Photochemistry and Photophysics of Coordination Compounds

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244 M.T. Indelli et al. intercalation through the major groove [146]. A series of systematic NMR and photocleavage studies clearly showed that the binding of complexes containing different ancillary ligands occurs at a different specific DNA sequence. This site specificity results from both shape-selective steric interactions as well as stabilizing van der Waals and hydrogen bonding contacts. In particular, Rh(NH3)4phi 3+ and related amine complexes bind to d(TGGCCA)2 duplex through hydrogen bonding between the ancillary amine ligands and DNA bases [130, 136]. Evidence for specific intercalation was found also for Rh(phen)2phi 3+ in the hexanucleotide d(GTCGAC)2 [134]. In this case Barton proposed that the site specificity was based upon shape-selection. Since thephenanthrolineligandsprovidestericbulkaboveandbelowtheplaneof the phi ligand, the stacking occurs at sites which are more open in the major grove. The most striking example of site-specific recognition by shape selection with bulky ancillary ligands was found for Rh(DPB)2phi 3+ (DPB =4,4 ′ -diphenylbpy) [140]. For all the complexes studied enantioselectivity favoring the intercalation of the ∆-isomer was observed [130, 147]. Further control of sequence specificity has been achieved by using derivatives of Rh(phen)(phi)2 3+ complexes where the nonintercalating phenanthroline ligand has been functionalized with pendant guanidinium group or with short oligopeptides [148, 149]. For metal-peptide complexes photocleavage experiments showed that the polypeptide chain is essential in directing the complex to a specific DNA sequence [149]. Among the rhodium intercalators explored as probes of DNA structure, Barton selected the Rh(bpy)2chrysi 3+ (chrysi = 5,6-chrysenequinone diimine, 31) complex as an ideal candidate for mismatches recognition [150–152]. The specific recognition is based on the size of the intercalating ligand: the chrysene ring system is too large to intercalate in normal B-form DNA but it can do so at destabilized mismatch sites. The authors point out that sterically demanding intercalators such as Rh(bpy)2chrysi 3+ may have application both in mutation detection systems and as mismatch-specific chemotherapeutic agents. Recently mixed-metal trimetallic complexes have been designed and studied by Brewer to obtain supramolecular system capable of DNA photocleavage [153, 154]. These complexes of general formula [{(bpy)2M(dpp)}2 RhCl2](PF6)5 with M = Ru(II) or Os(II) couple ruthenium or osmium chromophoric units to a central rhodium(III) core. When excited with visible light into their intense MLCT bands, these complexes exhibit DNA photocleavage
Photochemistry and Photophysics of Coordination Compounds: Rhodium 245 property. The authors discussed the role of the supramolecular architecture and in particular of the rhodium(III) unit on the photoreactivity 4.2 Rh(III) Complexes in DNA-Mediated Long-Range Electron Transfer DNA-mediated electron transfer has been an active and much debated topic of research [155]. Several articles have dealt with DNA-mediated photoinduced electron transfer reactions involving metal complexes as photoactive units [156]. In this context, of particular interest are the studies of Barton and coworkers that used rhodium(III) intercalator complexes not only as ground-state but also as excited-state electron acceptor in electron transfer (ET) reactions through the DNA [144]. 4.2.1 Rh(III) Complexes as Acceptors in Electron Transfer Reactions In 1993, Murphy et al. [157]. reported the surprising result that an efficient and rapid photoinduced electron transfer occurs over a large separation distance (> 40 ˚A) between DNA metallointercalators that are covalently tethered to opposite 5 ′ -ends of a 15-base pair DNA duplex (Fig. 13). In this oligomeric assembly Ru(phen)2dppz 2+ (dppz = dipyridophenazine) plays the role of excited electron donor and the Rh(phi)2(phen) 3+ is the electron acceptor. Both donor and acceptor bind to DNA with high affinities (> 10 6 M –1 ) by intercalation through the dppz [158, 159] and phi ligands, re- Fig. 13 A 15-base pair DNA duplex carrying covalently tethered Ru(II) and Rh(III) intercalators
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280 Topics in Current Chemistry Edi
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Photochemistry and Photophysics of
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X Preface nation Compounds. The ven
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Page 279 and 280: Subject Index Alkyne bridges 148 An
Page 281: Subject Index 273 Rh(III) cyclometa
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