Dissociation Constants of Lanthionine and Its Structure in Aqueous Solution

L‐ and meso‐isomers of lanthionine (3,3’‐thiodialanine) have been studied by poten‐tiometric, IR, NMR, and ORD techniques. Proton dissociation constants and thermodynamic values valid at 20° andμ‐0.37 (NaN03) are given. Unusually low values for all dissociation constants as well as the significant difference between the corresponding pK, and pKt values of both isomers have been recorded and attributed to the presence of two asymmetric centres in the molecule and to the bridging sulphur. atom. Marked differences between the L‐ and me so‐crystal structures have also been revealed, whereas no difference has been observed by NMR in acid solution. From temperature variation studies in acid, basic, and neutral aqueous solutions, the thioether group has been found to behave as a dissymmetric chromophore with a marked Kauzman‐Eyring effect. A cyclic structure stabilized by endo‐cyclic ionic or carboxylic interactions has been proposed as a possible model which would allow for rotational isomerism around the sulphur atom.