Scherz-Parson model

This is the most promising approach so far to a geometric model of a purple bacterial antenna complex [18]. Its basic ideas are simple and ingenious. If correct, it would explain much. However, the model has some significant problems. 

On the other hand, the model is not without its drawbacks. It requires a certain structural association of 4 BChl molecules. Although a number of conceivable structures would be consistent with the tenets of the model, many otherwise plausible structures are excluded. This in itself is not a drawback the structural restrictions may be viewed simply as predictions of the model. Indeed, the idea of a basic dimer of closely paired BChls enters other models as well (see Section 3.2). However, problems may arise if in an actual structure more than two dimers are present and interact with comparable energies. In other words, the success of the Scherz-Parson model may depend on each dimer having only one nearest neighboring dimer, a somewhat unlikely supposition on the grounds of both symmetry and energy-transfer requirements (see Section 3.2.). 

In spite of these criticisms, the Scherz-Parson model is interesting and clever, and should be considered very seriously. 

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To conclude the discussion of exciton theory, we present a brief account of the application of PCM-based exiton methods to studies of spectral shifts in bacteriochlorin (BC) dimers [57, 66]. This work was motivated by earlier exciton studies of a model methyl bacteriopheophorbide a (Me-BPheo-a) dimer reported by Scherz and Parson [67]. Using the point-dipole model to approximate monomeric charge and transition densities [54-56], they obtained a large (1330 cm" ) red shift of the band in a dimer whose geometry was defined as a translation of one monomer relative to the other by Ax = 0.5 A, Ay = 3.5 A, and Az = 3.6 A along the axes in Fig. 4. 

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