The Structure of Crystalline Tris: A Plastic Crystal Precursor, Buffer, and Acetylcholine Attenuator

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Abstract
The crystal and molecular structure of the widely used buffer tris(hydroxymethyl)aminomethane (tris) has been determined from single-crystal diffractometer data to a standard agreement factor ( R value) of 0.026 and bond length standard deviations of 0.002 angstrom. Tris crystallizes in the orthorhombic system, space group Pn 2 1 a , with four molecules per unit cell; a = 8.844(1) angstroms, b = 7.794(1) angstroms, and c = 8.795(1) angstroms. The center-to-center distances of tris molecules in the ordered phase range from 0.4 to 1.0 angstrom less than they do in the orientationally disordered (plastic) phase of similar molecules.