Mallotumides A-C: Potent Cytotoxic Cycloheptapeptides from the Roots of Mallotus spodocarpus.

The structures of potent cytotoxic cycloheptapeptides, mallotumides A-C (1-3, respectively) isolated from the roots of Mallotus spodocarpus Airy Shaw, were elucidated by extensive spectroscopic analysis. The absolute configuration of 1 was determined by single-crystal X-ray crystallographic data. All three cycloheptapeptides exhibited potent cytotoxicity against various cancer cell lines with IC50 values ranging from 0.60 to 4.02 nM.

Electrophilic difluoro(phenylthio)methylation: generation, stability, and reactivity of α-fluorocarbocations.

Electrophilic difluoro(phenylthio)methylation of allylsilanes has been achieved using bromodifluoro(phenylthio)methane (PhSCF2Br) and silver hexafluoroantimonate (AgSbF6). The structural assignment and observation of α-fluorocarbocation were substantiated by NMR and theoretical calculations. Detailed mechanistic and electronic studies have provided a fundamental understanding of the reactivity and stability of the difluoro(phenylthio)methylium cation (PhSCF2(+)).

Binding of nitric oxide to a synthetic model of iron-containing nitrile hydratase (Fe-NHase) and its photorelease: relevance to photoregulation of Fe-NHase by NO.

The activity of the non-heme iron enzyme nitrile hydratase (Fe-NHase) is modulated by nitric oxide (NO). The inactive (dark form) NO-bound enzyme is activated when exposed to light via the release of NO from the iron center. In order to determine whether oxygenation of active site Fe-bound Cys-S centers are involved in this process of NO regulation, a model complex (Et(4)N)[(Cl(2)PhPepS)Fe(NO)(DMAP)] (8) has been synthesized and structurally characterized. Complex 8 does not exhibit any NO photolability. However, following oxygenation of the Fe-bound thiolato-S centers to sulfinates (with the aid of oxaziridine), the resulting complex (Et(4)N)[(Cl(2)PhPep{SO(2)}(2))Fe(NO)(DMAP)] (9) releases NO readily upon illumination with visible light. Spectroscopic properties of 8 and 9 confirm that these species do mimic the active site of Fe-NHase closely, and the results indicate that NO photolability is related to S-oxygenation. Results of density functional theory and time-dependent DFT studies on both 8 and 9 indicate that S-oxygenation weakens Fe-S bonding and that strong transitions near 470 nm transfer an electron from a carboxamido-N/sulfinato-SO(2) MO to a dpi(Fe)-pi*(NO)/d(z)2(Fe)-sigma*(NO) antibonding orbital in 9. In case of 8, strong S-Fe-NO bonding interactions prevent the release of NO upon illumination. Together, the results of this work strongly suggest that oxygenated Cys-S centers play an important role in the process of NO regulation of Fe-NHases.

Thiolate S-oxygenation controls nitric oxide (NO) photolability of a synthetic iron nitrile hydratase (Fe-NHase) model derived from mixed carboxamide/thiolate ligand.

In order to determine the origin of the NO photolability of the active site of Fe-containing nitrile hydratase (Fe-NHase), a model complex of the NO-bound active site (dark form) has been isolated and structurally characterized. The model, NEt(4)[(Cl(2)PhPepS)Fe(NO)(DMAP)] (2), is derived from a tetradentate ligand comprising carboxamido N and thiolato S donor centers much like the donors present in the active site of Fe-NHase. This {Fe-NO}(6) nitrosyl effectively mimics the NO-bound active site in terms of structural and spectroscopic parameters. However, this model lacks the key property of NO photolability. Interestingly, S-oxygenation of the model complex results in formation of Na[(Cl(2)PhPep{SO(2)}(2))Fe(NO)(DMAP)] (3), in which the -S donors are oxygenated to -SO(2) moieties, and this species exhibits NO photolability. These results indicate that S-oxygenation could be the key reason for the observed NO photolability of the active site of the dark form of Fe-NHase.