Photoinduced Morphological Transformations of Soft Nanotubes

In water, synthetic amphiphiles composed of a photoresponsive azobenzene moiety and an oligoglycine hydrogen-bonding moiety selectively self-assembled into nanotubes with solid bilayer membranes. The nanotubes underwent morphological transformations induced by photoisomerization of the azobenzene moiety within the membranes, and the nature of the transformation depended on the number of glycine residues in the oligoglycine moiety (i.e., on the strength of intermolecular hydrogen bonding). Upon UV-light irradiation of nanotubes prepared from amphiphiles with the diglycine residue, trans-to-cis isomerization induced a transformation from nanotubes (inner diameter (i.d.) 7nm), several hundreds of nanometers to several tens of micrometers in length, to imperfect nanorings (i.d. 21-38nm). The cis-to-trans isomerization induced by continuous visible-light irradiation resulted in the stacking of the imperfect nanorings to form nanotubes with an i.d. of 25nm and an average length of 310nm, which were never formed by a self-assembly process. Time-lapse fluorescence microscopy enabled us to visualize the transformation of nanotubes with an i.d. of 20nm (self-assembled from amphiphiles with the monoglycine residue) to cylindrical nanofibers with an i.d. of 1nm; shrinkage of the hollow cylinders started at the two open ends with simultaneous elongation in the direction of the long axis.