## 3 Amyloid-like Assemblies Exhibiting Intrinsic Visible Fluorescence
### Proteins  
It is well known that proteins may undergo amyloid-like association independently of their initial functional structure. Even proteins whose native structure contains only \(\alpha\)-helices may form \(\beta\)-rich amyloid structures. The prototypal example in this context is represented by myoglobin, whose ability to form aggregates in slightly denaturing conditions was discovered twenty years ago [30; 31]. On the other hand, although the sequences of intrinsically disordered proteins seem to be specifically designed to prevent aggregation, they can also form amyloids. An inspection of Table 1 clearly shows that intrinsic visible fluorescence has been frequently detected in amyloids formed by proteins. Indeed, it has been demonstrated that more than thirty unrelated proteins are endowed with this peculiar property. Notably, this radiation is emitted in a variety of different physical states, which include solution, crystalline, liquid crystals, fibrillar, suspensions, and amorphous states. Most of these proteins emit fluorescence in the blue region of the visible spectrum (420-470 nm) upon excitation in the wavelength range of \(\sim\)320-380 nm. Green fluorescence, with a wavelength of the maximum typically centered at \(\sim\)520 nm and observed with excitation in the range of \(\sim\)400-480 nm, has been reported for the HET-s prion domain, \(\alpha\)-synuclein, \(\alpha\)-lactalbumin, \(\beta\)-lactoglobulin, insulin, human/bovine serum albumin, K18 tau, GRASP ancestor 4, and elastin. Among these, elastin presents peculiar behavior since the excitation at 325 nm induces fluorescence emission in 2 distinct regions of the spectrum with maxima at 415 (blue) and 505 (green) nm. The authors interpreted these data by assuming that the 415 and 505 nm peaks, whose intensities increase upon aging and likely aggregation, could be generated via aromatic residues and their interaction, respectively (N43). However, these findings may be alternatively interpreted by assuming that the emission in the blue region is the radiation generated by the association of \(\beta\)-rich structures, whereas the green fluorescence could have originated, as a sort of RET (resonance electron transfer), via the absorption of the blue radiation. Importantly, it has been recently reported that the emission spectra of protein amyloids can extend up to the red and the near infra-red (NIR) regions [32]. Although the intensity of this radiation is much lower than that measured for the blue emission, clear red/NIR fluorescence has been detected for the HET-s prion domain, insulin, lysozyme, \(\alpha\)-synuclein, \(\alpha\)-lactalbumin, \(\beta\)-lactoglobulin, human serum albumin, and transthyretin (Table 1). One of the most interesting results that emerged from the spectroscopic characterization of protein amyloids is the observation that the wavelength of the emitted fluorescence may systematically depend on the wavelength of the radiation used for the excitation. Indeed, the characterization of amyloid aggregates formed by insulin and \(\beta\)-lactoglobulin indicates that the increment of the excitation wavelength in the 310-450 nm range produces an emission with an increasing wavelength (maxima located in the range of -410-510 nm) [33]. This observation represents a violation of [PERSON]'s rule, which states that the emission of a photon in a molecular system is generated by the decay from the lowest excited state. The data observed for insulin and \(\beta\)-lactoglobulin were interpreted by the authors by invoking the red edge excitation shift (REES) phenomenon [34], which is generated via solventfluorophore interactions. If the relaxation of the solvent is faster than the fluorescence decay, a redshift of the emission wavelength, independent of the excitation, is observed.