Date Published: January 20, 2017
Publisher: Public Library of Science
Author(s): Charles O. Nwuche, Oguejiofo T. Ujam, Akachukwu Ibezim, Ifeoma B. Ujam, Mohammad Shahid.
The Antibiogram properties of 1-chloro-2-isocyanatoethane derivatives of thiomorpholine (CTC), piperazine (CPC) and morpholine (CMC) were evaluated by the approved agar well diffusion, the minimum inhibitory concentration (MIC) and in silico techniques. A total of fourteen microbial cultures consisting of ten bacteria and four yeast strains were used in the biological study while affinity of the compounds for DNA gyrase, a validated antibacterial drug target, was investigated by docking method. Results indicate that both thiomorpholine and piperazine had zero activity against the Gram negative organisms tested. With morpholine, similar result was obtained except that cultures of Escherichia coli (ATCC 15442) and Salmonella typhi (ATCC 6539) presented with weak sensitivity (7–8 mm) as shown by the inhibition zone diameter (IZD) measurement. The Gram positive organisms were more sensitive to morpholine than the other compounds. The highest IZD values of 15–18 mm were achieved except for Streptococcus pneumoniae (ATCC 49619) in which mobility of the compound stopped after 12 mm. S. pneumoniae was resistant to both thiomorpholine and piperazine. The yeast strains were not sensitive to any of the studied compounds investigated. The MIC tests evaluated against a reference antibiotic show that while morpholine was most active at 4 μg.ml-1 against both B. cereus ATCC (14579) and B. subtilis, the least active compound was thiomorpholine which inhibited S. aureus (ATCC 25923) at 64 μg.ml-1. The three compounds demonstrated high affinity for the target protein (DNA gyrase) ranging from -4.63 to -5.64 Kcal/mol and even showed better ligand efficiencies than three known antibiotics; chlorobiocin, ciprofloxacin and tetracycline. This study identified the studied compounds as potential antibiotic leads with acceptable physicochemical properties and gave the molecular basis for the observed interactions between the compounds and the target protein which can be harnessed in structural optimization process.
There has been increased interest in antimicrobial activity of compounds derived from multifunctional heterocyclic molecules especially those from thiomorpholine [1–3] and piperazine [4–6] and morpholine [7–9]. This is partly because they are privileged molecules for the preparation of bioactive compounds but mainly because they offer better solubility and pharmacokinetics [10–13]. Compounds of these moieties are known to be bioactive across a number of different therapeutic areas [14,8,2]. Therefore, there is a big scope in inventing novel antimicrobial agents with potential anti-pathogenic activities base on these heterocyclic compounds toward ameliorating the problem of antimicrobial drug resistance. Potentially, exploiting the synthetic scaffold offered by morpholine, thiomorpholine and piperazine through the heterocyclic amine functional group results in more functionalized derivatives that exhibit interesting anti-pathogen activities.
1-chloro-2-isocyanatoethane, thiomopholine, morpholine and piperazine were supplied by Sigma-Aldrich. Laboratory regent grade diethyl ether (EMD Chemicals) was used as the reaction solvent without further purification. ClCH2CH2NHC(O)N(CH2CH2)2S  (CTC), ClCH2CH2NHC(O)N(CH2CH2)2NC(O)NHCH2CH2Cl  (CPC) and ClCH2CH2NHC(O)N (CH2CH2)2O , (CMC) were synthesized according to literature procedures.
The test compound show in Fig 1 were synthesis by the reaction of the heterocyclic compounds thiomorpholine, piperazine, morpholine with 1-chloro-2-isocyanatoethane in diethylether.
The antimicrobial profiles of the three 1-chloro-2-isocyanatoethane derivatives CTC, CPC and CMC against selected potential pathogens are presented in this study. The results highlight the prospect of the three compounds as future drug candidates; hence the need for further research in toxicity testing and overall in vivo studies. Besides, additional insight is needed in designing effective strategies to enhance trans-membrane partitioning of antibiotics particularly through the lipopolysaccharide ‘firewall’ of the Gram negative bacterial cell walls. The ability of the three compounds to inhibit activity of bacteria DNA gyrase was investigated by means of docking simulation and their possible binding mode predicted. The theoretical free energy of binding obtained from docking calculations showed that all the compounds performed better than chlorobiocin (cocrystallized ligand) and ciprofloxacin, in terms of ligand efficiency. The described binding mode identified key residues which could be targeted in rational structure optimization process. This approach could in the long run reduce the huge sums spent yearly on drug prospecting and development while at same time empowering antibiotics to save more lives through the acquisition of broad spectrum coverage. Presently, the only solution to the menace of antibiotic resistance lies in the discovery of new and more efficient drugs and the foundation is laid in the present study.