A novel UV laser-induced visible blue radiation from protein crystals and aggregates: scattering artifacts or fluorescence transitions of peptide electrons delocalized through hydrogen bonding?

Proteins lacking prosthetic groups and/or cofactors are known to undergo electronic excitation transitions only upon exposure to UV-C (<280 nm) and UV-B (280–320 nm), but not UV-A (320–400 nm) photons. Here, we report the discovery of a novel excitation that peaks at ∼340 nm and yields visible violet-blue radiation with apparent band maxima at ∼425, 445, 470, and 500 nm. All proteins and large polypeptides examined in solid form, and in solutions, display this quenchable and photobleachable radiation which can be established not owing to aromatic sidechains. As a note of caution, we wish to state that we have not been able to completely eliminate the possibility that the radiation may be an artifact owing to second order effects such as, e.g., Raman scattering of Raman-scattered photons; however, we assert that all our experiments indicate that the radiation actually owes to some form of fluorescence. We propose that peptide electrons that have been delocalized through intramolecular or intermolecular hydrogen bond formation display these long-wavelength electronic transitions. If confirmed by future studies, this preliminary discovery may turn out to have important implications for biomolecular spectroscopy, protein crystallography, and materials science.