Structural studies by exciton coupled circular dichroism over a large distance: Porphyrin derivatives of steroids, dimeric steroids, and brevetoxin B

The present study (see ref 1) delineates the scope and limitations of porphyrin chromophores for structural studies by the exciton coupled circular dichroic (CD) method. A distance dependency of the porphyrin coupling was investigated in the range between 10 and 50 Angstrom. Over short interchromophoric distances, significant changes in the conformational distribution introduced by the bulky porphyrin chromophores were observed. Over longer distances, the porphyrins showed ca. 10-fold sensitivity increase over commonly used chromophores, and an effective direction for the interacting porphyrin transition moments was assigned by comparison. Porphyrins at the termini of dimeric steroids and brevetoxin B exhibited exciton coupling over interchromophoric distances up to 50 Angstrom. These results represent the porphyrins as promising reporter chromophores for extending the exciton coupled CD method to structural studies of biopolymers.