Solution equilibria in YbDOTMA, a chiral analogue of one of the most successful contrast agents for MRI, GdDOTA

The structure and the conformational equilibria in aqueous and methanolic s olutions of YbDOTMA (DOTMA = (1R,4R,7R,10R)-alpha,alpha',alpha",alpha""-tet ramethyl-1,4,7,10-tetraaza- cyclododecane-1,4,7,10-tetraacetic acid) are in vestigated and the dynamics studied by means of NMR methods. The presence o f two isomeric forms in slow exchange is confirmed and quantitatively chara cterized by EXSY. The exchange peaks are assigned to the inversion of the c onformation of the macrocycle ring. No evidence is obtained of a reorientat ion of the methyl acetate substituents. The protons switch between position s with the same geometrical factors, i.e. positions that would have the sam e pseudocontact shifts were it not for the different magnetic susceptibilit y anisotropies of the two species; steady-state NOE spectra on the major fo rm allow unequivocal assignment of the conformations of the two isomers.