Biomedical subjects
Publications and source records attributed to Cristiano Zonta.
[structure: see text] Benzotriazole forms complexes of different stoichiometries with amines and phenols. Four of them have been characterized by single-crystal X-ray diffraction. The trends of donor-acceptor hydrogen-bond distances between corresponding molecular entities in the different complexes are related to induction-mediated cooperativity effects.
[reaction: see text] A novel method for the synthesis of new highly functionalized cyclotrimers is described. The method consists of an original synthesis of beta-dibromosubstituted pyrroles, metalation, cycloaddition, and cyclotrimerization. The sequence is highly compatible with common functional groups and allows the construction of cup-shaped molecules functionalized both at the upper and bottom rim. This feature makes the newly formed structures useful scaffolds for the development of supramolecular receptors.
A new version of the double mutant cycle approach has been used for the evaluation of weak noncovalent interactions in transition states.
Chemical double mutant cycles have been used to quantify cation-pi interactions in chloroform as a function of the nature of the counteranion. The cation-pi interaction is -2.5 +/- 0.4 kJ mol(-1) and independent of the anion, even though the overall stability of the complexes varies by an order of magnitude due to competition of the anion for alternative binding sites.
Complexation-induced changes in proton chemical shifts provide a potent tool for conformational analysis, being highly dependent on intermolecular orientation. An important contribution to these shifts arises from the molecular magnetisability anisotropy, or more specifically from the anisotropy of certain groups, such as aromatic rings and unsaturated bonds. While the influence of aromatic rings has been well characterised via the ring current effect, unsaturated bonds have received much less attention and prediction of complexation shifts is hampered by the lack of accurate anisotropy parameters for these bonds. We have therefore used ab initio calculations at the HF/aug-cc-pVDZ level to obtain bond anisotropies for C-H, N-H, C=O, C=C, C triple bond N, N=N, C triple bond C, and C triple bond N. Fitting the anisotropies to bond magnetic dipoles (the McConnell equation) gives non-transferable values for C-H and N-H bonds. We have therefore expanded in terms of bond magnetic dipoles, quadrupoles, and octopoles for double and triple bonds only, obtaining highly accurate shielding surfaces in all cases. The transferable nature of the anisotropies is confirmed by comparing with shifts obtained in larger molecules containing unsaturated bonds.
Experimental measurements of edge-to-face aromatic interactions have been used to test a series of molecular mechanics force fields. The experimental data were determined for a range of differently substituted aromatic rings using chemical double mutant cycles on hydrogen-bonded zipper complexes. These complexes were truncated for the purposes of the molecular mechanics calculations so that problems of conformational searching and the optimisation of large structures could be avoided. Double-mutant cycles were then carried out in silico using these truncated systems. Comparison of the experimental aromatic interaction energies and the X-ray crystal structures of these truncated complexes with the calculated data show that conventional molecular mechanics force fields (MM2, MM3, AMBER and OPLS) do not perform well. However, the XED force field which explicitly represents electron anisotropy as an expansion of point charges around each atom reproduces the trends in interaction energy and the three-dimensional structures exceedingly well. Collapsing the XED charges onto atom centres or the use of semi-empirical atom-centred charges within the XED force field gives poor results. Thus the success of XED is not related to the methods used to assign the atomic charge distribution but can be directly attributed to the use of off-atom centre charges.
A synthetic supramolecular complex has been adapted to quantify cation-pi interactions in chloroform by using chemical double-mutant cycles. The interaction of a pyridinium cation with the pi-face of an aromatic ring is found to be very sensitive to the pi-electron density. Electron-donating substituents lead to a strong attractive interaction (-8 kJ/mol(-1)), but electron-withdrawing groups lead to a repulsive interaction (+2 kJ/mol(-1)).
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