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The three main fractions of clupeine, the protamine extracted from herring sperm, have been investigated by 13C nuclear magnetic resonance techniques. The dynamic behaviour, examined through the evaluation of the spin lattice relaxation times (T1) of individual carbon resonances assigned to both backbone and side chains, reveals interesting features. The relaxation times of backbone alpha-carbons, interpreted on the basis of an axially symmetric ellipsoid, point to the clupeine fractions as being essentially extended in aqueous solution. These times remain constant along the polypeptide chain and are of the order of 0.16 +/- 0.02 s. Conversely, the side chains show different flexibilities in the presence of monophosphate counterions, thus demonstrating a diverging behaviour which may be biologically relevant. In particular, the side-chain flexibilities of fraction YI decrease, while those of fractions Z and YII are either constant or increase. Comparison of these data with the viscosity measurements helps in explaining the viscosity changes observed in the presence of phosphate.
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