¹This work was supported by grants (NOS 95-YU-15 and 39570170) from the China Committee for Science and Technology.

²To whom correspondence should be addressed. Tel: +86-10-64888493, Fax: +86-10-64872026, E-mail: jingenx@public.east.cn. net

Abbreviations: SNase, staphylococcal nuclease; CD, circular dichroism; UV, ultraviolet; pdTp, thymidine-3',5'-bisphosphate; ANS, 1-anilinonaphthalene-8-sulfate.