Helium (HeI) and (HeII) photoelectron spectra of alloxazines and isoalloxazines

Eweg, J.K.; Müller, Felix; Dam, Henk H.; Terpstra, A.; Oskam, A.

doi:10.1021/ja00521a010

Cited by 38 publications

(25 citation statements)

References 10 publications

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“…The substantial difference in character between the isoalloxazine S1 and S2 electronically excited states [28] is probably the reason why the dichroism of the So+S2 transition is not fully compensated in the complex of FMN with the apoprotein. In previous work [28], we showed this transition to contain both nn* and nn* character and it was found that rotation of methyl substituents considerably affects a-n separation in orbitals localized on the isoalloxazine carbonyl groups [27]. It is well known that carbonyl nn* transitions have large intrinsic magnetic transition moments [41], hence for the isoalloxazine So+S2 we encounter the situation of a relatively small p (m* component more or less sign-forbidden [28]) and a relatively large m. This situation is particularly sensitive to external perturbation, which may explain the difference in circular dichroic absorption observed for the two long-wavelength bands in the native enzyme (Fig.…”

Section: Resultsmentioning

confidence: 83%

“…Electron paramagnetic resonance spectra were measured on a Bruker 200-tt ESR spectrometer equipped with a liquid helium cooling system. CNDO/S-CI calculations on isoalloxazine-phenol complexes were performed using the parameters and isoalloxazine molecular geometry from previous work [27,28]. The phenol molecular structure was taken from the literature [32,33].…”

Section: Methodsmentioning

confidence: 99%

“…From detailed comparison of this calculational method with a large variety of experimental data, we already had a good impression of its reliability [27,28]. Considering alternatives for the charge-transfer explanation of old yellow enzyme's spectral behaviour [3], the idea of tautomerization arose, i.e.…”

Section: Theoretical Datamentioning

confidence: 99%

“…Additionally, complexes of 'ideal' isoalloxazine [27,28] and the phenolate anion in stacked conformations with parallel molecular planes (cf. Materials and Methods) were calculated.…”

Section: Theoretical Datamentioning

confidence: 99%

“…The notation Ipq/ is an abbreviation for the normalized singly excited singlet configuration 2-' (Ipql-Ipql) in which an electron is promoted from orbital p to orbital q. The corrections according to Bigelow [27,28] were not applied since we are dealing with anionic closed shell systems.…”

Section: Theoretical Datamentioning

confidence: 99%

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On the Enigma of Old Yellow Enzyme's Spectral Properties

Eweg

Müller

Berkel

1982

European Journal of Biochemistry

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Old yellow enzyme (NADPH oxidoreductase) in the free and complexed state was thoroughly investigated by the following techniques : absorption, circular dichroism, fluorescence/phosphorescence and electron paramagnetic resonance spectroscopy and fluorescence/phosphorescence decay measurements, applied over a wide range of temperature (7 -293 K). The data obtained were interpreted by comparison with results from similar measurements on free FMN, existing spectral data on isoalloxazine model systems and theoretical data.The results clearly demonstrate the inadequacy of a simple phenolate-FMN donor-acceptor charge-transfer complex to explain the phenomena occurring upon the addition of phenols to old yellow enzyme. Instead it was found that the phenolate anion interferes strongly with an existing tight complex between FMN and the apoprotein, probably an H-bonded structure in which FMN is tautomerized and interacts with an L-chiral center. This is concluded from a separate electronic transition with an origin at 496 nm, thus far not recognized as such, and the circular dichroism observed.The emission is dominated by that of free FMN, although protein-bound FMN seems also to become luminescent in glassy solution at 143 K. A second fluorescence/phosphorescence emission appears upon excitation of both native and complexed old yellow enzyme in the ultraviolet. This emission is quenched by the addition of phenol to the enzyme, shows a large (3000-cm-') blue shift on going to a low-temperature glass and is tentatively assigned to excimers of nucleic acids. Long-wavelength excitation with a synchronously pumped, mode-locked Rhodamine 6-G dye laser revealed a third, extremely weak emission in both native old yellow enzyme and its complexes. It decays with a lifetime of about 3 ns at 143 K. Electron paramagnetic resonance spectra revealed the presence of a low amount of an unpaired spin in old yellow enzyme. Owing to an unusual relaxational behaviour it could only be observed below 15 K and, again, the signal was measured in both the native enzyme and its complexes. Possible assignment and consequences of this observation are discussed.In frozen aqueous solutions of the enzyme-phenolate complex, a phase transition was discovered at which the colour of the complex reverted to that of the native enzyme. Subsequent melting restored the original colour.The observed phenomena and existing literature data lead to the conclusion that the only model from which no apparent inconsistencies emerge is that of a very complicated network of hydrogen-bonded structures in the protein. These involve several, partly unknown, chromophores. Phenols interfere with this network, leading to the formation of the long-wavelength absorption band in old yellow enzyme.The spectral changes induced by the binding of a lowmolecular-weight compound to old yellow enzyme (NADPH oxidoreductase) were initially regarded to be sufficiently mysterious to refer to this unknown compound as 'regreening factor' [I]. After the identification of this 'regreening factor'...

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Section: Resultsmentioning

confidence: 83%

Section: Methodsmentioning

confidence: 99%

Section: Theoretical Datamentioning

confidence: 99%

“…Additionally, complexes of 'ideal' isoalloxazine [27,28] and the phenolate anion in stacked conformations with parallel molecular planes (cf. Materials and Methods) were calculated.…”

Section: Theoretical Datamentioning

confidence: 99%

Section: Theoretical Datamentioning

confidence: 99%

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