The Effect of pH on the Particle Stability of a Phosphotungstatestained Tobacco Necrosis Virus

When preparations of a tobacco necrosis virus were negatively stained with 2% sodium or potassium phosphotungstate, pH 6 or 7, immediately before viewing in an electron microscope, many of the virus particles were degraded or were penetrated by the stain. However, when the potassium phosphotungstate was at pH 3, 4 or 5 few of the particles were degraded or were penetrated by the stain. A correlation between the pH of stability and the isoelectric point of the virus is suggested. Fixation for 20 min. in 5, 10 or 20% formaldehyde effectively stabilized most of the particles of tobacco necrosis virus when stained with neutral potassium phosphotungstate, whereas fixation in 1.25 or 2.5% formaldehyde did not.