CHARACTERIZATION OF THERMAL-ISOMERIZATION AT THE SINGLE-MOLECULE LEVEL

Single-channel measurements were used to study the cis-trans isomeriza tion of a carbamate bond in gramicidin-ethylenediamine, a derivative o f the channel-forming peptide gramicidin. From the temperature depende nce of transition rates, Delta H-double dagger values of 64.9 +/- 2.5 and 58.2 +/- 3.0 kJ mol(-1) were calculated for isomerization from the trans and cis states, respectively. The entropies of activation were calculated to be near zero as expected for a unimolecular isomerizatio n process. This represents the first characterization of thermal isome rization at the single molecule level. The calculated activation param eters are in very good agreement with previous NMR studies of cis-tran s isomerization of carbamate bonds in small molecules. This correspond ence validates the use of ion channel measurements for the study of co nformational changes at the single molecule level.