Received;
10;
August;
2007;
accepted 15 December 2007 Projects 2004CB619205 supported by the National Basic Research Program of China;
50574099 by the National Natural Science Foundation of China and 06JJ30024 by the Natural Science Foundation of Hunan Province Corresponding author. Tel: +86-13973198490;
E-mail address: zhangjiecsu@126.com;