From Chiral Counterions to Twisted Membranes

In membranes, the chirality of the amphiphile constituents is sometimes expressed at a supramolecular scale of nanometers or micrometers. We have recently reported that membranes of nonchiral dicationic n−2−n amphiphiles can also be chirally twisted upon interacting with chiral tartrate counterions. Here, we demonstrate that the mechanism of the chiral induction by counterions involves specific anion−cation recognition and the induction of conformationally labile chirality in the cations. Single-crystal X-ray diffraction shows that the amphiphilic cations exist as a mixture of chiral conformers. 1H NMR data establish a specific recognition between tartrate and n−2−n cations and show that chiral conformers also exist in solution. Circular dichroism (CD) in the UV−vis shows a sharp conformational change of tartrate ions from anti to gauche when bound to the chiral cationic membranes. This is confirmed by CD in the infrared region which also shows concomitant induced CD bands in the vibrations of the n−2−n amphiphiles. These results represent the first example of the so-called Pfeiffer effect in a membrane. They provide a general framework for designing new tunable membrane systems. Our work also includes the first application of vibrational circular dichroism in the study of chiral conformations of amphiphiles in membranes and demonstrates the very high potential of this technique.