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The sedimentation coefficients of tobacco necrosis virus strain D (TNV d ) in sodium phosphate at pH 5 and pH 7 are 104S and 97S respectively. The change in sedimentation coefficient between pH 5 and 7 is fully reversible and the decrease on transfer to pH 7 is unaffected by the presence of Mg2+. TNV d is disrupted by Bacillus subtilis protease at pH 7.5 but not at pH 5; ribonuclease neither disrupts virions nor destroys the infectivity of TNV d at either pH 7.5 or pH 5. Virions are markedly unstable at alkaline pH but the effectiveness with which different buffers dissociate TNV d varies. Virus is largely resistant to dissociation by 1 m-NaCl, 1 m-urea, 10 m-ethanediol and 1% sodium dodecyl sulphate (SDS) at or below pH 6 and is largely susceptible to dissociation by these agents at or above pH 7. Our results suggest that as the pH is lowered some non-covalent bonds important for virion stability may be formed and/or some electronic repellent interactions causing virion instability may be weakened.
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