Contents lists available at ScienceDirect Bioorganic & Medicinal Chemistry Letters journal homepage: www.elsevier.com/locate/bmcl Caspase-3 mediated programmed cell death by a gold-stabilised peptide carbene Astha Gupta a,1 , Shivansh Nigam b,1 , Ilesha Avasthi c , Bikramjit Sharma c,2 , Bushra Ateeq b , Sandeep Verma c, ⁎ a Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur, India b Department of Biological Sciences and Bioengineering, Indian Institute of Technology Kanpur, Kanpur, India c Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur, India ARTICLEINFO Keywords: Thiazolylalanine Peptide Gold carbene Caspase-3 Programmed cell death ABSTRACT The synthesis of novel N-heterocyclic carbene complexes derived from a tripeptide ligand (L), containing non- natural amino acid, thiazolylalanine is described here. The peptide ligand was reacted with suitable precursors to generate gold and mercury carbene complexes. The plausible structures of both complexes were predicted by spectroscopic data and DFT calculations. The binding energy data was also analyzed to predict their stability. The gold carbene complex (1A), showed activity against MCF7 breast cancer cell line due to mitochondrial triggered caspase-3 mediated programmed cell death. Its internalization inside cells could be observed due to autofluorescence. This study affords a methodology for successful generation of peptide carbene complexes for their therapeutic potential. The landmark discovery of the stable carbene by Arduengo 1 opened multiple avenues of research in this field. Since then, gold carbene complexes have garnered a lot of attention for their promising and versatile applications as metallodrugs targeting cancer, malaria, bac- terial infections etc. 2–5 Auranofin, a gold based metallodrug that was initially used for rheumatoid arthritis is also being repurposed as an anti-tumor drug and undergoing clinical trials. 6 Gold complexes ex- isting in both + 1 2,3 and+3 3,7 oxidation states have been examined. The mechanism of action of gold complexes discussed so far includes inhibition of thioredoxin reductase, 8 PARP enzyme, 9,10 enhanced ex- pression of tumor suppressor p53 and p21 proteins 11 and interaction with telomeric G-quadruplex protein. 12 They offer various avenues of modification in their structure to generate a large pool of candidates for evaluation of their anti-tumor potential. The nature and structure of the coordinating ligand influences the mechanism and extent of activity. Thiazolyl heterocyclic ring occurs in biological systems as an im- portant cofactor, thiamine, that plays a crucial role in decarboxylation of α-ketoacids. 13,14 It invokes the ability of thiazolium ring for umpo- lung, to form thiazolin-2-ylidene as the active species that catalyzes the biochemical reaction mediated by transketolase enzyme in carbohy- drate metabolism. This active form is also known as Breslow inter- mediate to honor this path breaking discovery. 15,16 It is an important scaffold in various medicinal molecules e.g. bacitracin, penicillin and several other drugs. 17 In a recent report, thiazolyl free carbenes were used as catalysts for the N-formylation and N-methylation of amines in high yields and at ambient temperature. 18 Incorporation of peptides into N-heterocyclic carbene complexes makes them suitable for biomedical applications, by improving their biocompatibility and biodegradability. 19 Most of the previous reports discuss use of amino acids or oligopeptides for derivatization of carbene precursor, imidazole moiety. Anti-tumor activities of iridium, gold and platinum complexes of histidine and imidazole based ligands have been described. 20,21 Nolte et al. have described synthesis and demonstrated anti- cancer activity of organometallic complexes of Ru 2+ , Rh 2+ and Au + with dipeptides as ligands. 22,23 Previously, we have developed carbenoid species on a methox- yadenine rare imino-tautomer platform, which showed remarkable toxicity against A549 and MCF7 cell lines by targeting tubulin poly- merization. 24 Based on these observations, we decided to explore metal carbene complex with Phe-Phe dipeptide. Thus, we designed a tripep- tide containing a non-proteinogenic amino acid, thiazolylalanine (Z), at the N-terminus of Phe-Phe dipeptide to enable formation of metal- carbene complex on the thiazolium ring and to further facilitate π- stacking interactions in Phe-Phe dipeptide, to study morphological https://doi.org/10.1016/j.bmcl.2019.126672 Received 27 June 2019; Received in revised form 1 September 2019; Accepted 4 September 2019 ⁎ Corresponding author at: Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, Uttar Pradesh, India. E-mail address: sverma@iitk.ac.in (S. Verma). 1 These authors contributed equally to this work. 2 Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, Germany Bioorganic & Medicinal Chemistry Letters 29 (2019) 126672 Available online 12 September 2019 0960-894X/ © 2019 Published by Elsevier Ltd. T