Kuleshova et al. studied sugars as npCPAs for vitrification of
oocytes and embryos [311]. Glucose, fructose, and sorbitol were
equal in effectiveness to ethylene glycol, mole for mole, in supporting vitrification and raised T G . Trehalose, sucrose, and raffinose,
being larger molecules, required an increase in total solute concentration, when measured on a weight percent basis, to replace EG
mole for mole, thus depleting the solution of extra water, but also
raised T G , raffinose being particularly active in the latter respect.
3.7 Cryoprotectant
Toxicity Neutralization
There are cases in which the addition of a nontoxic concentration of
one cryoprotectant to a toxic concentration of another can neutralize the toxicity of the latter [312, 313]. This allows the otherwise
toxic agent to contribute to the glass-forming ability of the solution, in turn enabling the concentrations of the other component
(s) to be maintained at lower and safer levels while still allowing the
solution to vitrify [23, 227, 245, 248, 295]. The same principle has
also allowed improved recovery after freezing and thawing [136].
Cryoprotectant toxicity neutralization (CTN) is thus far
restricted to the neutralization of amide toxicity by Me 2 SO, and
even for amides, the effect is not universal [313]. CTN is strong for
formamide and urea, weak for acetamide and N-methylformamide,
and
non-existent
for
dimethylformamide
and
Nmethylacetamide [313].
The mechanisms that underlie these effects are presently
unknown. The idea that a “compatible solute” effect might be
involved analogous to the protection of proteins against urea by
solutes in nature that have a protein-stabilizing effect sufficient to
offset the protein-denaturing effect of urea [227, 314–317] was
not experimentally supported [295]. Pursuant to the original suggestion [318] that CTN may involve physical interaction between
amides and Me 2 SO, it was determined that the magnitude of the
exothermic heats of mixing when Me 2 SO is mixed with formamide,
ethylene glycol, and propylene glycol has the same rank order as the
viability of kidney slices exposed to mixtures of Me 2 SO with these
same solutes (formamide > ethylene glycol > propylene glycol),
and the heat of mixing between Me 2 SO and N-methylformamide,
whose toxicity is only marginally neutralized by Me 2 SO [313], was
minimal [227]. On the other hand, when similar experiments were
done in the presence of water, the affinity between Me 2 SO and
water is so strong that the interaction between Me 2 SO and formamide in aqueous solution is “thermochemically repulsive” (endothermic) [295]. Still, there remains a correlation between the
effectiveness of CTN for a given amide and its strength of interaction with Me 2 SO in aqueous solutions: as the interaction becomes
more thermochemically repulsive, CTN becomes weaker and eventually disappears (cf. [295] and [313]).
Principles of Vitrification
71
oocytes and embryos [311]. Glucose, fructose, and sorbitol were
equal in effectiveness to ethylene glycol, mole for mole, in supporting vitrification and raised T G . Trehalose, sucrose, and raffinose,
being larger molecules, required an increase in total solute concentration, when measured on a weight percent basis, to replace EG
mole for mole, thus depleting the solution of extra water, but also
raised T G , raffinose being particularly active in the latter respect.
3.7 Cryoprotectant
Toxicity Neutralization
There are cases in which the addition of a nontoxic concentration of
one cryoprotectant to a toxic concentration of another can neutralize the toxicity of the latter [312, 313]. This allows the otherwise
toxic agent to contribute to the glass-forming ability of the solution, in turn enabling the concentrations of the other component
(s) to be maintained at lower and safer levels while still allowing the
solution to vitrify [23, 227, 245, 248, 295]. The same principle has
also allowed improved recovery after freezing and thawing [136].
Cryoprotectant toxicity neutralization (CTN) is thus far
restricted to the neutralization of amide toxicity by Me 2 SO, and
even for amides, the effect is not universal [313]. CTN is strong for
formamide and urea, weak for acetamide and N-methylformamide,
and
non-existent
for
dimethylformamide
and
Nmethylacetamide [313].
The mechanisms that underlie these effects are presently
unknown. The idea that a “compatible solute” effect might be
involved analogous to the protection of proteins against urea by
solutes in nature that have a protein-stabilizing effect sufficient to
offset the protein-denaturing effect of urea [227, 314–317] was
not experimentally supported [295]. Pursuant to the original suggestion [318] that CTN may involve physical interaction between
amides and Me 2 SO, it was determined that the magnitude of the
exothermic heats of mixing when Me 2 SO is mixed with formamide,
ethylene glycol, and propylene glycol has the same rank order as the
viability of kidney slices exposed to mixtures of Me 2 SO with these
same solutes (formamide > ethylene glycol > propylene glycol),
and the heat of mixing between Me 2 SO and N-methylformamide,
whose toxicity is only marginally neutralized by Me 2 SO [313], was
minimal [227]. On the other hand, when similar experiments were
done in the presence of water, the affinity between Me 2 SO and
water is so strong that the interaction between Me 2 SO and formamide in aqueous solution is “thermochemically repulsive” (endothermic) [295]. Still, there remains a correlation between the
effectiveness of CTN for a given amide and its strength of interaction with Me 2 SO in aqueous solutions: as the interaction becomes
more thermochemically repulsive, CTN becomes weaker and eventually disappears (cf. [295] and [313]).
Principles of Vitrification
71