Peptide nanotubes (PNT) of diphenylalanine (FF) have attracted considerable attention from researchers in the last decades. The chirality of FF monomers determines the kinetics of PNTs’ self-assembly and their morphology. The helical symmetry of PNTs causes significant intrinsic polarization and endows them with a unique combination of mechanical, electronic, and optical properties, as well as a strong piezoelectric effect useful for various applications. In this work, we used a combination of computer modeling and quantum chemical calculations to study the photoelectronic properties of FF PNTs of different chiralities. Using semiempirical methods implemented in the HyperChem and MOPAC packages, we calculated HOMO and LUMO energy levels and a band gap and their variations under the action of external and internal electric fields. We demonstrated that the photoelectronic properties of l- and d-FF PNTs are slightly different and may be related to the intrinsic electric field arising due to the internal polarization. The band gap of FF PNTs is within the ultraviolet range (400–250 nm) and can be tuned by an external electric field. These results open a way to create FF PNT-based solar-blind ultraviolet photodetectors and other electro-optic and electronic devices.