X-Message-Number: 8436
From: Brian Rowley <>
Subject: adonitol vs. glycerol hydrated radius
Date: Sat, 2 Aug 1997 11:15:28 -0700

Doug Skrecky wrote:
>Adonitol has a molecular weight of 152,
>while glycerol's is 92.09. Adonitol perfuses well for a molecule of its
>size because its cross section is no larger than that of glycerol, so it
>can pass through pores in cell membranes.

Right; just because a molecule or ion is bigger doesn't always mean it will
permeate less effectively. A classic example is sodium vs. potassium.
Potassium has an atomic weight of 39, sodium has an atomic weight of 23 but
sodium ions are bigger in solution than potassium ions (!), and sodium
channels are wider than potassium channels in nature. The reason is that the
molecular mass is not as important as the hydrated radius--the hydrated
radius is the radius of the molecule + affiliated water molecules. Sodium is
a more compact molecule but has a bigger hydrated radius because it attracts
more associated water molecules (fewer electrons, a tighter electron shell
and less repulsion of electronegative oxygen molecules of water). Wonky but
true; ask any good electrophysiologist.

Water also perfuses well, but according to biologists at UBC no one knows
why. Tissue membranes are phospholipid bilayers, and there is no obvious
reason why water should get through so easily. They might have solved that
puzzle by now, I was given the information about 7 years ago (probably a
special pore or something).

Lastly, something that is hydrophobic (e.g., less polar) will cross a
membrane a lot easier than something that is hydrophilic as a rule,
regardless of molecular mass. It might be profitable to compare the
hydrophobicity of adonitol with that of glycerol. Cheers,

Brian Rowley

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