the permeation process of small molecules through lipid bilayers. We perform molecular-
dynamics simulations using three kinds of lipid molecules with different chain length:
dilauroylphosphatidylcholine, dimyristoylphosphatidylcholine, and
dipalmiltoylphosphatidylcholine. Free-energy profiles of O 2, CO, NO, and water molecules
are calculated by means of the cavity insertion Widom method and the probability ratio …