Crystal and molecular structure of a 4,5-dimethyoxbilindione derived from etiobiliverdin-IVγ: A possible model compound for the phytochrome chromophore

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The structure of a synthetic bile pigment 3,8,12,17-tetraethyl-4,5- dimethoxy-2,7,13,18-tetramethyl-4,5-dihydrobilin-1,19(21H,24H)-dione, C 33H44N4O4, has been determined using three-dimensional diffractometer data. It crystallizes in the triclinic space group P1 with a = 12.755 (2), b = 12.944(s), c = 12.615(2)A, α = 112.27°(2), δ = 113.76°(2), γ = 75.45°(2), Z = 2. There is one-half of an n-hexane molecule of crystallization present per formula unit. The structure was solved by direct methods and refined by full-matrix least squares to yield a final R factor of 0.105. The compound, which could be considered as a model for the far-red form (Pfr) of the phytochrome chromphore, belongs to the class of bile pigments known trivially as bilipurpurins. Three of the four pyrrole rings form a conjugated system which is not strictly planar, but is quasi-helical in nature. The fourth ring which is joined to the others by an sp3 carbon atom, is considerably out of the plane of the other three. Bond lengths and angles show a large degree of bond fixation. There is intermolecular hydrogen bonding so that the molecules form hydrogen-bonded dimers.

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Journal of the Chemical Society, Perkin Transactions 2

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