
118
Second Harmonic Generation Imaging
Considering for example the two main constituents of muscle (i.e., myosin and actin), the α-helices
present in G-actin display an orientational dispersion preventing SHG, while myosin is endowed with
some extraordinarily long
α
-helices highly aligned, especially in the tail portion. Clearly, a single pro-
tein would produce too low an intensity of SHG to be detected. us, a third level of structural orga-
nization is required in which SHG-emitting proteins are arranged with a symmetry allowing further
summation of the signal up to a detectable level. ese considerations provide basis of interpreta-