ESR and ENDOR studies of sterically hindered porphyrins and chlorins provide clear evidence of the flexibility of the porphyrin skeleton, in agreement with crystallographic results. Examples considered are the cation radicals of zinc tetraphenyl‐octaethylporphyrin and of the zinc and cobaltous nitrosyl complexes of octaethyl‐ and methyl octaethyl‐chlorins. Extrapolation of the above results suggests that, in vivo, the protein pocket into which the chromophores fit and neighboring residues provide ready means of altering the conformations and properties of the pigments. These considerations can be applied to the primary acceptors of photosystems I and II: a combination of theoretical calculations, model studies and ENDOR results for the chlorophyll and pheophytin acceptors of photosystems I and II, respectively, suggests specific orientation of the 2‐vinyl substituents of the chromophores and, in addition, supports the existence of hydrogen bonding at the 9‐keto group. The implications of these results are that the protein environment helps control the orientations of the macrocycles and substituents, and thereby optimizes the relative orientations of donors and acceptors to facilitate electron transfer.
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