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Sizing the Bacillus anthracis PA₆₃ Channel with Nonelectrolyte Poly(Ethylene Glycols)

Brian J. Nablo ^*, Kelly M. Halverson , Joseph W. F. Robertson ^*, Tam L. Nguyen , Rekha G. Panchal , Rick Gussio , Sina Bavari , Oleg V. Krasilnikov ^¶ and John J. Kasianowicz ^*

^* Electrical and Electronics Laboratory, Semiconductor Electronics Division, National Institute of Standards and Technology, Gaithersburg, Maryland; United States Army Center for Environmental Health Research, Fort Detrick, Frederick, Maryland; Target Structure-based Drug Discovery Group, SAIC-Frederick, National Cancer Institute Frederick, Frederick, Maryland; United States Army Medical Research Institute of Infectious Diseases, Fort Detrick, Frederick, Maryland; and ^¶ Laboratory of Membrane Biophysics, Department of Biophysics and Radiobiology, Federal University of Pernambuco, Recife, Pernambuco, Brazil

Correspondence: Address reprint requests to John J. Kasianowicz, Electronics and Electrical Engineering Laboratory, Semiconductor Electronics Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8120. E-mail: john.kasianowicz{at}nist.gov.

Nonelectrolyte polymers of poly(ethylene glycol) (PEG) were used to estimate the diameter of the ion channel formed by the Bacillus anthracis protective antigen 63 (PA₆₃). Based on the ability of different molecular weight PEGs to partition into the pore and reduce channel conductance, the pore appears to be narrower than the one formed by Staphylococcus aureus -hemolysin. Numerical integration of the PEG sample mass spectra and the channel conductance data were used to refine the estimate of the pore's PEG molecular mass cutoff (1400 g/mol). The results suggest that the limiting diameter of the PA₆₃ pore is <2 nm, which is consistent with an all-atom model of the PA₆₃ channel and previous experiments using large ions.