorate crystallized in the monoclinic system with the space group P21/c. The second-order susceptibility of the investigated crystal was measured by inducing the crystal by external laser field. The results obtained are interesting and discussed in detail. We have established that the optical second harmonic generation achieves its maximum (about 0.84 pm/V) at photoinducing power density about 1430 kW/cm2 for the fundamental wavelength 1064 nm. The maximal output second harmonic generation (SHG) was achieved at an angle of about 35° between the pumping and fundamental power beam's polarizations. Such maximum may be a consequence of a competition between the so formed anharmonic photothermal expansions and the thermal destruction that occurred because of the heating of the crystal surfaces. Our independent verifications have shown that the photoinduced surfaces were heated up to 8–12° C. This may cause occurrence of photothermal relaxation processes. The output SHG exists only immediately after the act of photoinduction. The obtained dependences are caused by specific features of the Raman spectra of the investigated crystals. Copyright © 2008 John Wiley & Sons, Ltd.

Antonio Panfil

Papers

Synthesis and characterization of a novel copper(ii)-cytosine complex : tetrakis(cytosine)copper(ii) chloride bis(dimethylacetamide) solvate

Complexes of Nickel(II) with creatinine: X-ray crystal structures and spectroscopic studies

Structural and spectroscopic studies of Ni (II) creatinine complexes