1. Center for Structural Molecular Biology, Brazilian Synchrotron Light Laboratory, LNLS, Rua Giuseppe Maximo Scolfaro 10000, P.O. Box 6192, CEP13083-970, Campinas SP, Brazil, and Department of Biochemistry, Institute of Chemistry, University of São Paulo, Av. Prof. Lineu Prestes, 748, São Paulo, SP, 05508-900, Brazil