High Mobility of N-Terminal Parts of A and B Subunits of Ricin

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Abstract
1H-NMR spectra (500 MHz) of ricin and its isolated A and B subunits have been analyzed in this study. It is shown that together with tight packing of the polypeptide chain there also exist flexible highly mobile segments in the structure of the proteins. Examination of the line-widths of resonances and the identification of sharp signals with protons of amino acids indicates that N-terminal peptide segments of A and B subunits are free and undergo rapid segmental motions. Spin-echo sequence (90-τ-180-τ) was used to suppress an intensive unresolved background signal from protons involved in the globular part of ricin, thus permitting selective identification of the unusually sharp signals from mobile side chains.