13C relaxation studies of the structure and flexibility of the carboxylic ionophore lasalocid A

Abstract

Carbon-13 spin lattice relaxation times (T₁) were determined for all the protonated carbon atoms of the carboxylic ionophore lasalocid A in non-hydroxylic (CDCl₃) and hydroxylic (CD₃OD) media. In CDCl₃, the motions of the protonated backbone carbon atoms C-4 to C-24 (inclusive) are all equally restricted. The correlation time for overall molecular reorientation, τ_eff, calculated from an average NT₁ value of 565 ms, is 8.6 × 10^–11 s, where N is the number of attached protons. The carbon atoms in the side chains are more mobile than those in the backbone. The carbon backbone in CD₃OD is characterized by segmental motion at the aromatic end, as evidenced by increased NT₁, values for C-4, C-5, C-7, and C-8. The reminder of the backbone appears to be rigid and to have a τ_eff virtually identical with that observed for the entire backbone in CDCl₃. These results are discussed in terms of the mechanism that has been proposed for ion complexation and transport by carboxylic ionophores in biomembranes.