Molecular Pharmacology, Vol 19, 44-48, Copyright © 1981 by the American Society for Pharmacology and Experimental Therapeutics

Ring Stacking in Solutions of Norepinephrine and the 4:1 Norepinephrine·ATP Complex

¹ Department of Pharmacology, University of Illinois College of Medicine, Chicago, Illinois 60680

In D₂O (pD 6.8), increasing concentrations of norepinephrine were accompanied by an upfield shift of the phenyl proton signal. At all concentrations the addition of 1 mole of ATP for 4 moles of the amine caused the signal to move further upfield. To delineate stacking from other electronic processes in solutions of norepinephrine, dimethyl sulfoxide (DMSO), a pure hydrogen bond acceptor, was utilized. In this solvent, increasing concentrations of norepinephrine were accompanied by a downfield shift of the phenyl proton signal, whereas phenylethylamine, a compound incapable of hydrogen bonding, showed no change. The downfield shift observed with norepinephrine was explained by increased hydrogen bonding between catechol OH groups in the more concentrated solutions. If hydrogen bonding were the only factor influencing the magnetic field, the replacement of DMSO by D₂O, a hydrogen bond donator and acceptor, should cause a continuous downfield shift of the phenyl proton signal. In 0.5 M norepinephrine the signal moved downfield until the D₂O reached 80 volumes/100 volumes but then reversed its course. The upfield shift was attributed to stacking of the aromatic rings. In all mixtures of DMSO and D₂O, the phenyl proton signal of norepinephrine was moved upfield by the addition of adenosine. These shifts were explained by the formation of complexes between the adenine and catechol rings. In D₂O, ATP produced a greater upfield shift than did adenosine. This observation indicated augmented ring stacking in the catecholamine· ATP complexes. In D₂O, increasing the temperature of the norepinephrine·ATP complex from 28° to 80° caused a downfield shift of the phenyl—H and adenine ²H signals, whereas the adenine ⁸H showed no change. The phenyl—H signal of norepinephrine alone was not affected by temperature. These observations are in harmony with a Dreiding model of the 4:1 norepinephrine·ATP complex in which the adenine ²H is located between two catechol rings.