  Standard models of ion channel voltage gating require substantial movement of
one transmembrane segment, S4, of the voltage sensing domain. Evidence comes
from the accessibility to external methanethiosulfonate (MTS) reagents of the
positively charged arginine residues (R) on this segment. These are first
mutated to cysteines (C), which in turn react with MTS reagents; it is assumed
that the C is passively carried in the S4 movement, becoming accessible on one
side or the other of the membrane. However, the Rs were salt bridged to
negatively charged residues on other transmembrane segments, or hydrogen
bonded, while C reacts as a negative ion. The space available for MTS is fairly
close to the difference in volume between the large R residue and much smaller
C, so the MTS is not severely sterically hindered. A reagent molecule can reach
a C side chain; the C can react if not repelled by a negative charge from the
amino acid to which the R had been salt bridged. Nearby protons may also make
reaction possible unless the C itself is protonated. Therefore interpretation
of the C substitution results requires reconsideration. To test the idea we
have done quantum calculations on part of a mutated S4 and the nearby section
of the channel. The mutation is R300C of the 2A79/3Lut structure, a mutation
that would be done to test MTS reagent access; there is a large cavity where
the R is replaced by C. Two quantum calculations show a substantial difference
in the structure of this cavity with 2 water molecules compared to 4. This
suggests that the structure, and presumably reaction probability, could depend
on water molecules, very likely also protons, in or near the cavity that the
R300C mutation produces.
