Toward an Amphiphilic Bilirubin: The Crystal Structure of a Bilirubin E-Isomer Sanjeev K. Dey and David A. Lightner* Department of Chemistry, UniVersity of NeVada, Reno, NeVada 89557
[email protected] ReceiVed December 6, 2007
A new bilirubinoid analog (1) with two methoxy β-substituents on the lactam ring of each dipyrrinone was synthesized and examined spectroscopically. It is more soluble in CH3OH and CHCl3 than bilirubin, which is insoluble in CH3OH but soluble in CHCl3. The solubility of 1 is ∼10 µg/mL in CH3OH (vs e1 µg/mL for bilirubin) and ∼3 mg/mL in CHCl3 (vs ∼0.6 mg/mL for bilirubin). Vapor pressure osmometry indicates that 1, like bilirubin, is monomeric in CHCl3, and NMR studies show that the most stable structure has the syn-4Z,syn-15Z configuration, with the pigment’s dipyrrinones engaged in intramolecular hydrogen bonding to the propionic acid carboxyl groups. And, like bilirubin, Z,Z-1 adopts a conformation that is bent in the middle into a ridge-tile shape. For the first time, a crystal structure of a bilirubin E-isomer has been obtained. Crystallization of 1 under dim room lighting gave an X-ray quality crystal of the anti-4E,syn-15Z-(photo) isomer, in which only the Z-dipyrrinone half is engaged in intramolecular hydrogen bonding to a propionic acid. Hydrogen bonding is nearly completely disengaged in the E-dipyrrinone half; yet, the ridge-tile conformation persists.
Introduction Bilirubin (Figure 1A), the end product of heme metabolism in mammals,1 is a lipophilic tetrapyrrole dicarboxylic acid,2 the yellow neurotoxic pigment of jaundice,1,2 and a powerful antioxidant.3 Bilirubin and its blue-green biogenetic precursor, biliverdin, are formed copiously in healthy humans by catabolism of heme and other heme proteins, and they are eliminated from circulation by the liver, crossing into bile.1,4 Unlike biliverdin, which is polar and excreted intact across the liver, bilirubin is nonpolar and requires glucuronidation of at least (1) Chowdhury, J. R.; Wolkoff, A. W.; Chowdhury, N. R.; Arias, I. M. Hereditary jaundice and disorders of bilirubin metabolism. In The Metabolic and Molecular Bases of Inherited Disease; Scriver C. F., Beaudet A. L, Sly W. S., Valle D., Eds.; McGraw-Hill: New York, 2001; Chapter 125, pp 3063-3101. (2) (a) Lightner, D. A.; McDonagh, A. F. Acc. Chem. Res. 1984, 17, 417-424. (b) McDonagh, A. F.; Lightner, D. A. Pediatrics 1985, 75, 443455. (c) McDonagh, A. F.; Lightner, D. A. Semin. LiVer Dis. 1988, 8, 272283. (3) (a) Dore, S.; Takahashi, M.; Ferris, C. C.; Hester, L. D.; Guastella, D.; Snyder, S. H. Proc. Natl. Acad. Sci. 1999, 96, 2445-2450. (b) Stocker, S.; Yamamoto, Y.; McDonagh, A. F.; Glazer, A. N.; Ames, B. N. Science 1987, 235, 1043-1046.
one propionic acid group for transhepatic transport.1,4 Some 65 years ago, Hans Fischer determined their constitutional structures by degradation methods and total synthesis and indicated linear structures for the pigments without designating the configurational stereochemistry of the exocyclic CdC bonds5 at C(4), C(10) and C(15). Lemberg6 suggested that an all-Z configuration would follow logically from the porphyrin precursor; yet, he used “linear” representations in his classic (1949) book7 with Legge, apparently because they “save space and are more readily visualized.” Thus, from the late 1930s to present times linear structural representations of bilirubin and biliverdin (Figure 1B) have persisted in the literature and biochemistry textssoften (4) (a) McDonagh, A. F. Bile pigments: Bilatrienes and 5,15-biladienes. In The Porphyrins; Dolphin D., Ed.; Academic Press: New York, 1979; Vol VI, Chapter 6. (b) Schmid, R.; McDonagh, A. F. Hyperbilirubinemia. In The Metabolic Basis of Inherited Disease, 4th ed.; Stanbury, J. B., Wyngaarden, J. B., Fredrickson, D. S., Eds.; McGraw-Hill: New York, 1978; pp 1221-1257. (5) Fischer, H.; Plieninger, H. Hoppe-Seyler’s Z. Physiol. Chem. 1942, 274, 231-260. (6) Lemberg, R. Austral. Chem. Int. J. Proc. 1939, 6, 170-180. (7) Lemberg, R.; Legge, J. W. Hematin Compounds and Bile Pigments; Interscience Publ., Inc.: New York, 1949. 10.1021/jo702593x CCC: $40.75 © 2008 American Chemical Society
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Toward an Amphiphilic Bilirubin
FIGURE 1. (A) Catabolism of heme to biliverdin and bilirubin, both shown in porphyrin-like representations. The most stable conformation of biliverdin is porphyrin-like and helical. (B) Linear representations of biliverdin and bilirubin, both unstable conformations. (C) The most stable conformation of bilirubin is neither linear nor porphyrin-like but folded into a half-opened book or ridge-tile shape that is stabilized by 6 intramolecular hydrogen bonds. (Only one enantiomer is shown.)
