The effect of internal motion on the quality of a protein structure derived from nuclear magnetic resonance (NMR) cross relaxation has been investigated experimentally. Internal rotation of the tyrosine-31 ring of turkey ovomucoid third domain was found to mediate magnetization transfer; the effect led to underestimation of proton-proton distances in its immediate neighborhood. Experimental methods that distinguish pure cross relaxation from chemical exchange mediated cross relaxation were used to separate true distances from distorted ones. Uncorrected and corrected sets of distances, where the corrections took internal motion into account, each were used as input to a distance geometry program for structural modeling. Each set of distances yielded a family of similar (converged) structures. The two families of structures differed considerably (2 Å) in the region of tyrosine-31. In addition, differences as large as 1 Å were observed at other positions throughout the structure. These results emphasize the importance of analyzing the effects of internal motions in order to obtain more accurate NMR solution structures.
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