Sigma- and Pi-Electron Delocalization: Focus on Substituent Effects

The increasing prevalence of antibiotic-resistant pathogens reinforces the need for structures of antibiotic-ribosome complexes that are accurate enough to enable the rational design of novel ribosome-targeting therapeutics. Structures of many antibiotics in complex with both archaeal and eubacterial ribosomes have been determined, yet discrepancies between several of these models have raised the question of whether these differences arise from species-specific variations or from experimental problems. Our structure of chloramphenicol in complex with the 70S ribosome from Thermus thermophilus suggests a model for chloramphenicol bound to the large subunit of the bacterial ribosome that is radically different from the prevailing model. Further, our structures of the macrolide antibiotics erythromycin and azithromycin in complex with a bacterial ribosome are indistinguishable from those determined of complexes with the 50S subunit of Haloarcula marismortui, but differ significantly from the models that have been published for 50S subunit complexes of the eubacterium Deinococcus radiodurans. Our structure of the antibiotic telithromycin bound to the T. thermophilus ribosome reveals a lactone ring with a conformation similar to that observed in the H. marismortui and D. radiodurans complexes. However, the alkyl-aryl moiety is oriented differently in all three organisms, and the contacts observed with the T. thermophilus ribosome are consistent with biochemical studies performed on the Escherichia coli ribosome. Thus, our results support a mode of macrolide binding that is largely conserved across species, suggesting that the quality and interpretation of electron density, rather than species specificity, may be responsible for many of the discrepancies between the models.

https://www.pnas.org/content/pnas/107/40/17158.full.pdf

Revisiting the structures of several antibiotics bound to the bacterial ribosome.

Cyclopropenylium cations, cyclopropenones, and heteroanalogues-recent advances.

The exothermic nonradiative relaxation processes from photoexcited nitrobenzene (NB) are studied by the picosecond time-resolved transient grating method. The decay rate constants and energy of the excited NB are determined. The lifetime of the lowest excited singlet state is found to be very short ( e10 ps) and a surprisingly short lifetime of the lowest excited triplet state (480 ps) is detected. From quantitative measurements of the thermal energies released from the excited states of NB, the lowest excited triplet state is determined to be located at (22-27) 10 3 cm -1 . The triplet formation is very efficient and its quantum yield is found to be grater than 0.80. From the temperature and solvent dependence of the triplet lifetime, it is suggested that motions of the nitro group may cause the surprisingly rapid relaxation from the lowest excited triplet state to the ground state. The photophysical properties and excited states of monosubstituted nitrobenzenes are also discussed.

Nonradiative Relaxation Processes and Electronically Excited States of Nitrobenzene Studied by Picosecond Time-Resolved Transient Grating Method

Optimized geometries and energies in the ground (S0) and excited states of nitrobenzene were calculated using the CAS-SCF method. The optimized geometries in the S1, S2, T1, T2, and T3 states are very different from that in the S0 state. Most significantly, the nitro group is largely bent out of the phenyl plane in the excited states. The potential curves along the nitro rotation coordinate around the C−N bond and the out-of-plane bending mode of the nitro group in the S0, S1, T1, and T2 states were calculated in relation to the excited states dynamics. It is found that the potential curves along the bending mode in the excited states are very flat compared with that in the S0 state. The mechanisms for the fast relaxation from the S1 and T1 states are discussed based on the ab initio results.

Geometries and Energies of Nitrobenzene Studied by CAS-SCF Calculations

The crystal and molecular structure of nitrobenzene was determined at 103 K by X-ray diffraction, yieldingR=0.034 and a highly precise geometry of the molecule (esds of bond lengths ≤0.001 A and bond angles ≤0.1°). The bond angles in the ring agree very well with additive scheme based on angular substituent parameters. X-X electron density maps support the view that the C-N bond does not exhibitπ-character, and, hence, the nitro group interacts with the ring mostly by inductive effects.

Low temperature crystal and molecular structure of nitrobenzene

Small Rings, 74[1]. - Hexa-tert-butyl-3,3′-bicyclopropenyl



Title compound 3 can be prepared by reductive coupling of tri-tert-butylcyclopropenylium tetrafluoroborate (4a) with lithium. Upon treatment with AgBF43 does not undergo valence isomerization to the Dewar form of hexa-tert-butylbenzene (2) but is reoxidized under backformation of substrate 4a. The photochemical behavior of 6 is also unusual: Irradiation at room temperature results in gas evolution. However, instead of 7, the s-diketone 5 is isolated. Monoketone 7 is only formed if 6 is irradiated in an organic matrix at -196°C.

Michael Nussbaumer

Papers

Low temperature crystal and molecular structure of nitrobenzene

Kleine Ringe, 74. Hexa‐tert‐butyl‐3,3′‐bicyclopropenyl