Elucidating electronic and structural changes in metal porphyrins by 1-n-alkyl-3-methylimidazolium cations

Metal porphyrins act as the reactive center of the ubiquitous monooxygenase, cytochrome P-450. As such, both their native and functionalized forms have been successfully utilized in studying biodegradation of a plethora of molecular substrates. Ionic liquids, although quite promising and possessing benign physicochemical properties, are poorly understood in terms of their biodegradability from a theoretical standpoint. In this article, a quantum mechanical treatment of ionic liquids 1-n-alkyl-3-methylimidazolium (n = 2, 4, 6, 8, and 10) ([C_nmim]⁺) is carried out in the presence of a variety of metal porphyrins to understand their binding, which is the first step in the well-known catalytic cycle of cytochromes. Our treatment concerns the interaction strength between the cations and porphyrin molecules and we quantify it in terms of binding energy calculations of concerted electronic and geometrical effects. The conformations having the alkyl chain of these IL cations facing the porphyrin molecules are destabilized to the greatest extent as the chain length is enhanced along the homologous series. Structures of cation-porphyrin complexes are further analyzed by employing vibrational analysis on the active site molecules to deduce key structural features of the complexes. The reductive abilities of the metal porphyrins considered are also inferred by invoking conceptual DFT in our work through electrophilicity and Fukui reactivity indices. We uncover that the delocalized nature of the electrophilicity extends to the pyrrolic nitrogen atoms upon addition of an electron in the subsequent step to the binding. It is also demonstrated that geometrically, the core size is significantly affected upon inclusion of the cationic ligand with NiP showing the highest out of plane displacement. The motions governing the symmetry of the porphyrin macrocycle are perturbed from the native D_4h to D_2h in case of FeP, CoP, NiP and CuP exhibiting saddling and ruffling character and even B_1u and B_2u for MnP and ZnP showing doming character upon binding with [C_nmim]⁺.