Analytical Currents: A dissection of cross section.

Conformation by electrochemistry Electrochemistry is rarely thought of as a technique for determining structures of biomolecules. Nevertheless, because it is a much simpler and less expensive method than traditional structure-determining approaches such as NMR, MS, and X-ray diffraction, electrochemistry is certainly worth developing for conformational analysis. This is exacdy what Janet G. Osteryoung of North Carolina State University and Malgorzata Ciszkowska of Brooklyn College are doing, using a theoretical model that relates measured diffusion coefficients for counterions of highly charged polyions to charge-spacing values. In this work, the researchers investigated the anionic polysaccharide K-carrageenan (K-car), which undergoes a transition from a coil to a double helix conformation as the temperature decreases. The polysaccharide consists of one charge per repeating disaccharide unit. The probe ion is TV. Using steady-state voltammetry, the diffusion-limited current (»D) for the reduction of Tl in an aqueous solution of 20 mM Na„K-car was determined at various temperatures. A change in iD over the 10-25 °C range, indicative of the conformation change for the polysaccharide, was observed. Plugging the measured values into the theoretical model a value of 0.38 for the uniform spacing between the polyion charges in the double helix form was determined which agrees with Xray data. The charge spacing for the coil form is 0.89 nm, which is somewhat lower than the 1.0 nm values from extended molecular models. (J. Am. Chem. Soc. 19199 121,1617-18)