can be attributed to the different degrees of charge transfer character in the three
compounds that differ for electron-donor character of the substituent and not for
geometry. This is a nice example of how peripheral substituents can modulate a
number of CPL relevant parameters: transition wavelength, Stokes shift, rotational
strength sign, and magnitude.
10.2.2 Fluorescence and CPL Probes
In this section, we investigate how a fluorescent probe is able to report on molecular
or supramolecular phenomena that are either determined by chirality or related to
it. Sensing in these circumstances is caused by a noncovalent interaction of the
fluorescent molecule with the environment: aggregates, metal ions, pH; such interaction either turns on or turns off chiroptical responses, like CD or CPL. We present
three examples of the phenomenon and will compare the CPL response with other
chiroptical responses, mainly electronic CD (ECD), activated at the same time.
10.2.2.1 Sensing Fibrils
The first example regards sensing of fibril aggregation by a fluorescent nonchiral
probe, namely thioflavin T (ThT). Fibril aggregation is one of the numerous phenomena of macromolecular assembling in which two types of ordering, M- and Pinsulin fibrils in this case, coexist, and can be separated through a bifurcation process
[43, 44]. In the present case the two species were indeed separated depending on
temperature and pH condition while vortexing [43]. In Fig. 10.4, we report the ECD
and UV-absorption spectra and the CPL and fluorescence spectra of ThT staining
insulin fibrils of either P or M helicity.
One may see that, for the 1:10 (ThT:Insulin) molar ratio, g abs ¼
(ΔAbs/Abs) ¼ 1 Â 10
–2 , and also for g lum ¼ (ΔI/I) ¼ 1 Â 10
–2 , thus g lum /g abs ¼ 1.
For 1:2 (ThT:Insulin) molar ratio, comparison of CD and absorption spectra shows an
evident shift that can be explained by considering that absorption contains contributions from bound and nonbound thioflavin; on the contrary there is no shift of the CPL
band with respect to the fluorescence band since only ThT molecules bound to
amyloid exhibit both fluorescence and CPL. Investigating the origin of the phenomenon, two possibilities were theoretically tested [45] both in the ground and excited
state, namely: a chiral twist of ThT is stabilized by the fibril or particular orientations
of (even flat) thioflavin with respect to nearby aromatic amino acids; the two situations
provided theoretical chiral responses of the same order of magnitude.
10.2.2.2 Sensing Metal Cations
The second example regards the possibility to reveal the presence of metal ions in a
solution of special organic molecules or oligomers based on ortho-oligo-phenylene226
G. Longhi and S. Abbate
compounds that differ for electron-donor character of the substituent and not for
geometry. This is a nice example of how peripheral substituents can modulate a
number of CPL relevant parameters: transition wavelength, Stokes shift, rotational
strength sign, and magnitude.
10.2.2 Fluorescence and CPL Probes
In this section, we investigate how a fluorescent probe is able to report on molecular
or supramolecular phenomena that are either determined by chirality or related to
it. Sensing in these circumstances is caused by a noncovalent interaction of the
fluorescent molecule with the environment: aggregates, metal ions, pH; such interaction either turns on or turns off chiroptical responses, like CD or CPL. We present
three examples of the phenomenon and will compare the CPL response with other
chiroptical responses, mainly electronic CD (ECD), activated at the same time.
10.2.2.1 Sensing Fibrils
The first example regards sensing of fibril aggregation by a fluorescent nonchiral
probe, namely thioflavin T (ThT). Fibril aggregation is one of the numerous phenomena of macromolecular assembling in which two types of ordering, M- and Pinsulin fibrils in this case, coexist, and can be separated through a bifurcation process
[43, 44]. In the present case the two species were indeed separated depending on
temperature and pH condition while vortexing [43]. In Fig. 10.4, we report the ECD
and UV-absorption spectra and the CPL and fluorescence spectra of ThT staining
insulin fibrils of either P or M helicity.
One may see that, for the 1:10 (ThT:Insulin) molar ratio, g abs ¼
(ΔAbs/Abs) ¼ 1 Â 10
–2 , and also for g lum ¼ (ΔI/I) ¼ 1 Â 10
–2 , thus g lum /g abs ¼ 1.
For 1:2 (ThT:Insulin) molar ratio, comparison of CD and absorption spectra shows an
evident shift that can be explained by considering that absorption contains contributions from bound and nonbound thioflavin; on the contrary there is no shift of the CPL
band with respect to the fluorescence band since only ThT molecules bound to
amyloid exhibit both fluorescence and CPL. Investigating the origin of the phenomenon, two possibilities were theoretically tested [45] both in the ground and excited
state, namely: a chiral twist of ThT is stabilized by the fibril or particular orientations
of (even flat) thioflavin with respect to nearby aromatic amino acids; the two situations
provided theoretical chiral responses of the same order of magnitude.
10.2.2.2 Sensing Metal Cations
The second example regards the possibility to reveal the presence of metal ions in a
solution of special organic molecules or oligomers based on ortho-oligo-phenylene226
G. Longhi and S. Abbate