This work was partly supported by the Spanish Ministry of Science and Innovation (PSI2010-15115) and Portuguese Fundação para a Ciência e a Tecnologia (grants PDCT/SAU-OBS/59821/2004, PTDC/QUI/ 67522/2006 and PTDC/SAU-OSM/105572/2008, and fellowship SFRH/ BD/22612/2005 to M. Pingarilho).