Abstract: According to the experimental results on the influences of biological activity due to the point-mutant of ricin A-chain' S functional region (Tyr 80 Tyr 123, Glu 177, Arg180), the functional region of RTA and its mutants are studied theoretically by semi-empirical quantum chemical AM1 method.The intrinsic reason of the influences on biological activity due to functional region's point-mutant is analyzed theoretically by the change of frontier molecular orbital's property and energy level.The mutant(Tyr 123→Trp) which possesses a higher biological activity than RTAis predicted theoretically.

Key words: Ricin A-chain (RTA), Functional region, Semi-empirical quantum chemical AM1, Frontier molecular orbital