The unnatural amino acid analogue 2-fluorohistidine (2-FHis) was incorporated into protective antigen to produce a protein which resists protonation at physiological pH by reducing the side-chain pKa. The protein structure was unperturbed by the incorporation of fluorinated histidine residues, and the heptameric (2-FHisPA.sub.63).sub.7 could form ion conducting channels, and bind to the PA-binding domain of LF (LF.sub.N), but translocation of LF.sub.N in planar lipid bilayers was blocked. Further, while (2-FHisPA.sub.63).sub.7 could bind to host cells and in vitro to the host cellular receptor, pore formation in the presence of the receptor was blocked, and LF.sub.N-DTA mediated cytotoxicity in CHO-K1 cells was blocked. The modified PA is useful as both a vaccine and an antitoxin, providing epitopes for the production of antibodies against PA, but preventing key steps in pathogenesis (pore formation, translocation).