Photoinduced Electron vs. Concerted Proton Electron Transfer Pathways in Sn^IV (l‐Tryptophanato)₂ Porphyrin Conjugates

Abstract

Aromatic amino acids such as l-tyrosine and ltryptophan are deployed in natural systems to mediate electron transfer (ET) reactions. While tyrosine oxidation is always coupled to deprotonation (proton-coupled electrontransfer, PCET), both ET-only and PCET pathways can occur in the case of the tryptophan residue. In the present work, two novel conjugates 1 and 2, based on a Sn IV tetraphenylporphyrin and Sn IV octaethylporphyrin, respectively, as the chromophore/electron acceptor and l-tryptophan as electron/proton donor, have been prepared and thoroughly characterized by a combination of different techniques including single crystal X-ray analysis. The photophysical investigation of 1 and 2 in CH 2 Cl 2 in the presence of pyrrolidine as a base shows that different quenching mechanisms are operating upon visible-light excitation of the porphyrin component, namely photoinduced electron transfer and concerted proton electron transfer (CPET), depending on the chromophore identity and spin multiplicity of the excited state. The results are compared with those previously described for metal-mediated analogues featuring Sn IV porphyrin chromophores and l-tyrosine as the redox active amino acid and well illustrate the peculiar role of ltryptophan with respect to PCET.), while the subsequent hole transfer process to a terminal tryptophan residue (W306) is accompanied by deprotonation yielding a neutral TrpN * species (PCET). ET-only processes are observed also in long-range electron transfer within protein mutants mediated by tryptophan residues. [5] PCET reactions involving redox active amino acids may occur either through stepwise ET-PT/PT-ET pathways or via a concerted mechanism (concerted proton electron transfer, CPET). [2,3,6] The latter implies the transfer of both electron and proton in a single, concerted kinetic event. CPET is usually favored on thermodynamic basis as it avoids the formation of high energy intermediates, while it may present kinetic barriers associated to the simultaneous involvement of both electron and proton motion. [2] Many approaches have been adopted to study PCET reactions and understand the related mechanistic requirements.

Releasing and Assessing the Toxicity of Polycyclic Aromatic Hydrocarbons from Biochar Loaded with Iron

“… 48 Notably, the intensity of tryptophan was significantly enhanced with the increase of Fe loading, which was also in line with previous studies. 49 Tryptophan was reported to be played an important role in the electron transfer process, 50 which could be in favor of the reaction between Fe and PAHs and decrease the PAH concentration. Besides, three DOM substances can be classified into two main groups: protein-like and soluble microbial byproduct-like substances, which have a high potential for microbial utilization and can serve as a significant source of carbon for microbial communities.…”

Sequential proton coupled electron transfer events from a tetraruthenium polyoxometalate in photochemical water oxidation

“…Proton coupled electron transfer (PCET) is a pervasive process in many biological and artificial chemical transformations. 1–4 PCET events are often associated with photoinduced charge separation, 5–10 which is a primary step in chemical processes associated with the photosynthetic conversion of small molecules, while recently, its interest has broadened to synthetic organic chemistry. 11–15…”

“…Proton coupled electron transfer (PCET) is a pervasive process in many biological and articial chemical transformations. [1][2][3][4] PCET events are oen associated with photoinduced charge separation, [5][6][7][8][9][10] which is a primary step in chemical processes associated with the photosynthetic conversion of small molecules, while recently, its interest has broadened to synthetic organic chemistry. [11][12][13][14][15] In the eld of water oxidation, investigation of PCET was pioneered by T. J. Meyer, who recognized that for the [Ru II (H 2 -O)(py)(bpy) 2 ] 2+ coordination complex (where py = pyridine; bpy = 2,2 0 -bipyridine) two stepwise oxidative PCET events occurring in a narrow potential window of 110 mV lead to the formation of a Ru IV -oxo species; 16 these ndings were pivotal in the design of the blue dimer [(bpy) 2 Ru III (H 2 O)(m-O)Ru III (H 2 O)(bpy) 2 ] 4+ as the rst molecular water oxidation catalyst.…”