**5. Acknowledgement**

Financial support from Fundação para a Ciência e Tecnologia (FCT) through grant no. PEst-C/EQB/LA0006/2011 and by European Union through FEDER and QREN 2007- 2013 programs (project PTDC/AAC-AMB/104882/2008) is acknowledged. H.M. Oliveira also thanks post-doctoral grant SFRH/BPD/75065/2010.
