The conformational, electrical, and optical intrinsic properties of L-phenylazophenylalanine (L-PAP), a nonproteinogenic photoresponsive amino acid used to modulate the binding affinity and activity of peptides and proteins, have been systematically investigated using quantum mechanical calculations, with special emphasis being put on the trans-to-cis isomerization of the azobenzene side group. Analyses of the conformational maps and the minimum-energy conformations, which were obtained using density functional theory calculations at the B3LYP/6-311þþG(d, p) level, indicate that the semiextended β is the most favored conformation for both the trans and cis isomers in the gas phase. However, water tends to stabilize the helical backbone arrangement, but only for the cis isomer since this is a sterically forbidden conformation for the trans one. On the other hand, time-dependent density functional theory calculations at the BMK/6-311þG(d,p) level indicate that the peptide backbone does not induce significant changes in the optical properties of the chromophore. This feature was evidenced by both the small dependence of the πfπ* and nfπ* transition wavelengths with the backbone dihedral anglesjandψand the resemblance between the transition wavelengths determined for L-PAP and free azobenzene. In contrast, the dipole moment has been identified as a key property for this photoresponsive amino acid because of its large dependence on both the peptide backbone and the isomerization state. ; Peer Reviewed ; Postprint (published version)