Biomedical Engineering Reference
In-Depth Information
Fig. 2.21 Structural
morphology of hexagonal ice
directions [010] and [001]. The characteristic morphology of ice grown from most
insect AFP solutions is hexagonal plates [ 5 , 92 ]. In contrast to the structural or
the natural morphology, these ice crystallites reveal a strongly pronounced basal
face (001) and a morphologically much weakened primary prism (100). In all ice
crystallites grown as basal and primary prismatic forms, resulting from layer-by-
layer growth, triggered, e.g., by the spruce budworm AFP, no deviation from the
f 001 g and f 100 g indices has been observed under all experimental conditions. One
presumed exception to the hexagonal disk form is found in the action of the TmAFP,
the presence of which gives rise to a pyramidal ice habit. It is traditionally held
that the action of the insect TmAFP is deviant in causing a pyramidal rather than
a disk-shaped morphological modification, whereas all other known insect AFPs
cause a hexagonal plate habit. The question arises in the case of the TmAFP as to
whether the mechanism of morphological modification is exceptional; that means,
whether the ice binding surface of the TmAFP acts on secondary ice surfaces by
an exceptional mechanism or on primary ice surfaces by surface pinning through
2D PBC matching, just like the remaining insect-type ice binding surfaces. What
this question amounts to is whether the pyramidal ice form observed in the presence
of the TmAFP is one of the secondary pyramids with variable indices ( h 0 l )orthe
primary pyramid (101). It is found that the ice bipyramid triggered by the TmAFP
[ 93 ] has consistently a stubby lemon shape, showing no variation in the apical angle,
so that the TmAFP gives rise to a pyramid of fixed indices. The lemon-shaped ice
bipyramid produced by the TmAFP is not elongated like the predominant (201)
or higher-indexed pyramids observed in connection with the fish AFPs. From the
assortment of pyramidal shapes, it can be seen that this bipyramid matches well
the primary surface (101). Also in all ice crystallites grown as primary pyramidal
forms triggered by the TmAFP, resulting from layer-by-layer growth, no deviation
from the f 101 g indices has been observed under all experimental conditions. Thus,
the TmAFP action is by no means exceptional. According to Liou et al. [ 94 ], the
TmAFP has a high tendency to form hydrogen bonds with an ice surface. This result
agrees with the theoretical observation that there are 30% more unbonded hydrogen
atoms on the (101) than in the other primary surfaces, pointing to an increased
interaction between the ice binding surface of the TmAFP and the (101) primary
pyramid [ 80 ].
Lastly, it is worth mentioning that although freezing inhibition is always accom-
panied by a modification of the morphology of the ice crystallites, the converse does
not hold true: a morphological modification of ice does not necessarily imply that
Search WWH ::




Custom Search