## 3 Results and Discussion  
### Antibacterial Activity and QSAR Studies  
Table 1 provides the inhibitory potencies of TPZ (**1**) and a set of its 3-amino substituted derivatives (compounds **2-10**) against a panel of _E. coli_, _S. enterica_ and _S. aureus_ strains in terms of minimum inhibitory concentrations (MICs), which varied in the range of 1.1 \(\upmu\)M (0.25 \(\upmu\)g/mL)-412.6 \(\upmu\)M (128.0 \(\upmu\)g/mL). Note that the activities of the antibacterial compounds are considered significant when the MIC \(\leq\) 10 \(\upmu\)g/mL, moderate when the MIC varies in the range of 10-100 \(\upmu\)g/mL, and insignificant when the MIC \(>\) 100 \(\upmu\)g/mL [33; 34]. TPZ (**1**) showed significant activity towards Gram-negative _E. coli_ and _S. enterica_ strains (with MICs of 44.9 \(\upmu\)M (8.0 \(\upmu\)g/mL)), which was twice as high as that as defined against the Gram-positive _S. aureus_ strain (MIC = 89.8 \(\upmu\)M (16.0 \(\upmu\)g/mL)). Of the whole set of the compounds tested, TPZs (compounds **2-6**) exhibited the highest activities towards all the strains (MICs varied in the range of 1.1-32.0 \(\upmu\)M (0.25-8.0 \(\upmu\)g/mL)) (Table 1), being close to or markedly higher than those of tirapazamine (**1**) along with the reference drugs chloramphenicol (CAM) and nitrofurantoin (NFT) (MICs of 8.4-24.8 \(\upmu\)M or 2.0-8.0 \(\upmu\)g/mL) and were less potent than ciprofloxacin (Cipro) (MICs of 0.04-0.08 \(\upmu\)M or 0.02-0.06 \(\upmu\)g/mL). Amid the most potent TPZs (**2-6**), the compound bearing an acetylamino group (**2**) showed the strongest activity against _E. coli_ and _S. enterica_ strains (MICs of 1.1 \(\upmu\)M (0.25 \(\upmu\)g/mL)), and two-fold lower potency against _S. aureus_ (MIC = 2.2 \(\upmu\)M (0.5 \(\upmu\)g/mL)). The compound with a methoxycarbonylamino substituent (**4**) was the second most active agent against _E. coli_ with a MIC of 4.2 \(\upmu\)M (1.0 \(\upmu\)g/mL) and with MICs of 2.1 \(\upmu\)M (0.50 \(\upmu\)g/mL) against _S. enterica_ and _S. aureus_. The compound bearing a propionylamino group (**3**) was the third most active agent (MICs of 8.5 \(\upmu\)M (2.0 \(\upmu\)g/mL)) towards _E. coli_ and _S. enterica_ strains and a MIC of 4.3 \(\upmu\)M (1.0 \(\upmu\)g/mL) against _S. aureus_, which was followed by the ethoxycarbonylamino-TPZ (**6**), with MICs of 32.0 \(\upmu\)M (8.0 \(\upmu\)g/mL) against _E. coli_ and 16 \(\upmu\)M (4.0 \(\upmu\)g/mL) against _S. enterica_ strain, while its potency against _S. aureus_ was markedly higher (MIC of 4.0 \(\upmu\)M (1.0 \(\upmu\)g/mL)), close to that of the highly active compound **4**. Due to its electron-withdrawing characteristic and attractive stereo-electronic profile, the trifluoromethyl group (-CF\({}_{3}\)) is applied as one of the most important substituents in pharmaceutical research ([34], and refs. therein). Nonetheless, the substitution of the methyl group of the most active compound **2** with a -CF\({}_{3}\) group (compound **7**) led to markedly lower activity towards _E. coli_ and _S. enterica_ (MICs of 58.4 \(\upmu\)M (16.0 \(\upmu\)g/mL)) and _S. aureus_ (MIC of 116.7 \(\upmu\)M (32.0 \(\upmu\)g/mL)). The substitution of the -COCF\({}_{3}\) in compound **7** with -SO\({}_{2}\)CF\({}_{3}\) (compound **8**) resulted in a further decline in activity against _E. coli_ and _S. enterica_ strains (MICs of 412.6 \(\upmu\)M (128.0 \(\upmu\)g/mL)), while the activity of this compound towards _S. aureus_ was four-fold higher, close to that of compounds **9** and **10**. The negative contribution of the  
-CF\({}_{3}\) group is clearly exemplified by comparing the activity of compound **8** with that of compound **9** against Gram-negative bacterial strains. Overall, as shown in Table 1, di-\(N\)-oxides showed virtually the same inhibitory potencies with respect to both of the Gram-negative strains, with an exception of compounds **4-6**, which showed two-fold higher activities against _S. enterica_. In addition, the activities of the whole set of TPZs towards the Gram-positive _S.