Timescale of Protonation and Deprotonation across micelle layers
Permeation of protons, potassium ions, and small polar molecules through phospholipid bilayers as a function of membrane thickness


  Reported values for proton permeability coefficients vary over
  a
  wide
  range,
  from
  $10^{-2} \, \mathrm{cm/s}$ to $10^{-7} \, \mathrm{cm/s}$
  (Nichols
  and
  Deamer,
  1980;
  Nichols
  et
  al.,
  1980;
  Biegel
  and
  Gould,
  1981;
  Elamrani
  and
  Blume,
  1983;
  Grzesiek
  and
  Dencher,
  1986;
  Perkins
  and
  Cafiso,
  1986;
  Norris
  and
  Powell,
  1990).
  This
  variation
  relates
  to
  the
  use
  of
  different
  techniques,
  lipids,
  and
  bilayer
  systems
  (liposomes
  and
  planar
  bilayers)
  or
  different
  pH
  ranges
  (Perkins
  and
  Cafiso,
  1986).
  The
  proton
  data
  of
  this
  study
  agree
  with
  the
  higher
  reported
  values.
  
  As revealed by Fig. 4, proton permeability coefficients strongly
  depend on membrane thickness, starting at a maximum value of $1.3 \cdot 10^{-2} \, \mathrm{cm/s}$ for the shortest lipid and steadily decreasing to a value of $4.9 \cdot 10^{-5} \mathrm{cm/s}$ for the longest lipid. It follows that proton permeability decreases by a factor of approximately 250 as the thickness of the
  hydrophobic region is increased from $20 \, \mathring{\mathrm{A}}$ to $37 \, \mathring{\mathrm{A}}$.