The influence of hydrogen bonds on NIAD-4 for use in the optical imaging of amyloid fibrils

The fast and accurate detection of amyloid fibrils, which are associated with many neurodegenerative diseases, is important for their early diagnosis. {[50-(p-Hydroxyphenyl)-2,20-bithienyl-5-yl]-methylidene}-propanedinitrile (NIAD-4) is a new promising fluorescent marker for amyloid fibrils, and the photophysical behaviour of NIAD-4 is controversial. Nonadiabatic dynamic simulations, density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were performed to determine the influence of the environment on NIAD-4 and the photophysical behaviour of NIAD-4. The results indicate that NIAD-4 is in the NIAD-4·3H2O compound form in the ground state in water. The torsion process of NIAD-4 proposed by Hu et al. (Phys. Chem. Chem. Phys. 2016, 18, 28) does not occur in the excited state. In addition, the fluorescence behaviour of NIAD-4 is sensitive to a hydrogen bonding environment, the maximum fluorescence wavelengths of NIAD-4 show considerable red-shifts, and the fluorescence intensity of NIAD-4 increases significantly in a hydrogen bonding environment. Intermolecular hydrogen bonds are vital for the phenomenon observed in the experiment because the fluorescence intensity of NIAD-4 becomes unusually high with increasing solvent polarities. Therefore, the influence of the intermolecular hydrogen bond should be carefully taken into consideration when NIAD-4 is used to probe the amyloid fibrils in hydrogen-bonding surroundings, especially in complex bioenvironments.