Interaction between spin-labelled penicillins and lanthanide ions

The penicillin (1b) [spin-labelled with a nitroxide group] has been prepared by reacting 6-amino-penicillanic acid (1e) with 3,5-dibromo-4-oxo-2,2,6,6-tetramethylpiperidin-1-oxyl (2). Penicillin (1b) is a potential probe for proteins (e.g. penicillinases) which bind penicillins. Because it is intended to use paramagnetic metal ions as additional probes in studies of proteins with (1b), an investigation has been carried out of the interactions of (1b) and the related ester (1d) with a series of lanthanide ions. It is shown that a direct interaction between the nitroxide group of (1b) [or (1d)] and the metal ion leads to contact quenching of the EPR spectrum of (1d).