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High-field 13C-NMR of 13C-enriched compounds in dilute aqueous d6-Me2-SO solutions provides a simple, accurate method for measuring pK(a)s of sparingly soluble carboxylic acids. Using this method, we found the pK(a)s of mesobilirubin-XIII alpha to be 4.2 and 4.9, much lower values than reported recently for bilirubin, and of mesobiliverdin-XIII alpha to be 3.9 and 5.3.
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