The structure of cyclo[(-L-Phe^1-D-Ala^2)~n-] and cyclo-[(-L-Phe^1-D- ^M^eN-Ala^2)~n-](n=3～6) were completely optimized by using molecular mechanics and semi-empirical molecular orbital method AM1. The structural characteristics of these molecules for self-assembly were revealed. The influences of the substituent and ring size on molecular structure and self-assembly process were discussed in detail. The computational results show that [(-L-Phe^1-D-^M^eN-Ala^2) ~n-](n=3,5,6) cannot form disklike conformations, and further predict that [(-L-Phe^1-D-Ala^2)~n-] (n=3,5,6) can constitute nanotubes in addition to [(-L-Phe^1-D-Ala^2)~4-].

Key words: CYCLIC COMPOUNDS, STRUCTURE, MOLECULAR MECHANICS, MOLECULAR ORBITAL THEORY