Monitoring proton dissociation and solution conformation of chiral ytterbium complexes with near-IR CD

The ytterbium complex [Yb((S)-THP)](3+) ((S)-THP = (IS,4S,7S,10S-tetrakis(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane) is investigated in solution through NMR, near-IR absorption, and CD spectroscopy. Quantitative analysis of the paramagnetic pseudocontact NMR shift shows A helicity of the ligand cage around the metal. The NIR CD spectrum recorded at acidic pH is found to be very similar to that of [Yb((R)-DOTMA)] (R)-DOTMA = (IR,4R,7R,10R)-alpha,alpha ',alpha '',alpha '''-tetramethyl-1,4,7,10tetraazacyclododecane-1,4,7,10-tetraacetic acid), which in solution assumes a twisted square antiprism (`FSA) conformation. The similarity of the NIR CD spectra is discussed, and it is the first proof of the A(lambda,lambda,lambda,lambda) conformation of [Yb((S)-THP)](3+). NIR CD spectra recorded in the pH range of 2-9 allow one to easily follow proton dissociation and to calculate the pK of this equilibrium in water (PKA = 6.4 +/- 0.1). This value agrees well with that determined for [Lu((S)-THP)](3+) using potentiometric methods. This demonstrates once again that NIR CD spectroscopy is a powerful technique for investigating the solution structure and dynamics of these complexes. Chirality 17:201-211, 2005. (c) 2005 Wiley-Liss, Inc.