X-Message-Number: 31309
Date: Wed, 31 Dec 2008 19:04:07 -0800 (PST)
From: 
Subject: Hydration index better explains ice crystalization

J Am Chem Soc. 2008 Dec 24;130(51):17494-501.
Hydration index--a better parameter for explaining small molecule hydration
in inhibition of ice recrystallization.
  Tam RY, Ferreira SS, Czechura P, Chaytor JL, Ben RN. Department of
Chemistry, D'Iorio Hall, 10 Marie Curie, University of Ottawa, Ottawa,
Ontario, Canada, K1N 6N5.
  Several simple mono- and disaccharides have been assessed for their
ability to inhibit ice recrystallization. Two carbohydrates were found to be
effective recrystallization inhibitors. D-galactose (1) was the best
monosaccharide and D-melibiose (5) was the most active disaccharide. The
ability of each carbohydrate to inhibit ice growth was correlated to its
respective hydration number reported in the literature. A hydration number
reflects the number of tightly bound water molecules to the carbohydrate and
is a function of carbohydrate stereochemistry. It was discovered that using
the absolute hydration number of a carbohydrate does not allow one to
accurately predict its ability to inhibit ice recrystallization.
Consequently, we have defined a hydration index in which the hydration
number is divided by the molar volume of the carbohydrate. This new
parameter not only takes into account the number of water molecules tightly
bound to a carbohydrate but also the size or volume of a particular solute
and ultimately the concentration of hydrated water molecules. The hydration
index of both mono- and disaccharides correlates well with experimentally
measured RI activity. C-Linked derivatives of the monosaccharides appear to
have RI activity comparable to that of their O-linked saccharides but a more
thorough investigation is required. The relationship between carbohydrate
concentration and RI activity was shown to be noncolligative and a 0.022 M
solution of D-galactose (1) and C-linked galactose derivative (10) inhibited
recrystallization as well as a 3% DMSO solution. The carbohydrates examined
in this study did not possess any thermal hysteresis activity (selective
depression of freezing point relative to melting point) or dynamic ice
shaping. As such, we propose that they are inhibiting recrystallization at
the interface between bulk water and the quasi liquid layer (a semiordered
interface between ice and bulk water) by disrupting the preordering of
water.
PMID: 19053462

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