Effects of pi-pi interactions on the 1H-NMR spectra and solution structures of pheophytin a and a' dimers
1993
Hynninen, P.H. | Lotjonen, S.
A comparative investigation of pheophytin a and a' aggregates in acetone solution is described for the first time using 1H-NMR spectroscopy. The determined aggregation shifts (delta values) were found to attain largest values in the vicinity of ring B of the macrocycle and to be very similar for both epimers. The similarity of the delta values was interpreted as indicating equivalency in aggregation behaviour for the epimers. An aggregation map was constructed on the basis of the ring-current model and the delta values for pheophytin a'. Replacement of the circle in the map with another pheophytin a' molecule, yielded two dimer structures showing a parallel, slipped arrangement for the two phorbin macrocycles, held together by pi-pi interactions. The more likely symmetrical dimer structure shows clearly that the asymmetric centres, C-13(2), C-17 and C-18, with their bulky substituents, remain far from the overlapped area of the macrocycles and at opposite sites of the dimer. Thus, no additional steric crowding exists in either epimer dimer. This dimer geometry explains why the pheophytin epimers are equivalent in aggregation behaviour. Finally, the effects of solvent, concentration and temperature on pheophytin aggregation are considered, and the nature of pi-pi interactions is discussed in the light of recent experimental findings and theoretical models for porphyrin aggregation.
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