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| Content Provider | PubMed Central |
|---|---|
| Author | Wasielewski, Michael R. Norris, James R. Shipman, Lester L. Lin, Chih-ping Svec, Walter A. |
| Abstract | The chlorophyll a (Chl a) special-pair model of the primary donor of photosystem I (P700) does not account in a completely adequate fashion for the magnetic resonance properties observed for P700+. Moreover, P700 is at least 420 mV easier to oxidize than is Chl a in vitro. Neither Chl a dimer formation nor selective ligation of Chl a can account for this potential difference. Enolization of the Chl a ring V β-keto ester results in a very different π electronic structure. The Chl a enol can be trapped as a silyl enol ether. In addition, the enol analog 9-desoxo-9,10-dehydro-Chl a can be prepared. Both the trapped enol and its 9-H analog are ≈350 mV easier to oxidize than Chl a. The ESR spectrum of the cation radical consists of a single 6.1-G gaussian line that is line narrowed relative to that of Chl a + in a manner similar to P700+. Electron-nuclear double resonance (ENDOR) spectroscopy resolves only a 3.5-MHz hyperfine splitting for the 3-methyl-group. The remaining splittings are all less than 3.5 MHz. The second moment of the ESR line of fully 13C-enriched 9-desoxo-9,10-dehydro-Chl a + agrees with that of [13C]P700+ to within 10%. Application of the special-pair model to the [13C]P700+ second-moment data yields a 100% error. Ab initio molecular orbital calculations on ethyl chlorophyllide a enol cation bear out the ESR and ENDOR data. We conclude that a monomeric Chl a enol model provides a better description of the magnetic resonance parameters and oxidation potential of P700 than a Chl a special-pair model. |
| Starting Page | 2957 |
| File Format | |
| ISSN | 10916490 |
| e-ISSN | 10916490 |
| Journal | Proceedings of the National Academy of Sciences of the United States of America |
| Issue Number | 5 |
| Volume Number | 78 |
| Language | English |
| Publisher Date | 1981-05-01 |
| Access Restriction | Open |
| Subject Keyword | Research in Higher Education |
| Content Type | Text |
| Resource Type | Article |
| Subject | Multidisciplinary |
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