PCET of the amino acid pair: (a) \(Glu^{-}_{235}...Lys_{91}\) structure and (b) \(Ser_{178}...Glu^{-}_{184}\) structure. The proton transfer potential energy surface is (a1 and b1) in vacuum and (a2 and b2) in aqueous solution, TS of (a3 and b3) the singlet state and (a4 and b4) the triplet state in which the spin up (down) is colored by black (red).