Biomedical Engineering Reference
In-Depth Information
rewarming of the bilayers after they had been cooled through their Tm. These
results, which suggest hydrophobic interactions in the stabilization mechanism, are
interesting, as hydrophobicity has also been identified as a key property required
for thermal hysteresis. Studies using the synthetic derivatives XXXX2KE with
different lipid mixtures have been recently reported to clarify the mechanism by
which TTTT interacts with membranes [
128
]. The study showed that the overall
charge and mutations in XXXX (X
D
T, A, V, I) had a profound effect on the
ability of the peptide to stabilize membranes. While TTTT stabilized the model
membranes to leakage, all four peptides XXXX2KE destabilized the membranes
to leakage. TTTT2KE and AAAA2KE interacted preferentially with the DGDG
in the lipid mixture, while VVVV2KE showed no preference for either lipid. The
results are consistent with interactions involving the hydrophobic face of AFP I
and the model bilayers, which is the same face of the protein that is responsible for
antifreeze properties. The different effects correlate with the helicity of the peptides,
suggesting that the solution conformation of the peptides has a significant role in
determining the effects of the peptides on thermotropic phase transitions of the
membrane.
AFGPs was proposed by Tomczak and Crowe [
129
] to form a monolayer
coverage of the surface of the membrane to prevent leakage from phospholipid
membranes, blocking transient leakage across the membrane as it is cooled through
its thermal phase transition (
T
m
). In contrast, direct interactions between the '-
helical AFP I, TTTT, and liposomes containing plant lipid bilayers (that alter the
order of the alkyl chains in the hydrophobic core) were proposed as a mechanism
for stabilization, via partial insertion of the N-terminal residues of the peptide into
the membrane during chilling.
The proposed role of the hydrophobic residues in the four synthetic analogues
(XXXX2KE X
D
T, A, I, V) in interactions with lipids [
130
] is consistent with the
ice growth inhibition properties of these compounds. This ice growth inhibition
is directly correlated to the hydrophobic face of the helix being oriented toward
the ice at the ice/water interface [
131
]. However, the results with XXXX2KE [
86
]
and liposomes are also informative in terms of the design of synthetic helical
AFPs. While TTTT2KE and VVVV2KE retain antifreeze properties (i.e., thermal
hysteresis and modification of ice crystal growth) comparable to the native winter
flounder AFP TTTT, the introduction of the additional salt bridges in XXXX2KE
leads to a destabilization of membranes, clearly an undesirable property when the
goal is enhancement of stabilization of membranes to leakage. Further experiments
are required to establish whether the destabilization is due to the large size of the
additional 2KE side chains, or the increased hydrophilicity of one face of the helix.
2.4.3
Food Technology
Ice crystal morphology plays an important role in the textural and physical
properties of frozen and frozen-thawed foods and in processes such as freeze
