Fluorescence Emission of Self‐assembling Amyloid‐like Peptides: Solution versus Solid State

Abstract

Analysis of the intrinsic UV-visible fluorescence exhibited by self-assembling amyloid-like peptides in solution and in solid the state highlights that their physical state has a profound impact on the optical properties. In the solid state, a linear dependence of the fluorescence emission peaks as a function of excitation wavelength is detected. On the contrary, an excitation-independent emission is observed in solution. The present findings constitute a valuable benchmark for current and future explanations of the fluorescence emission by amyloids.

Solid-state optical properties of self-assembling amyloid-like peptides with different charged states at the terminal ends

“…The PL results collected on two amyloidogenic systems, F6 and Aβ 16–21 , allow to conclude that all the generated peptide films are able to emit multicolor PL profiles regardless their terminal charge state. The observed optical properties provide further support to the notion that proton transfer between the C- and the N-terminal of two adjacent strands in a β-sheet structure is not essential for generation of the intrinsic PL exhibited by amyloid-like systems.These results are compatible with the hypothesis that the emission is a consequence of the decrease of the excitation energy and stabilization of the peptide bond due to a deplanarization of amide groups 27 , although this explanation has to demonstrate its validity in interpreting the fluorescence exhibited by helical peptides 40 as well as the its dependence from the physical state of the sample 36 .…”

“…3 c, d and S3 ). Also, very recent outcomes have shown the occurring of this same effect in solid-state polyethylene glycol-derivatized F6 films 36 . The variation of the emission wavelength upon increasing the excitation wavelength exhibited by these peptides represents an interesting violation of the Kasha’s rule, according to which “the emitting level of a given multiplicity is the lowest excited level of that multiplicity” 31 .…”

A Comprehensive Analysis of the Intrinsic Visible Fluorescence Emitted by Peptide/Protein Amyloid-like Assemblies

“…In highly rigid environments, such as those occurring in amyloid aggregates, the tight association of the molecules makes the relaxation of the solvent molecules, if present, particularly long, and high-energy emissions can also be detected; thus, a wavelength-dependent fluorescence emission is observed. Similar effects have also been observed for amyloids formed by peptides, even in solution states [35,36]. Although the three-dimensional structures of some of these amyloids have been reported in the Protein Data Bank (PDB) (see Figure 3 for representative examples), caution should be taken in deriving correlations between the structure and the spectroscopic properties.…”

Differently N‐Capped Analogues of Fmoc‐FF

“…As reported for other β‐rich peptide aggregates, the resulting fibers exhibited unexpected photoluminescence (PL) in the blue spectral region [24] with an emission maximum at 420 and 460 nm upon excitation at 340 and 380 nm for PEG2‐FF and PEG6‐FF, respectively. A REES (Red Edge Excitation Shift) phenomenon, generally verified at solid state, [25] was also detected, thus suggesting a high degree of flexibility for PEGylated diphenylalanines (Figure 4D).…”