Dynamic studies of a fibronectin type I module pair at three frequencies: Anisotropic modelling and direct determination of conformational exchange.

A detailed analysis of the (15)N relaxation of a pair of modules from fibronectin is presented. The overall dimensions of the protein structure can be approximated by a cylinder with an axial ratio D(∥)/D(⊥) of 1.9. T(1), T(2) and NOE data, collected at three (15)M frequencies (50.6, 60.8 and 76 MHz), can be fitted satisfactorily to a Lipari-Szabo model, taking anisotropy into account. A method for analysing the exchange contribution to relaxation is presented. This contribution depends upon the predicted B (inf0) (sup2) frequency dependence in the fast exchange limit of these exchange terms. Using this analysis, relatively slow conformational exchange contributions are detected around one of the disulphide bonds in the first module of the pair.