Self-assembled helical ribbon and tubes of alanine-based amphiphiles induced by two different formation mechanisms

Three long chain alanine-based amphiphiles (1-3) possessing either a saturated long alkyl chain group or unsaturated groups as the self-assembling unit of a highly organized molecular architecture were synthesized. Their self-assembling properties were investigated using EF-TEM, SEM, CD, XRD, and FTIR. The d-form (1) and the l-form (2) enantiomers showed the opposite CD signals. Furthermore, electron micrographs of the self-assembled 1 and 2 exhibited right- and left-handed helical structures, respectively. The helical structures of amphiphiles 1 and 2 were developed into tubular structures by slow cooling. On the other hand, in the helical ribbon growth process, the helical pitch and ribbon width were changed in fast cooling process. The findings strongly imply that the helical ribbon growth of alanine-based amphiphiles can be induced by two different pathways. This mechanism, quite different from that observed for sugar- and cholesterol-based tubes, is a rare example for a tube formation process.