Acknowledgements
This research was supported by the Program for New Century Excellent Talents in China (NCET-07-0507), the Natural Science Foundation of Shandong Province of China (Z2008D01) and the Project of Science and Technology Development Plan of Shandong Province of China (2007GG2009011).
Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.