with the wrong double bond stereochemistry. The double bond configuration was clarified as Z approximately 30 years ago by X-ray crystallography,8,9 and we now know that (4Z,10Z,15Z)verdins adopt a porphyrin-like conformation while the most stable conformation of (4Z,15Z)-rubins is neither linear or porphyrin-like.10 In bilirubin the two dipyrrinones may in principle rotate freely about the central 10-CH2 like propeller blades to generate a multitude of conformations, of which the linear and porphyrin-like are highest energy and one resembling a half-opened book (Figure 1C) minimizes intramolecular nonbonded steric repulsions.11 It is in such a “ridge-tile” shape9 that bilirubin is further stabilized by intramolecular hydrogen bonding between the propionic acid COOH groups and the opposing dipyrrinone lactam and pyrrole groups (Figure 1C),9,11 a conformation that explains its polarity and solubility that differs from biliverdin: insoluble in CH3OH (biliverdin is soluble) and solubility in CHCl3 (biliverdin is insoluble).2 It also explains bilirubin’s lipophilicity and correlates with the pigment’s inability to be excreted intact by the liver.12 However, when some of the hydrogen bonds are broken, as in the E-isomers of bilirubin (8) (a) Sheldrick, W. S. Israel J. Chem. 1983, 23, 155-166. (b) Sheldrick, W. S. J. Chem. Soc., Perkin 2 1976, 1457-1462. (9) (a) Bonnett, R.; Davies, J. E.; Hursthouse, N. B.; Sheldrick, G. M. Proc. R. Soc. London, Ser. B 1978, 202, 249-268. (b) LeBas, G.; Allegret, A.; Mauguen, Y.; DeRango, C.; Bailly, M. Acta Crystallogr. 1980, B36, 3007-3011. (c) Becker, W.; Sheldrick, W. S. Acta. Crystallogr., Sect. B 1978, B34, 1298-1304. (d) Mugnoli, A.; Manitto, P.; Monti, D. Acta Crystallogr. 1983, C39, 1287-1291. (10) Falk, H. The Chemistry of Linear Oligopyrroles and Bile Pigments; Springer-Verlag: Wien, 1989. (11) Person, R. V.; Peterson, B. R.; Lightner, D. A. J. Am. Chem. Soc. 1994, 116, 42-59. (12) (a) McDonagh, A. F.; Lightner, D. A. Cell. Mol. Biol. 1994, 40, 965-974. (b) McDonagh, A. F.; Lightner, D. A.; Nogales, D. F.; Norona, W. FEBS Lett. 2001, 506, 211-215.
formed during phototherapy for jaundice in the newborn,2 or in adults exposed to natural light,13 the pigment can be excreted intact s and glucuronidated at the propionic acid still engaged in intramolecular hydrogen bonding. Although X-ray crystal structures of natural 4Z,15Z-bilirubin9 and some of its synthetic analogs have been obtained,8,14 no crystal structures of a bilirubin E-isomer have been reported. Bilirubin E-isomers are important because by breaking a set of intramolecular hydrogen bonds, the pigment becomes more polar and excretable,2,15 which is of fundamental importance to the success of the widely used phototherapy for the jaundiced newborn.1,2 Earlier, we explored the influence of non-ionizable substituents on the lipophilicity of bilirubins in which intramolecular hydrogen bonding is maintained. Thus, we designed and synthesized analogs that were capable of intramolecular hydrogen bonding but were either more or less lipophilic than bilirubin by: (1) changing the lactam β-substituents from methyl and vinyl to two ethyls,16 or to a methyl and n-butyl,17 which increased the pigment’s lipophilicity; (2) adding a gem-dimethyl group to C(10), which increased the pigment’s solubility in nonpolar as well as polar (e.g., CH3OH) solvents;16a and (3) synthesizing analogs with R-F or R-OCH3 groups on the (13) McDonagh, A. F. N. Engl. J. Med. 1986, 314, 121-122. (14) (a) Ghosh, B.; Catalano, V. J.; Lightner, D. A. Monatsh. Chem. 2004, 135, 1305-1317. (b) Tipton, A.; Lightner, D. A. Monatsh. Chem. 2002, 133, 707-716. (c) Kar, A. K.; Tipton, A. K.; Lightner, D. A. Monatsh. Chem. 1999, 130, 833-843. (15) McDonagh, A. F.; Lightner, D. A. Semin. LiVer Dis. 1988, 8, 272283. (16) (a) Xie, M.; Holmes, D. L.; Lightner, D. A. Tetrahedron 1993, 49, 9235-9250. (b) Ghosh, B.; Lightner, D. A.; McDonagh, A. F. Monatsh. Chem. 2004, 135, 1189-1999. (17) Brower, J. O.; Lightner, D. A.; McDonagh, A. F. Tetrahedron 2000, 56, 7869-7883.
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propionic acid chains,18 which increased the pigment’s polarity and decreased its lipophilicityseven making the former watersoluble. In our quest to obtain a truly amphiphilic bilirubin, more recently we synthesized a water-soluble bilirubin attached to polyethyleneglycol (PEG), where the pigment’s intramolecular hydrogen bonding ability and intrinsic lipophilicity remained unperturbed.19 However, except at pH
