The pore of sodium channels contains a selectivity filter made of negatively charged amino acid residues, which attract the positive Na+ ion and keep out negatively charged ions such as chloride. The cations flow into a more constricted part of the pore that is 0.3 by 0.5 nm wide, which is just large enough to allow a single Na+ ion with a water molecule associated to pass through. The larger K+ ion cannot fit through this area. Ions of different sizes also cannot interact as well with the negatively charged glutamic acid residues that line the pore.