On the aggregation of bilirubinoids in solution as evidenced by VCD and ECD spectroscopy and DFT calculations

Abstract

Vibrational and electronic circular dichroism (VCD and ECD) spectra of 3 optically active bilirubin analogs with propionic acid groups replaced by (1) 1-(S)-methylpropyl groups, (2) 3-acetoxy-1-(S)-methylpropyl groups, and (3) 1-(S)-2-(R)-dimethyl-2-(methoxycarbonyl)ethyl groups have been recorded at different concentrations in chloroform. The aliphatic chains attached to C-8 and C-12 of the 3 chosen mesobilirubins were modified so as to possess no OH group. The variation of the VCD spectra with concentration is consistent with the formation of dimers at high concentration. Density functional theory and time-dependent density functional theory calculations on monomeric and dimeric forms support such a conclusion. Comparing with previous VCD (ECD) and IR (UV) studies of other mesobilirubin molecules, it is concluded that here, the key feature for aggregation is the missing OH groups on the propionic acid chains. The latter, in synergy with the polar groups of lactam moieties, appear to be involved in intramolecular phenomena and thus favor monomeric forms. Investigation of ECD and UV spectra of the same compounds in mixed DMSO/chloroform solutions provide further clues to the proposed picture.

l-Stercobilin-HCl and d-Urobilin-HCl. Analysis of Their Chiroptical and Conformational Properties by VCD, ECD, and CPL Experiments and MD and DFT Calculations

“…The features at ca. 1690 cm –1 host the CO stretching normal mode transitions of the lactam rings, as shown for bilirubin systems. − The different number of nearby stereogenic carbon atoms may justify the nonperfect mirror image of this region in the two molecules studied herein. The calculated CO stretchings of propionic acids do not give appreciable VCD contributions, but it is clear that the corresponding absorption band is calculated at higher wavenumbers (about 1780 cm –1 ) with respect to the experimental ones: a problem already encountered in other bilirubinoids. , …”

“…1610 cm –1 , one finds a mono-signate negative VCD feature for l -stercobilin and a positive feature for d -urobilin. According to previous studies on peptides , and on bilirubin derivatives, − the two regions just described are determined by normal modes containing CO stretching and NH bendings. These two regions could well be important for the study of intramolecular H-bonds. In correspondence to the strong IR band at ca.…”

“…The calculated CO stretchings of propionic acids do not give appreciable VCD contributions, but it is clear that the corresponding absorption band is calculated at higher wavenumbers (about 1780 cm −1 ) with respect to the experimental ones: a problem already encountered in other bilirubinoids. 42,43 The mono-signate negative VCD feature for l-stercobilin and the positive feature for d-urobilin at 1610 cm −1 are determined by complex normal mode transitions, involving NH− and C(10)−H in-plane bendings and ring vibrations. This band, as well as the bands in the previously described spectroscopic region, is affected by the number and nature of intramolecular hydrogen bonds; for this reason, they are common to all spectra obtained by the MD/ONIOM method of Figure 5.…”

“…Generally, however, especially in some important regions of the spectra, the information is quite precise and appropriately sheds light on the conformations of some parts of the molecule. Bilirubin and biliverdin, which are intrinsically achiral, were investigated by VCD either in the presence of chiral complexing agents, such as peptides, cyclodextrin, and metals, ,− or as appropriate chemical derivatives where chiral pendant groups had been chemically inserted. , However, to the best of our knowledge, no VCD study exists for the title metabolites l -stercobilin and d -urobilin. Given that computational analysis is an essential aspect of the VCD spectral interpretation, with these molecules one cannot be content with DFT calculations alone because interaction with the solvent may play a non-negligible role.…”

Biliverdin chiral derivatives as chiroptical switches for pH and metal cation sensing

“…some synthetic chiral bilirubin derivatives and the natural compounds L-stercobilin and D-urobilin. [36][37][38] In Fig. 1, we report the VCD and IR spectra of 1-6 in CDCl 3 solution (panels A and B, respectively) and display them beside their ECD and UV-Vis spectra in CHCl 3 (panels C and D, respectively), which had been recorded previusly 12 but were not shown there, since only the maximum/minimum value of molar ellipticity was tabulated.…”