Influence of chemical structure on nitroxyl spin label magnetic relaxation characteristics

An attempt was made to develop guidelines for the design of new contrast agents using nitroxyl spin labels (NSL). The structural parameters of ring size and of substituents were correlated with the stability towards reduction and the relaxation effectiveness using 5 piperidine and 5 pyrrolidine nitroxyls containing the same substituents. The susceptibility of NSL to reduction was assessed by EPR spectroscopy. The relaxation effectiveness of NSL on protons in buffer and plasma solution was measured on a NMR spectrometer. The ring size and substituents has a decisive effect on the stability of NSL, whereby the ring size effect was dominant. In the case of spin-lattice relaxivities (R₁) and spin-spin relaxivities (R₂), the ring size and the substitution effect were marginal in buffer solution, while in plasma these effects were more pronounced. A number of guidelines were proposed for the design of suitable NSL contrast agents for MRI.