The ionic properties of the filamentous bacteriophages Pf1 and fd

The surface charge of the filamentous bacteriophages Pf1 and fd was determined by polyelectrolyte titration covering a range from pH 3 to 9. The isoelectric point was measured at pH 4.0 for Pf1 and at pH 4.2 for fd. Close agreement of experimental and calculated data was found, confirming the hypothesis that charged residues in the DNA are neutralized by oppositely charged residues of the coat proteins. Almost perfect consistence is reached when structural details of the viruses are included.