Intramolecular Exciton Coupling and Induced Circular Dichroism From Bilirubin-Ephedrine Heteroassociation Complexes. Stereochemical Models for Protein Binding
Bichromophoric (4Z,15Z)-bilirubin-IXa, the cytotoxic and yel-:
low-orange pigment of jaundice, prefers to adopt either of two
enantiomeric intramolecularly hydrogeri-bonded conformations that
are in dynamic equilibrium in solution. In the presence of optically
active amino-alcohols, particularly ephedrines, the pigment solutions
exhibit intense bisignate circular dichroism in the region of
the bilirubin long wavelength UV-visible absorption band. The
most intense circular dichroism Cotton effects, I !\u27lE; i --+ 200, are
induced by O-methylephedrines, exceeding even those generally
exhibited by bilirubin complexes with serum albumin and other
proteins. Like serum albumin and other proteins, the optically
active amino alcohols act as chiral templates, inducing an asymmetric
transformation of bilirubin, whose induced bisignate circular
dichroism Cotton effect originate from exciton sphtting of
its two component pyrromethenone chromophores. The amines are
thought to serve as agents for chiral molecular recognitlon by forming
.diastereorneric salts with the pigment. And the complementary
action of fJ-aryl and proximal hydroxyl and methoxyl group s
provides insight into the binding forces important in bilirubinprotein
binding
