Porphyrins: Powerful Chromophores for Structural Studies by Exciton-Coupled Circular Dichroism

Abstract

We introduce porphyrins as long-range interacting chromophores that extend the applicability of the exciton-coupled circular dichroic method to configurational studies of molecules with remote stereogenic centers and possibly to conformational studies of biopolymers. The interaction between excited states of chromophores in chiral environments give circular dichroism (CD) curves with split Cotton effects, Le., exciton-coupled CD.' In this method for determining the absolute stereochemistry of organic molecules in solution, the signs and shapes of the characteristic CD curves are defined by the absolute skewness of interacting chromophores.' The extent of chromophoric coupling, Le., the amplitudes of split Cotton effects, is inversely proportional to the square of interchromophoric distance2 and proportional to the square of extinction coeff i c i en t~~ of the coupled chromophores. Therefore, the intensity of chromophoric absorptions is of prime importance in increasing the sensitivity over the large distance between interacting transition moments. Furthermore, when the original substrate contains a chromophore, new chromophores with bathochromic intense absorbances, E = 31 000-58 000, may be used to deliberately avoid coupling with preexisting chromophore^.^ For the purpose of developing intense red-shifted chromophores, we had investigated cyanine dye chromophores; although unique from a spectroscopic viewpoint$b they were unsuited for practical applications due to their instability, nontrivial microscale preparation, etc. This led to the introduction of other redshifted chromophores which have wide and diverse applicab i l i t i e ~ . ~ ~ . ~ In the following, we show that porphyrins further enhance the sensitivity of the exciton-coupled CD method by almost 10-fold and extend the applicability of the method to molecules that could not be studied so far. Thus, 5-substituted 10,15,20-triphenylporphyrins, e.g., 1, with their intense redshifted Soret band at 414 nm, E = 350 000: provide powerful exciton-coupled CD chromophores for absolute configurational studies of natural products with remote stereogenic centers 3550 8, apart6 as well as conformational studies of biopolymers, e.g., ligand-receptor interactions. Snejana Novkova, Irena Philipova, and Blagoy Blagoev