- A joint structural, kinetic, and thermodynamic investigation of substituent effects on host-guest complexation of bicyclic azoalkanes by β-cyclodextrin
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Derivatives of the azoalkane 2,3-diazabicyclo[2,2,2]oct-2-ene (1a) with bridgehead 1,4-dialkyl (1b), 1,4-dichloro (1c), 1-hydroxymethyl (1d), 1-aminomethyl (1e), and 1-ammoniummethyl (1f) substituents form host-guest inclusion complexes with β-cyclodextrin. They were employed as probes to assess substituent effects on the kinetics and thermodynamics of this complexation by using time-resolved and steady-state fluorimetry, UV spectrophotometry, induced circular dichroism (ICD) measurements, and 1H NMR spectroscopy. The kinetic analysis based on quenching of the long-lived fluorescence of the azoalkanes by addition of host provided excited-state association rate constants between 2.6 × 108 and 7.0 × 108 M-1 s-1. The binding constants for 1a (1100 M-1), 1b (900 M-1), 1c (1900 M-1), 1d (180 M-1), 1e (250 M-1), and 1f (ca. 20 M-1) were obtained by UV, NMR, and ICD titrations. A positive ICD signal of the azo absorption around 370 nm was observed for the β-cyclodextrin complexes of 1a, 1d, and 1f with the intensity order 1a ? 1d ≈ 1f, and a negative signal was measured for those of 1b, 1c, and 1e with the intensity order 1c 1b ≈ 1e. The ICD was employed for the assignment of the solution structures of the complexes, in particular the relative orientation of the guest in the host (co-conformation).
- Zhang, Xiangyang,Gramlich, Gabriela,Wang, Xiaojuan,Nau, Werner M.
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- Exploiting long-lived molecular fluorescence
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Fluorophores based on the azo chromophore 2,3-diazabicyclo[2.2.2]oct-2-ene, referred to as fluorazophores, display an exceedingly long fluorescence lifetime. Besides the use in time-resolved screening assays, where the long-lived fluorescence can be time-gated, thereby reproving the signal to background ratio, a distinct application of fluorazophores lies in the area of biopolymer dynamics. For this purpose, one chain end is labeled with a fluorazophore and the other one with an efficient fluorescence quencher. The fluorescence lifetime of the probe/quencher-labeled peptide then reflects the kinetics of intramolecular end-to-end collision. Applications to polypeptides are described and control experiments which establish the nature of the quenching mechanism as a diffusive process requiring intimate probe/quencher contact are described.
- Nau, Werner M.,Huang, Fang,Wang, Xiaojuan,Bakirci, Huseyin,Gramlich, Gabriela,Marquez, Cesar
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p. 161 - 167
(2007/10/03)
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