Surface Monolayers of Poly(l-glutamic Acid)-Functionalized Amphiphiles: Effect of Attachment of Spiropyran to the...

A novel poly(l-glutamic acid)-functionalized amphiphile (2) containing spiropyran groups in the side chain was successfully prepared. Surface monolayers of 2 were affected by isomerization of the spiropyran group; i.e., 2sp, which is in the spiro form, gave an expanded monolayer comparable with that of the spiropyran-free 1, while 2mc in the cyanine form gave a well-condensed monolayer similar to the 1 monolayer. CD measurements for these deposited monolayers onto quartz plates showed that the helix content was higher for the 2sp monolayer than for the 2mc monolayer. Upon isomerization from the cyanine to the spiro form, the 2mc monolayer had a tendency to expand gradually. When d-tryptophan was added into the subphase, such a monolayer expansion was completely suppressed due to electrostatic interactions between zwitterions, whereas no effect of the addition of l-tryptophan on the monolayer was observed. This result suggests that the assembled structure of alpha-helical poly(l-glutamic acid) would provide a suitable site for capturing alpha-amino acid enantioselectively.