Proton nmr relaxation study of the dynamics of anthopleurin‐A in solution

Spin–spin and spin–lattice ¹H‐nmr relaxation times of the sea anemone polypeptide anthopleurin‐A were measured at frequencies of 200, 300, 400, and 500 MHz. Relaxation times were fitted iteratively by least squares regression to the isotropic tumbling model, Woessner's model for anisotropic motion, and Lipari and Szabo's “model‐independent” model. Data for aromatic and aliphatic methine protons could not be fitted satisfactority using the isotropic model. Good fits were obtained, however, using the model‐independent approach,‐indicating that high‐frequency internal motions of the polypeptide backbone were significant. In addition, a range of τ_C values from 2.2 to 3.2 ns was obtained for various methine protons, suggesting that overall rotational reorientation of the molecule was anisotropic. Methyl group relaxation data were fitted satisfactorily by Woessner's model. Some assessment has been made of the effect of experimental errors on the quality of fit to the data, as well as of the contribution of experimental values at certain frequencies to definition of the spectral density function.