[1] Univ Complutense, Fac Quim, Dept Quim Fis, E-28040 Madrid, Spain
[2] CSIC, Inst Estructura Mat, E-28006 Madrid, Spain
[3] Univ Politecn Madrid, ETS Ingn Montes, Dept Quim Gen & Bioquim, E-28040 Madrid, Spain
[4] Harvard Univ, Dept Chem & Chem Biol, Cambridge, MA 02138 USA