Insight into structural aspects and study of reaction kinetics of model [oxo(salen)iron(IV)] complexes with dipeptides Periyakaruppan Karuppasamy a,⇑ , Dharmaraj Thiruppathi a , Jeyaraj Vijaya Sundar b , Muniyandi Ganesan a , Thangamuthu Rajendran c , Sher Singh Meena e , Seenivasan Rajagopal d,⇑ , Veluchamy Kamaraj Sivasubramanian a,f,⇑ , Varatharaj Rajapandian g,⇑ a Post Graduate and Research Department of Chemistry, Vivekananda College, Tiruvedakam West, Madurai 625 234, Tamil Nadu, India b CSIR-Central Leather Research Institute, Adyar, Chennai 600 020, Tamil Nadu, India c Department of Chemistry, PSNA College of Engineering & Technology, Dindigul 624 622, Tamil Nadu, India d School of Chemistry, Madurai Kamaraj University, Madurai 625 021, Tamil Nadu, India e Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India f Department of Chemistry, K.L.N College of Information Technology, Pottapalayam, Sivagangai 630 611, Tamil Nadu, India g Department of Chemistry, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641 020, Tamil Nadu, India article info Article history: Received 30 August 2020 Accepted 17 November 2020 Available online 24 December 2020 Keywords: Biomimitic oxidation Oxo(salen)iron(IV)-radical cation Mössbauer study Cyclic voltammetric technique Methionyl di-peptides DFT abstract Iron(III)-salen complexes (salen = N,N’-bis(salicylidene)ethylenediaminato) react with H 2 O 2 in aqueous CH 3 CN medium to form short lived intermediate [oxo(salen)iron(IV)] radical cation as an active oxidant. The present study devotes to establish the nature and reactivity of this active oxidant by spectral (UV–vis and Mössbauer), electrochemical (Cyclic voltammetric) and density functional theory (DFT) method. Mössbauer spectral study gives a clue to the formation of [oxo(salen)iron(IV)] radical cation with doublet (S = 2) spin multiplicity as an intermediate. Geometry optimization of oxo-iron species using DFT method, Mulliken charges and spin densities uphold that the doublet (S = 2) state is more stable and less in energy than the quartet (S = 4) and sextet (S = 6) spin states. The calculated average Fe@O bond dis- tance for all the [oxo(salen)iron(IV)] complexes is 1.63 Å which is in good agreement with X-ray crystal- lography value of 1.65 Å. The CV study shows a clear oxidation wave at 1.18 (±0.02) V with increase in the oxidation peak current for Fe III /Fe IV redox couple. Kinetics of oxygenation of four dipeptides (eg. Met-Gly, Met-Ala, Met-Leu, Met-Ser) with [oxo(salen)iron(IV)] radical cation into the corresponding sulfone follow Michaelis-Menten kinetics. The reaction rate is sensitive to the nature of the substituent present in the phenolic part of salen ligand and side chain present in the peptides. To account for spectral, kinetic and DFT results a suitable mechanism has been proposed. Ó 2020 Elsevier Ltd. All rights reserved. 1. Introduction Iron complexes carrying heme and nonheme ligands are poten- tial oxidants in many biotransformations [1–10]. High valent oxo- iron species are implicated as the active oxidant in a series of bio- logical oxygenation processes including CAH activation, dealkyla- tion, epoxidation, hydroxylation and sulfoxidation reactions [11– 15]. Cytochromes P450 are significant superfamily of heme-con- taining enzymes which form the high valent oxoiron(IV)-porphyrin radical cation presumed as Compound I in the presence of oxygen source, which effects oxygenation reactions. 2 Likewise in nonheme iron enzymes catalysis these high valent oxoiron(IV) complexes are cited as reactive intermediates [16]. In the above reaction, the oxoiron(IV) intermediates usually possess a ground state as a https://doi.org/10.1016/j.poly.2020.114952 0277-5387/Ó 2020 Elsevier Ltd. All rights reserved. Abbreviations: Salen, bis(salicylidene)ethylenediamine; ROS, reactive oxygen species; ESI-MS, electronspray ionization mass spectrometry; GCMS, gas chro- matography with mass spectrometry; HEPES, 4-(2-hydroxyethyl)-1-piperazi- neethanesulfonic; Met-Gly, Methionine glycine dipeptide; Met-Ala, Methionine alanine dipeptide; Met-Leu, Methionine leucine dipeptide; Met-Ser, Methionine serine dipeptide; DFT, density functional theory; TBAP, tetrabutylammoniumper- chlorate; PBS, phosphate buffer solution. ⇑ Corresponding authors at: Post Graduate and Research Department of Chem- istry, Vivekananda College, Tiruvedakam West, Madurai 625 234, Tamil Nadu, India (V.K. Sivasubramanian). E-mail addresses: periyakaruppankaruppasamy@gmail.com (P. Karuppasamy), rajagopalseenivasan@yahoo.com (S. Rajagopal), sivavk1957@gmail.com (V.K. Siva- subramanian), vrajapandian@hotmail.com (V. Rajapandian). Polyhedron 196 (2021) 114952 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly