ESR spectroscopic characterization of spin labeled procaine in homogeneous solutions and membrane mimetic systems

Abstract Procaine labeled at the aromatic amino group with the paramagnetic 2,2,5,5-tetramethylpyrrolin-1-oxyl moiety (sl-PRC) has been used as a model compound in several studies on interactions of local anesthetics with lipid membranes, but these works were not preceded with a detailed spectral characterization of the compound in simple, well defined systems. To fill the gap we examine here ESR spectra of sl-PRC solutions in solvents encompassing a large range of dielectric constants, as well as in aqueous solutions containing sodium dodecylsulfate (SDS) micelles, sodium bis(2-ethylhexyl)sulfosuccinate (AOT) vesicles or small unilamellar liposomes of 1,2-dipalmitoyl- sn -glycero-3-phospho-(1′- rac -glycerol) (DPPG, sodium salt). Results obtained for the homogeneous systems are discussed in terms of the effect of solvent properties (polarity, electrophilicity, H-bonding ability, and viscosity) on the 14 N hyperfine splitting parameters and on the rotational mobility of the spin label. For the microheterogeneous systems amphiphile/water partition coefficients of the drug have been evaluated from ESR spectra. Their values indicate that binding of procaine (protonated at biological pH) to the anionic aggregates is determined by both electrostatic and hydrophobic interactions. The local polarity at the nitroxide group site varies in the order SDS > DPPG > AOT, and the trend of the local viscosity change is DPPG ≫ SDS > AOT. From these findings it is concluded that in DPPG and AOT bilayers the sl-PRC molecule is oriented radially and the nitroxide group penetrates well below the Stern layer, while in SDS micelles it assumes a different location with the label inserted closer to the interface.