Methylvinylmaleimide from bilirubin photooxidation.

Irradiation of an oxygenated methanolic solution of bilirubin IXalpha in the absence of known singlet oxygen sensitizers gave methylvinylmaleimide among other products.

Blue light and bilirubin excretion.

Blue light converts bilirubin in the skin of jaundiced rats to metastable geometric isomers that are transported in blood and excreted in bile. The same reaction probably occurs in jaundiced babies exposed to light, particularly during treatment with phototherapy. Excretion of unisomerized bilirubin is prevented by intramolecular hydrogen bonding, and the pigment has to be metabolized to more polar derivatives to be excreted efficiently.

Electrochemical reduction of the biliverdin-serum albumin complex as monitored by absorption and circular dichroism spectroscopy.

The cathodic reduction at the mercury electrode of a biliverdin IX alpha-serum albumin complex at physiological pH in an aqueous buffer containing percentages of DMSO ranging from 4% to 20% is studied by cyclic voltametry and controlled potential coulometry. The progression of pigment disappearance and the (stereochemical) nature of the product are monitored by chromatography, UV-visible absorption and circular dichroism spectroscopy. Upon reduction, albumin-bound biliverdin IX alpha, with a slight preference for the P-helicity, affords the corresponding bound bilirubin IX alpha -with an M-chirality conformation. The complex is reduced at -0.64 V (vs. SCE; 8% DMSO), only a little shifted compared to reduction of free biliverdin IX alpha under the same conditions. In contrast, an analogous bilirubin IX alpha-serum albumin complex is essentially inert towards cathodic reduction under conditions where free bilirubin IX alpha is reduced, indicating a better shielding by the protein of the bilirubin IX alpha molecule from the electrode surface. The presence of relative position (as in the biliverdins IX alpha and XIII alpha) or absence (as in mesobiliverdin IX alpha) of vinyl groups in the pigment does not have a significant effect upon its electroreduction behaviour, indicating that the process is not sensitive to the subtle differences imposed by vinyl groups upon the structure of the corresponding biliverdin-albumin complexes.

Biliary excretion of a stretched bilirubin in UGT1A1-deficient (Gunn) and Mrp2-deficient (TR-) rats.

The metabolism and biliary excretion of a stretched bilirubin analog with a p-xylyl group replacing the central CH2 hinge were investigated in normal rats, Gunn rats deficient in bilirubin conjugation, and TR- rats deficient in bilirubin glucuronide hepatobiliary transport. Unlike bilirubin, the analog was excreted rapidly in bile unchanged in all three rat strains after intravenous administration. In TR- rats biliary excretion of the analog was diminished, but still substantial, demonstrating that the ATP-binding cassette transporter Mrp2 is not required for its hepatic efflux. These effects are attributable to differences in the preferred conformations of bilirubin and the analog.

'Like a shrivelled blood orange'--bilirubin, jaundice, and phototherapy.

The biochemistry of bilirubin is reviewed with particular reference to newborn infants. The formation, properties, and metabolism of bilirubin are summarized and the importance of molecular shape, hydrogen-bonding, and polarity on the biologic disposition of bilirubin is emphasized. The chemical basis for the subtle influence of visible (blue) light on bilirubin structure and metabolism is explained, and recent concepts of the mechanism of phototherapy are presented. A glossary of current jargon is appended.

A new photoisomerization of bilirubin.

Photoirradiation of solutions of natural (4Z,15Z)-bilirubin-IX alpha in chloroform with approximately 366 nm UV light leads rapidly to a new photoisomer that has been characterized by NMR spectroscopy and by its (ground-state reaction) adducts with methanol and other protic nucleophiles. Unlike the previously described Z-->E geometric isomerization important in phototherapy of neonatal jaundice, the new photoisomerization involves regiospecific photoautomerization to afford 2-ethenyl-18-ethylidene- 1,10,19,22,24-pentahydro-2,7,13,17-tetramethyl-1,19- dioxo-21H-biline-8,12-dipropanoic acid.

The octant rule VIII. Variable temperature circular dichroism spectra of alpha-methyl- and methoxyl-substituted 5 alpha-cholestan-2- and -3-ones.

2 alpha- and 2 beta-Methyl- and methoxy-5 alpha-cholestan-3-ones and 3 alpha- and 3 beta-methyl- and methoxy-5 alpha-cholestan-2-ones have been synthesized and their variable temperature circular dichroism spectra obtained and analyzed. Rotatory strength (R) values for alpha-axial and equatorial CH3 and OCH3 groups are determined by difference measurements with the parent ketone. The (small) equatorial CH3 R-values do not consistently follow the Octant Rule. Axial OCH3 groups do not obey the Octant Rule ("anti-octant" behavior) and impose a bathochromic shift on the C = O n-pi transition. Equatorial OCH3 groups do not consistently follow octant or "anti-octant" behavior.

Structure and metabolism of natural and synthetic bilirubins.

The secondary structure of bilirubin, with a ridge-tile shape and six intramolecular hydrogen bonds, is more stable than any other conformation and perhaps the most important determinant of its solubility and properties in solution and its hepatic metabolism/excretion. Uncoupling the intramolecular hydrogen bonding, increasing the acidity of the propionic acid, or even increasing its length can decrease the pigment's hydrophobicity and permit its excretion intact across the liver into bile; less intuitively obvious, so can subtle modifications at C(10), the pivotal carbon in the ridge tile.

