[1] Univ Florence, Magnet Resonance Ctr CERM, I-50019 Sesto Fiorentino, Italy
[2] Univ Florence, Dept Chem, I-50019 Sesto Fiorentino, Italy
[3] Northwestern Univ, Dept Chem & Biochem, Evanston, IL 60208 USA
[4] Northwestern Univ, Dept Mol Biol, Evanston, IL 60208 USA
[5] Northwestern Univ, Dept Cell Biol, Evanston, IL 60208 USA