The nature of the strong interaction between biotin (1) and the glycoprotein avidin has been examined by X-ray analysis of two model compounds containing the biotin ring system separated by a trimethylene bridge from an indole ring that represents the tryptophan molecules present in the binding site of avidin. Crystals of Biot-C3-Ind (2) hemihydrate are orthorhombic (P212121) with a = 46.229 (13), b = 7.533 (2), and c = 9.012 (2) Å, and the structure containing two molecules (2 and 2′) in the asymmetric unit was refined to an R factor of 0.067 on 2285 nonzero reflections. The crystals of Biot(SO)-C3-Ind (6) are also orthorhombic (P212121) with a = 10.634 (3), b = 13.641 (2), and c = 10.847 (3) Å, and the structure has been refined to an R factor of 0.045 on 1595 nonzero reflections. One of the molecules (2) has a gauche arrangement around the central bond of the trimethylene bridge, while 2′ and 6 have a fully extended arrangement. There is no evidence for a significant intramolecular interaction between the biotin and the indole rings in any of the three molecules, nor is there any evidence for a strong intermolecular hydrogen bonding interaction between the rings. In the crystal of Biot-C3-Ind, one of the molecules, 2′, fits snugly into a hydrophobic V-shaped crevice formed by two indole rings. It is postulated that such an interaction may be significant in the avidin-biotin complex.
|Original language||English (US)|
|Number of pages||9|
|Journal||Journal of the American Chemical Society|
|State||Published - 1979|
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