A natural crystalline urobilinogen composed of d- and l-components of differing molecular weight.

In 19661 we described the isolation of a crystalline optically inactive urobilinogen (Ubg) from feces (case M). The crystals were homogeneous in habit and in other physical characteristics were identical with those of synthetic mesobilirubinogen (Mbg).2 On (lehydrogenation of the natural Ubg with iodine, the resulting urobilin yielded optically active components-one, strongly dextrorotatory which crystallized, the other, levorotatory, remaining in the mother liquor."" The dextrorotatory crystals exhibited an FeCl3 oxidation pattern of a dipyrrolinone or pure glaucobilin (GI) type, now recognized as characteristic of urobilins of 588 or 590 molecular weight.3 The i-component remaining in the mother liquor exhibited an entirely different FeCl3 oxidation pattern of Mv-S type: mesobiliviolin (Mv), and S or unreacted pigment with absorption at 490 m,u (S = stercobilin, the dipyrrolidone prototype of stability toward FeCl3 oxidation). The basis of this behavior was unexplainied. M\1ass spectral analysis was not then available to us, nor was it possible to crystallize the small amount of levorotatory urobilin in the mother liquor. It seemed unlikely that the two optically active componienits represented enantiomers, as the marked differenee in FeCl3 oxidation pattern would be incompatible with a natural racernie mixture of RR and SS forms." 4 Sueh a natural racemie urobilin of moleeular weight 590 has now been isolated repeatedly, and it is elear that both dand i-forms are of dipyrrolinone or GI type on FeCl3 oxidation.5 It has also been shown that on further alkaline racemization of this eompound to yield RS,SR as well as RR,SS diastereoisomers, the behavior toward FeCl3 oxidation is unehanged; henee, the differing FeCl3 patterns of the optically aetive urobilins from case 1\1 are not related to the presenee of RS,SR diastereoisomers. At the time of the earlier paper' this eharaeteristie had not been ascertained. The possibility was then eonsidered that the Ubg of case M was composed of optieally aetive components of differing molecular weight. Recently, we described a new levorotatory urobilinoid of molecular weight 592, which was given the t:rivial name "half-stercobilin" (i-HS)6 because of its relative stability to FeCl3 oxidation. and other evidence that it is a pyrrolinonepyrrolidone type; in other words, one of its end rings is saturated as in 1-stereobilin (a dipyrroli.done), the other unsaturated as in. dor i-urobilin. (dipyrrol.inonles). The behavior and properties of I-HS were reminiscent of the levorotatory component of the above urobilin obtained from the crystalline Ubg of case 1\Y. The question arose as to whether crystals of this chromogen included two components, one of molecular weight 592 yielding d-urobilin of 590, the other of molecular weight 594, yielding the half-stercobilin 592 just referred to.