Aminoglycosides are clinically relevant antibiotics that participate in a large variety of molecular recognition processes involving different RNA and protein receptors. The 3-D structures of these policationic oligosaccharides play a key role in RNA binding and therefore determine their biological activity. Herein, we show that the particular NH2/NH3(+)/OH distribution within the antibiotic scaffold modulates the oligosaccharide conformation and flexibility. In particular, those polar groups flanking the glycosidic linkages have a significant influence on the antibiotic structure. A careful NMR/theoretical analysis of different natural aminoglycosides, their fragments, and synthetic derivatives proves that both hydrogen bonding and charge-charge repulsive interactions are at the origin of this effect. Current strategies to obtain new aminoglycoside derivatives are mainly focused on the optimization of the direct ligand/receptor contacts. Our results strongly suggest that the particular location of the NH2/NH3(+)/OH groups within the antibiotics can also modulate their RNA binding properties by affecting the conformational preferences and inherent flexibility of these drugs. This fact should also be carefully considered in the design of new antibiotics with improved activity.

A complete characterisation of the protonation equilibrium that accompanies the molecular recognition of neomycin-B by a specific RNA receptor has been achieved by employing simple NMR measurements.

The molecular recognition of streptomycin by Bacillus subtilis aminoglycoside-6-adenyl transferase has been analysed by a combination of NMR techniques and molecular dynamic simulations. This protein inactivates streptomycin by transferring an adenyl group to position six of the streptidine moiety. Our combined approach provides valuable information about the bioactive conformation for both the antibiotic and ATP and shows that the molecular recognition process for streptomycin involves a conformational selection phenomenon. The binding epitope for both ligands has also been analysed by 1D-STD experiments. Finally, the specificity of the recognition process with respect to the aminoglycoside and to the nucleotide has been studied.

Catalytic converter: A (salen)iron(III) complex (salen*=asymmetric bis(salicylidene)ethylenediamine ligand) can be used as a catalyst system for the asymmetric oxidation of aryl sulfides by iodosylbenzene (see scheme). An iodosylbenzene(salen)iron(III) species has been identified as the active intermediate by EPR and NMR spectroscopy. R=aryl, R′=alkyl.