The conformation of epinephrine in polar solvents: an NMR study

Epinephrine (Epi) is a physiologically important catecholamine. Molecular conformation of Epi controls the interactions with other molecules and its biological effects. There have been a number of theoretical studies addressing conformation and hydrogen bonding of Epi in different solvents, but experimental data are scarce. Herein, we applied 1H NMR, 1H-1H COSY, 1H-15N HSQC, and NOESY to examine and compare the conformation of Epi in polar solvents—dimethyl sulfoxide (DMSO) and water. The main differences were observed for NH2 and CH2 groups. Both showed chemical nonequivalence of protons in DMSO that was not present in water. The analysis of the effects of increasing temperature and solvent substitution on NMR signals showed that one of the protons in amine group forms a strong intramolecular hydrogen bond with aliphatic OH group, which is H-donor in another hydrogen bond with DMSO. NOESY provided data on the spatial positions of protons in the side chain, allowing for 3D model of the structure of Epi in DMSO to be built. In close, Epi molecule forms an additional 5-membered ring that encompasses bifurcate intra-/intermolecular hydrogen bonds, and acquires conformation that resembles the shape of a “scorpion”—the catechol ring representing the body and the side chain being a forward-curved tail. The conformation of Epi in water lacks the intramolecular hydrogen bond and most likely largely depends on hydrogen bonds with water molecules.