Lanthanide Induced Shift and Relaxation Rate Studies of Aqueous L-Proline Solutions

The structure of aqueous L-proline at pH 3 has been studied with use of the lanthanide-induced-shift technique. Paramagnetic nuclear shifts and enhancement of nuclear-relaxation rates have been measured for all ¹³C and ¹H nuclei of L-proline in the presence of ten different lanthanide cations. The relaxation-rate data indicate that isostructural complexes are formed between proline and all ten lanthanide cations. An attempted separation of the observed lanthanide-induced shifts into pseudocontact and contact components with use of the Reilley method (C. N. Reilley, B. W. Good, and R. D. Allendoerfer, Anal. Chem., 48, 1446 (1976)) gave inconsistent results for the α and carboxyl carbons indicating that either the shifts at these nuclei have a nonaxial dipolar contribution or the hyperfine coupling constant is lanthanide ion dependent. The pseudocontact shifts at C_β, C_γ, and C_Δ and at all protons conform well to the axial symmetry model with the lanthanide ion principal symmetry axis intersecting a single oxygen atom 2.9 ±0.1 Å from the lanthanide cation.