Structure and conformation of the nitroxyl spin-label ethyl 3-(2,2,5,5-tetramethylpyrrolinyl-1-oxyl)-propen-2-oate determined by electron nuclear double resonance: comparison with the structure of a spin-label substrate of carboxypeptidase A.

The conformation of the nitroxyl spin-label ethyl 3-(2,2,5,5-tetramethylpyrrolinyl-1-oxyl)-propen-2-oate has been determined by electron nuclear double resonance (ENDOR) spectroscopy and computer-based molecular modeling. From ENDOR spectra of the compound in frozen solution, we have assigned resonance absorption features for each class of protons, and we have identified their principal hyperfine coupling (hfc) components from analysis of the dependence of ENDOR spectra on the static laboratory magnetic field. The dipolar hfc components yielded estimates of the electron-proton separations for each class of protons of the ethyl propenoyl moiety. Torsion angle search calculations were carried out to determine the conformational space compatible with hard-sphere nonbonded constraints and with the ENDOR-determined distance constraints. Molecular graphics analysis revealed that the propenoyl side chain of the spin-label exhibits an extended trans conformation and that the ethyl moiety of the ester group deviates significantly from coplanarity with the carboxylate--COO--atoms. The conformation of this molecule is compared with that of an analogous compound O-[3-(2,2,5,5-tetramethylpyrrolinyl-1-oxyl)-propen-2-oyl]-L- beta- phenyllactate, which has been employed as a spectroscopic substrate probe of carboxypeptidase A (L. C. Kuo, J. M. Fukuyama, and M. W. Makinen (1983) Journal of Molecular Biology 163, 63-105). The rotamer conformation of the free spin-label ester in solution, as determined in this study, and that of the enzyme-bound spin-labeled phenyllactate are compared. Differences in rotamer structure are discussed in terms of stereoelectronic principles that govern the pathway of substrate hydrolysis catalyzed by carboxypeptidase A.

Effect of highly potent antipseudomonal beta-lactam agents alone and in combination with aminoglycosides against Pseudomonas aeruginosa.

The activities of ticarcillin, cefsulodin, ceftazidime, aztreonam, and imipenem, formerly known as N-formimidoyl thienamycin, were evaluated alone and in combination with aminoglycosides against 56 clinical isolates of Pseudomonas aeruginosa, which were characterized by aminoglycoside susceptibility and content of aminoglycoside-modifying enzymatic activities. All beta-lactam agents were highly active against aminoglycoside-susceptible isolates, and with few exceptions the aminoglycoside-resistant isolates were susceptible to all of the beta-lactam agents except ticarcillin. Combinations of the beta-lactam agents with aminoglycosides frequently acted synergistically, but the effect of different beta-lactam agents in combination with an aminoglycoside against individual strains was unpredictable. The presence or absence of an aminoglycoside-modifying enzymatic activity had no observed effect on synergism with tobramycin. Killing-curve experiments with strains in the presence of concentrations of a beta-lactam and an aminoglycoside that were not bactericidal alone (one-fourth the minimal bactericidal concentration) showed synergistic bactericidal action against four strains that were tested. The results demonstrate the great activity of these newer antipseudomonal beta-lactam agents and their potential for synergism with aminoglycosides.

Antimicrobial Properties of α-Dicarbonyl and Related Compounds.

By surface plating on plate count agar, 0.005 M (430 ppm) of the α-dicarbonyl compound, diacetyl, inhibited 28 of 40 organisms with the medium at pH 6 but only 11 at pH 8. Diacetyl was more effective against gram-negative bacteria and yeasts than non-lactic gram-positive bacteria and least effective against lactic acid bacteria. Acetoin, butanedioldiacetate and five butanediol isomers were considerably less effective than diacetyl, although all were more effective at pH 6 than 8. Diacetylmonoxime and diacetyldioxime were more effective than the diols and were less affected by pH of medium than diacetyl. Phenylglyoxal (PG) and 1,2-cyclohexanedione (CHD) ranked closest to diacetyl but, unlike the latter, they were more effective against gram-positive bacteria and less so against fungi and gram-negative bacteria. All 12 compounds were more effective against selected organisms by pour plating and in broth culture than by surface plating. Like diacetyl, CHD and PG are α, α-dicarbonyls and are widely used as arginine reactive agents resulting in the blockage of enzyme-substrate reactions. Although their antimicrobial activity may be due to this property, they apparently affect different enzymes than diacetyl because their antimicrobial spectra were different. The possibility of a class of food-use antimicrobial agents bearing α-dicarbonyl groups is suggested by the findings of this study.