Biomedical Engineering Reference
In-Depth Information
x-ray diffraction (Reconditi et al., 2003) produces a modeled γ
rig
in good agreement with the experimen-
tal value. The γ
rest
value, on the other hand, can be reproduced by several conformations, all character-
ized by S1 heads oriented along the fiber axis.
The validity of this approach is also confirmed on a simple and static protein: collagen. From collagen
atomic structure (Berisio et al., 2002), the value of γ can be computed, equal to 1.39. This value is in excel-
lent agreement with the average of published experimental measurements of γ (Tiaho et al., 2007; Isaac
Freund, 1986): 1.47 ± 0.10 (see Figure 5.15).
5.8 conclusions
In this chapter, we have illustrated the physical origin of SHG from HRS and how coherent summation
establishes the sensitivity of this signal to the order and geometrical orientation of the emitters. The use
of this sensitivity through SHG polarization anisotropy measurements has been illustrated. Further,
we describe the molecular origin of HRS within proteins and provide a full mathematical model for
the interpretation of SPA data in terms of molecular order and protein conformation within the focal
volume. The power of these techniques was illustrated with examples applied to collagen imaging and to
the study of myosin conformation in skeletal muscle.
The capability of probing protein conformation and the geometrical distribution of proteins within
ordered lattices in a tissue, coupled with the μm-scale resolution in deep tissue (allowed by the nonlinear
nature of SHG), makes of SHG microscopy a unique technique for biomedical applications ranging from
basic research to the development of novel diagnostic tools, as illustrated in other chapters of this topic.
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