Chirality inversion in the bilirubin molecular exciton

Abstract

The bichromophoric pigment bilirubin acts as a molecular exciton in its UV-visible and circular dichroism (CD) spectroscopy. In both polar and nonpolar solvents, an optically active analog, (beta R,beta 'R)-dimethylmesobilirubin-XIII alpha (1), exhibits intense bisignate CD Cotton effects in the region of its long wavelength UV-vis absorption near 400 nm: Delta epsilon(434)(max) + 337, Delta epsilon(389)(max) - 186 (CHCl(3)), and Delta epsilon(431)(max) + 285, Delta epsilon(386)(max) - 177 (CH(3)OH). However, introduction of an amine into a CHCl(3) solution of 1 causes the Cotton effect signs to become inverted, e.g., after addition of NH(3), Delta epsilon(433)(max) - 345, Delta epsilon(389)(max) + 243, and after addition of ethylene diamine, Delta epsilon(435)(max) - 420, Delta epsilon(390)(max) + 299. The sign inversions imply inversion of molecular chirality of the bilirubin and the phenomenon appears to be general for amines, including alpha,omega-diamines. 1,8-Diaminooctane was found to be more effective than longer or shorter chain analogs in producing CD sign inversion.

Hemin and bile pigments are the secondary structure regulators of intrinsically disordered antimicrobial peptides

“…Such an exciton CD couplet was reported by several times for macromolecular complexes of bilirubin and reflects the enantioselective binding of the pigment molecules to the chiral host. [25][26][27][28][29][30] In line with the so-called exciton chirality rule, [31][32][33] the CD signal pattern shown in Figure S2 indicates the slight binding preference of the M-helical (left-handed) conformer ( Figure S3). Besides the main VIS absorption band between 340 and 550 nm, bilirubin ditaurate also shows light absorption in the UV region ( Figure S4).…”

Vibrational and Electronic Circular Dichroism of Dimethyl Mesobilirubins-XIIIα

“…Indeed, in Figure 4, one can see first that the experimental ECD spectra are well reproduced by the TDDFT-calculated spectra (and the UV spectra as well), except for the overestimate of electronic transition frequencies; second, one learns that the observed large ECD couplet is due to just two transitions with large and opposite rotational strengths (in the figure, the latter are proportional to the bars underneath the bands); the corresponding wide UV band is instead due mostly to one of the two π → π* transitions, the other bearing weaker dipole strength. Thus, the ECD spectrum, which had been previously and fruitfully interpreted in terms of an exciton couplet from dipyrrinone chromophores, 9,10 The very striking effect on the ECD spectrum due to DMSO added to the solutions of (αR,α′R)-and (βS,β′S)-dimethylmesobilirubin-XIIIα in CHCl 3 (Figure 3) clearly suggests a reorientation of the dipyrrinones themselves. Their relative orientation is governed by their ability to rotate about the central CH 2 group to which they are conjoined, with limitations imposed by intramolecular hydrogen bonds.…”

“…Also, chiral derivatives of mesobilirubin with stereogenic centers, for example, (βR,β′R)-dimethylmesobilirubin-XIIIα, have been used successfully to investigate conformational properties and conformational changes in solution. 9,10 The same exciton system detected by induced CD of bilirubin, as observed in refs 7 and 8, was also found in natural ECD of their chiral derivatives described in refs 9 and 10, and changes in the CD spectra were then used as indicators of conformational change occurring in most bilirubin systems.…”

Solvent‐dependent inversion of circular dichroism signal in naproxen: An unusual effect!

“…5 Electronic circular dichroism (ECD) is very sensitive to the presence of solvent, 6,7 temperature, 8 and molecular exciton formation. 9 The equilibrium between enantiomers can lead to interesting effects, and some studies are being done to understand the solvent-solute interactions using the polarizable continuum model 10 and most simple chiral alkane molecules. 11 In this work, we studied the solvent effect on ECD spectrum of naproxen using experimental and computer simulation approaches.…”