This is the intermembrane space and this is the matrix of the mitochondria. Now we know along the matrix we have a low concentration of protons relative to the intermembrane space because remember, complexes one, three and four of the electron transport chain basically use the movement of electrons to generate that proton electrochemical gradient. So we have a proton rich environment in the intermembrane space and we have a proton poor environment in the matrix of the mitochondria. So in the first step, what happens is we have this hydrophilic half channel is open to the matrix of the mitochondria. And at the center of that half channel is this aspartate 61 residue that bears a negative charge. And so the proton will move from a high concentration through this half channel and it will bind onto that aspartate 61 residue found at the center of this C subunit that lies along this half channel.