S-adenosyl-L-methionine and S-adenosyl-L-homocysteine, an NMR study

An analysis of the 360-MHz super(1)H NMR spectra of the title compounds in super(2)H sub(2)O is presented. The super(3)J values for the ribose vicinyl protons of (S)-adenosyl-L-methionine are consistent with a predominantly C sub(3')-exo conformation and with one highly favored gauche-anti conformation about the C sub(4')-C sub(5') bond. The corresponding super(3)J values for S-adenosyl-L-homocysteine imply a similar C sub(3')-exo ribose ring conformation, but the orientation about the C sub(4')-C sub(5') bond is distributed between two gauche-anti rotamers. The methionine side chain of S-adenosyl-L-methionine has approximately equal populations of rotational isomers about the C sub( alpha )-C sub( beta ) and C sub( beta )-C sub( gamma ) bonds, whereas the side chain of S-adenosyl-L-homocysteine exhibits a conformational preference for the gauche-anti conformations about the C sub( alpha )-C sub( beta ) bond. super(1)H and super(13)C NMR spectra of commercially available samples of (-)-S-adenosyl-L-methionine consistently reveal the presence of a small amount of the (+)-sulfonium diastereomer. This assignment was confirmed by synthesis of both the super(1)H and super(13)C methyl derivatives of S-adenosyl-L-homocysteine.