1. Dipartimento di Chimica, Universita' di Firenze, Polo Scientifico, Via della Lastruccia 3, I-50019 Sesto Fiorentino (FI), Italy, LENS−European Laboratory for Nonlinear Spectroscopy, Polo Scientifico, Via N. Carrara 1, I-50019 Sesto Fiorentino (FI), Italy, Department of Biochemical Sciences, University “La Sapienza” 00185 Rome, Italy, and Department of Biology, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan