Deracemization of bilirubin as the marker of the chirality of micellar aggregates

Insights into the Structures of Bilirubin and Biliverdin from Vibrational and Electronic Circular Dichroism: History and Perspectives

In this work we review research activities on a few of the most relevant structural aspects of bilirubin (BR) and biliverdin (BV). Special attention is paid to the exocyclic C=C bonds being in mostly Z rather than E configurations, and to the overall conformation being essentially different for BR and BV due to the presence or absence of the double C=C bond at C-10. In both cases, racemic mixtures of each compound of either M or P configuration are present in achiral solutions; however, imbalance between the two configurations may be easily achieved. In particular, results based on chiroptical spectroscopies, both electronic and vibrational circular dichroism (ECD and VCD) methods, are presented for chirally derivatized BR and BV molecules. Finally, we review deracemization experiments monitored with ECD data from our lab for BR in the presence of serum albumin and anesthetic compounds.

Chiral recognition of bilirubin and biliverdin in liposomes and micelles

The structural formula of biologically important chiral pigments bilirubin and biliverdin differs only by one double bond. We showed that this results in dissimilar interactions with two models of membranes: cationic liposomes composed of 3β-[N-(N',N'-dimethylaminoethane)-carbamoyl]cholesterol and zwitterionic micelles from 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS). While the liposomes recognized the P-form of bilirubin, the micelles recognized its M-form. Both recognized the P-form of biliverdin. Our study also comprised ternary systems consisting of the pigments, model membranes and serum albumin (human and bovine). Bilirubin preferentially interacted with the albumins even in the presence of the liposomes. On the other hand, biliverdin preferred the liposomes. Remarkably, the presence of CHAPS completely changed the biliverdin binding to the protein. Because our study was oriented on different chiral interactions, a chiroptical method of electronic circular dichroism was chosen as the principal method to study our systems. As complementary methods, UV-vis absorption and fluorescence emission were used.

Unprecedented relationship between the size of spherical chiral micellar aggregates and their specific optical rotations

Transmission electron microscopy (TEM) images and fluorescence quenching methods indicated that lauryl ester of L-phenylalanine (LEP) and lauryl ester of L-tyrosine (LET) form spherical chiral micelles in the 50-200 mM range and their size increases with concentration. The number of molecules present in these spherical chiral aggregates varied from 80 to 160 for LEP and 80-100 for LET. The specific optical rotation, representing circular birefringence, for LEP at 405 nm and 32 °C is found to increase linearly from 37 deg cc g(-1) dm(-1) for an isolated molecule to 56 deg cc g(-1) dm(-1) for ∼200 nm size aggregate. A similar trend was found for temperatures up to 70 °C and at other visible wavelengths. A linear relation between specific optical rotation and the size of aggregate is also observed for LET. Circular dichroism, as measured in both the visible and infrared wavelength regions, however did not reveal any concentration dependent changes. The unique sensitivity uncovered for specific optical rotation as a function of the size of spherical chiral aggregates is unprecedented and opens new areas of enquiry for physical chemists.