Oxygenation of a dipyrromethene model for bilirubin: formation of a singlet oxygen-like product.

An oxodipyrromethene model compound for bilirubin is found to undergo oxidation to a blue tetrapyrrole and a water-propentdyopent on a silica gel thin layer chromatography plate. The reaction involves ground state oxygen and requires silica gel, although the propentdyopent is an expected product from reaction with singlet oxygen.

Semirubin. A novel dipyrrinone strapped by intramolecular hydrogen bonds.

(4Z)-9-(5-Carboxypentyl)-2,3,7,8-tetramethyl-(10H)-dipyrrin- 1-one (1, semirubin), a new dipyrrinone model for one-half of bilirubin, the yellow-orange neurotoxic pigment of jaundice, was synthesized following Friedel-Crafts acylation of 2,3,7, 8-tetramethyl-(10H)-dipyrrin-1-one (5) with the half-ester acid chloride of adipic acid. Unlike other dipyrrinone models for bilirubin, such as the xanthobilirubic acids, which engage only in intermolecular hydrogen bonding, 1 is unique in having been designed and found to engage in intramolecular hydrogen bonding, between the carboxylic acid and the dipyrrinone lactam and pyrrole. This important conformation-determining structural characteristic, shared by 1 and bilirubin, renders them less polar than their methyl esters and leaves them monomeric in nonpolar solvents, where their esters are dimeric. The corresponding 10-oxo analogue (3) of 1 serves as a model for 10-oxo-bilirubin, a presumed bilirubin metabolite in alternate pathways for bilirubin excretion. Like 1, 3 is found to engage in intramolecular hydrogen bonding. Unlike the methyl ester of 1, the ethyl ester of 3 is not intermolecularly hydrogen bonded in nonpolar solvents.

A C-H...O=C hydrogen bond? Intramolecular hydrogen bonding in a novel semirubin.

(4Z)-8-(5-Carboxypentyl)-9-butyl-2,3-diethyl-dipyrrin-1-one (1), a new analogue of xanthobilirubic acid, (4Z)-8-(carboxyethyl)-2,7,9-dimethyl-3-ethyl-dipyrrin-1-one, was synthesized in four steps from the known 2,3-diethyl-dipyrrin-1-one. Whereas xanthobilirubic acid (which is a model for one-half of bilirubin, the yellow pigment of jaundice) and its homologues with hexanoic and longer acid chains at C-8 engage only in intermolecular hydrogen bonding, 1 is found to engage in intramolecular hydrogen bonding. In CDCl(3) solution, dipyrrinone 1 adopts an anti-Z conformation, and its hexanoic acid COOH is hydrogen-bonded to the lactam H-N-C=O and to the pyrrole C(7)-H but not to the pyrrole NH. The latter constitutes an example of a hydrogen bond of the type C-H...O=C, weak and detected typically in crystals. Dipyrrinone 1 is found by vapor pressure osmometry to be monomeric in CHCl(3), but its methyl ester (2) tends toward being dimeric, like that of methyl xanthobilirubinate, which is dimeric.

On the structure of bilirubin in solution. 13C[1H] heteronuclear Overhauser effect NMR analyses in aqueous buffer and organic solvents.

Select hydrogen-carbon distances have been determined from 13C[1H] heteronuclear Overhauser effects observed in a 99% carbon-13 enriched 13CO2H bilirubin analog, [8(3), 12(3)-13C2]-mesobilirubin XIII alpha. Analysis of the data confirms that the propionic acid carbonyl lies within hydrogen bonding distance to the dipyrrinone lactam and pyrrole N-H groups in chloroform and indicates, surprisingly, that those distances are only slightly longer in dimethyl sulfoxide solvent or when the carboxyl group is ionized in pH 7.4 aqueous buffered solutions of the pigment. The data supports the presence and persistence of folded, intramolecularly hydrogen-bonded bilirubin conformations in solution.

Exciton chirality of bilirubin homologs.

Bilirubin, the yellow pigment of jaundice, is a bichromophoric tetrapyrrole that readily adopts either of two enantiomeric, folded conformations shaped like ridge-tiles and stabilized by a network of six intramolecular hydrogen bonds. Interconversion of these M and P helical chirality conformational enantiomers is rapid at room temperature but may be displaced toward either enantiomer by intramolecular non-bonded steric interactions. Introduction of a methyl group at the beta and beta' carbons of the propionic acid chains on the symmetric bilirubin analog, mesobilirubin-XIII alpha, shifts the conformational equilibrium toward the M or the P-chirality intramolecularly hydrogen-bonded conformer, depending only on the S or R stereochemistry at beta and beta', resulting in pigments with intense exciton coupling circular dichroism (CD) for the approximately 430 nm transition(s). Optically active synthetic analogs of bilirubin with propionic acid groups lengthened systematically to heptanoic acid (1-5) were synthesized and examined by spectroscopy to explore the influence of alkanoic acid chain length on conformation and intramolecular hydrogen bonding. In these diacids and their dimethyl esters (6-10), strong exciton chirality CD spectra are observed, and the data are correlated with molecular helicity.