Abstract
Enzyme applications in industry is often limited due to their high cost, restricted availability, instability, and low return on investment in separating these soluble biocatalysts from a reaction mixture. Inorganic complexes can be manipulated to operate as synthetic/artificial enzymes, mimicking the activity of an enzyme's function (such as peptidase) and thereby participating in comparable applications exclusive to that particular enzyme. Immobilising these inorganic complexes to solid supports bridges the gap to a new era of catalysts that combine all the beneficial aspects of recyclability, activity, and selectivity. Furthermore, compounds with the ability of amide/peptide bond cleavage in peptides and proteins have become ever more important, in numerous applications in the field of chemistry, biotechnology, proteomics and drug discovery.1–5
This study investigates the efficacy, as proof of principle, of polymer-supported Cobalt(III) amine complex and Schiff base transition metal complexes to catalyse the cleavage of small peptides, such as L-carnosine, under physiological conditions.
Cobalt(III) amine complexes were synthesised by complexation of tris(2-aminoethyl)amine (tren) with a suitable Cobalt precursor. The monobasic tridentate Schiff base (SB) ligands were derived by condensation of thiosemicarbazide/semicarbazide with substituted salicylaldehyde. Complexation of these SB ligands to transition metals, such as V(V), Co(III), Ni(II) and Pd(II), yields the corresponding SB metal complexes. The complexes were then heterogenised by covalently immobilisation to Merrifield resin by anchoring through the amine nitrogen of the thio-semi/semicarbazide moiety. Several analytical, spectroscopic, and thermal techniques characterised the un-supported and polymer-supported catalysts. Techniques such as SCXRD, CHNS, FTIR, UV–Vis, NMR, SEM, EDS and TGA (where applicable) analysis were used to establish the molecular structure of the ligands, complexes and supported complexes.
Where applicable, a crystallographic analysis was completed on the successfully crystallized materials (AB1_Imid, SB2, SB3, SB4, SB2_Ni, SB1_Co, SB2_Co, SB6_Co and SB3_Pd), where interesting structural properties and inter-molecular interactions responsible for the packing arrangements were observed.
Theoretical DFT calculations have been carried out on the cobalt(III) amine complexes, SB ligands and their corresponding SB metal complexes to study feasibility of this study's proposed structures and provide theoretical insights and aspects in possible chemical stability and reactivity.
The catalytic efficiency was monitored by periodic UV‒Vis (ninhydrin study) and LC‒MS measurements of the reaction mixture. Optimised conditions were accomplished by modifying various reaction parameters (catalyst amount, time, base addition, and temperature). The catalysts were recovered easily by filtration from the reaction mixture, washed well and dried to be used again for the next cycle.