This work was supported by a research project from the Fundação para a Ciência e Tecnologia (POCI/QUI/55350/2004). SRP, AGD and MSC would like to thank the Fundação para a Ciência e Tecnologia for financial support (POCI/SFRH/BPD/14067/2003, POCI/QUI/55350/2004, POCI/SFRH/BPD/24150/2005, respectively). NMR experiments were run at the CERM EU-NMR Research Infrastructure (Contract n° 026145), and SRP would like to thank the EU-NMR for the financial support during her visits to CERM.