Photodissociation of tert-butyl hypochlorite and decomposition of the tert-butoxy radical fragment

Thelen, Marc; Felder, P.; Frey, Jeremy G.; Huber, J. Robert

doi:10.1021/j100125a022

Cited by 17 publications

(25 citation statements)

References 6 publications

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“…An FAD-O-Cl intermediate could react via a one-electron mechanism to form • Cl as the chlorinating species. Photolysis of ROCl to • Cl and • RO has been observed (47,48). However, in vanadium-dependent haloperoxides, a radical-generating reaction has been ruled out in favor of a mechanism generating cationic bromine (30).…”

Section: Discussionmentioning

confidence: 99%

Robust in vitro activity of RebF and RebH, a two-component reductase/halogenase, generating 7-chlorotryptophan during rebeccamycin biosynthesis

Yeh

Garneau²,

Walsh³

2005

Proc. Natl. Acad. Sci. U.S.A.

256

278

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The indolocarbazole antitumor agent rebeccamycin is modified by chlorine atoms on each of two indole moieties of the aglycone scaffold. These halogens are incorporated during the initial step of its biosynthesis from conversion of L-Trp to 7-chlorotryptophan. Two genes in the biosynthetic cluster, rebF and rebH, are predicted to encode the flavin reductase and halogenase components of an FADH 2-dependent halogenase, a class of enzymes involved in the biosynthesis of numerous halogenated natural products. Here, we report that, in the presence of O 2, chloride ion, and L-Trp as cosubstrates, purified RebH displays robust regiospecific halogenating activity to generate 7-chlorotryptophan over at least 50 catalytic cycles. Halogenation by RebH required the addition of RebF, which catalyzes the NADH-dependent reduction of FAD to provide FADH 2 for the halogenase. Maximal rates were achieved at a RebF͞RebH ratio of 3:1. In air-saturated solutions, a k cat of 1.4 min ؊1 was observed for the RebF͞RebH system but increased at least 10-fold in low-pO 2 conditions. RebH was also able to use bromide ions to generate monobrominated Trp. The demonstration of robust chlorinating activity by RebF͞RebH sets up this system for the probing of mechanistic questions regarding this intriguing class of enzymes.enzymology ͉ natural product biosynthesis ͉ flavoenzyme

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Section: Discussionmentioning

confidence: 99%

Robust in vitro activity of RebF and RebH, a two-component reductase/halogenase, generating 7-chlorotryptophan during rebeccamycin biosynthesis

Yeh

Garneau²,

Walsh³

2005

Proc. Natl. Acad. Sci. U.S.A.

256

278

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“…The tert-butyl hypochlorite recoil kinetic energy distribution is about a factor of 2 broader than that measured here for CH 3 OCl (with a fwhm of 8 kcal/ mol). 20 The Thelen et al study of tert-butyl hypochlorite also observed decomposition of alkoxy radicals, which occurs with a low energy barrier of ∼20 ( 3 kcal/mol, indicating that a significant amount of the internal energy must have been partitioned to vibration. This is not a surprising result because in the impulsive dissociation the larger tert-butoxy radical, with more low-frequency vibrational modes than the CH 3 O, is likely to correspond more closely to the soft radical limit, whereas the CH 3 OCl photodissociation is closer to the hard radical limit.…”

Section: Discussionmentioning

confidence: 97%

“…Support for this conjecture is given by two experimental studies, one that examined UV photolysis of CH 3 -OCl in a time-of-flight mass spectrometer 19 and the other that performed photofragment translational spectroscopy on an analogous compound, tert-butyl hypochlorite. 20 Both studies observed only direct O-Cl bond fission. It is also worth noting that the qualitative shape of the UV absorption spectrum is quite similar for several alkyl hypochlorites studied, 13,20 which suggests that they access analogous repulsive excited electronic states and will exhibit similar behavior upon photolysis.…”

Section: Introductionmentioning

confidence: 99%

Photodissociation of CH₃OCl to CH₃O + Cl at 248 nm

et al. 2004

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This study examines the 248 nm photodissociation of methyl hypochlorite (CH 3 OCl), a molecule that serves as an atmospheric chlorine reservoir. The data show that the primary photodissociation channel is cleavage of the O-Cl bond to produce Cl atoms and CH 3 O radicals, a result consistent with the direct dissociation mechanism found by other computational and experimental studies of alkyl hypohalites. Photofragment translational spectroscopy with a crossed laser-molecular beam apparatus, coupled with tunable VUV photoionization detection, identified the momentum-matched products at m/e ) 35 (Cl + from Cl atoms) and m/e ) 29 (the CHO + daughter ion of CH 3 O). Products were formed with a narrow range of recoil kinetic energies, peaking at 48 kcal/mol in the center-of-mass reference frame, with a full-width half-maximum of 4 ( 2 kcal/mol. This kinetic energy distribution shows that the CH 3 O products are formed with a very narrow range of internal energies, and a simple model shows that to conserve angular momentum this internal energy, near 20 kcal/mol, is partitioned primarily to rotational energy. Thus CH 3 OCl could serve as a photolytic precursor of CH 3 O radicals with high and well-defined rotational and translational energies.

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“…These data are discussed in more detail in the subsequent sections. A full analysis of the photolysis of t-butyl hyopchlorite is in preparation [8].…”

Section: Transformationmentioning

confidence: 99%

“…The strictly monotonically decreasing constraint is Cl > C2 > C3 > "'" > CM (7) or, for monotonically non-decreasing p(E), r ~ C2 ~ C3 ~ "'" CM" (8) These constraints can be implemented when using the NAG routing E04UPF by the linear constraints on the c 1…”

Section: Monotonic Behaviourmentioning

confidence: 99%

Analysis of time-of-flight data constraints and error bars

Frey

1993

Molecular Physics

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Photodissociation of tert-butyl hypochlorite and decomposition of the tert-butoxy radical fragment

Cited by 17 publications

References 6 publications

Robust in vitro activity of RebF and RebH, a two-component reductase/halogenase, generating 7-chlorotryptophan during rebeccamycin biosynthesis

Robust in vitro activity of RebF and RebH, a two-component reductase/halogenase, generating 7-chlorotryptophan during rebeccamycin biosynthesis

Photodissociation of CH₃OCl to CH₃O + Cl at 248 nm

Analysis of time-of-flight data constraints and error bars

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Photodissociation of tert-butyl hypochlorite and decomposition of the tert-butoxy radical fragment

Cited by 17 publications

References 6 publications

Robust in vitro activity of RebF and RebH, a two-component reductase/halogenase, generating 7-chlorotryptophan during rebeccamycin biosynthesis

Robust in vitro activity of RebF and RebH, a two-component reductase/halogenase, generating 7-chlorotryptophan during rebeccamycin biosynthesis

Photodissociation of CH3OCl to CH3O + Cl at 248 nm

Analysis of time-of-flight data constraints and error bars

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Photodissociation of CH₃OCl to CH₃O + Cl at 248 nm