Add time:10/01/2019         Source:sciencedirect.com

The characteristic thermodynamically-favored intramolecularly hydrogen-bonded conformation adopted by bilirubin pigments is destabilized by substituting methyl groups on the C(10) central methylene. These methyl groups impose conformation-destabilizing methyl-methylene non-bonded steric interactions with the propionic acid β-CH2 groups at C(8) and C(12) when the propionic acids are engaged in intramolecular hydrogen bonding with the opposing dipyrrinones. Amphiphilic 10,10-dimethylbilirubins (1) and (2) are found to be more polar, but also more soluble than the parents (3) and (4) in organic solvents; yet, 1H-NMR studies in non-polar solvents indicate that a deformed but folded, intramolecularly hydrogen-bonded conformation is retained. The dimethyl esters of 10,10-dimethylbilirubins 1 and 2 did not exhibit the typical strong tendency of bilirubin dimethyl esters to form intermolecular hydrogen bonds in non-polar solvents such as chloroform and benzene.

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