Relative steric size of SCH(3), OCH(3), and CH(3) groups from circular dichroism measurements.

The relative steric size of methyl, methoxy, and methylthio groups was determined from circular dichroism (CD) spectroscopy using a sensitive system based on the bilirubin model. In the cyclohexane model, equatorial vs. axial orientation and conformational analysis led to quantitative measurements of orientation preference or steric demand: conformational A-values CH(3) > SCH(3) > OCH(3). A more sterically demanding model for assessing group size has been found in bilirubin analogs, which are yellow pigments that adopt a ridge-tile shape stabilized by a matrix of intramolecular hydrogen bonds. Optically active bilirubins have been shown to exhibit intense bisignate CD Cotton effects from exciton coupling of their two dipyrrinone chromophores held in either of two enantiomeric ridge-tile conformations. Interconversion of these M and P conformational enantiomers of helical chirality is rapid at room temperature but may be displaced toward either enantiomer by intramolecular nonbonded steric interactions that arise when substituents are introduced at equivalent sterically demanding sites, viz., the alpha or beta carbons of the pigment's propionic acid chains. Such substituents shift the conformational equilibrium toward the M or the P-chirality conformer, depending only on the S or R stereochemistry at the alpha and beta sites, and the resulting exciton CD for the approximately 430 nm transition(s) was used to evaluate the relative steric size, SCH(3) > CH(3) > OCH(3).