Rediscovering Pasteur's Quasiracemates

Abstract

Neu aufgerollt: Durch Wiederholung eines von Pasteur 1853 beschriebenen Experiments gelang es, zwei kristalline Phasen als Ammonium‐(+)‐hydrogentartrat (Mittelteil der gezeigten Kristalle) und quasiracemisches Ammonium‐(+)‐hydrogentartrat/(−)‐hydrogenmalat (Stirnseiten der Kristalle) zu identifizieren. Das Quasiracemat besteht aus annähernd inversionssymmetrischen Molekülanordnungen, in denen die Hydrogentartrat‐ und Hydrogenmalat‐Komponenten ein homomeres Catemermotiv bilden.

Design, Total Chemical Synthesis, and X‐Ray Structure of a Protein Having a Novel Linear‐Loop Polypeptide Chain Topology

“…Quasi-racemate crystals of small organic molecules were first described by Pasteur in 1853, [32] and their properties have more recently been explored by Curran [33] and Wheeler. [34] The quasi enantiomers that were used to form these crystals are related to each other by an almost mirror-image relationship and can be converted to the true enantiomers by slightly changing the chemical composition of one or more atoms or functional groups. Although small-molecule quasi-racemates are widely used in asymmetric organic synthesis and for organizing supramolecular arrays, utilization of macromolecular quasi-racemates was demonstrated only recently when we found that the incorporation of an additional -Se-CH 2 -in only the l enantiomer of snowflea antifreeze protein still facilitated crystallization with the d-protein molecule as was seen for the true racemate.…”

Failures of fractional crystallization: ordered co-crystals of isomers and near isomers

“…The advantages of removing the requirement that molecules be related by rotations of 2/n, n = 1, 2, 3 or 6, are more important than the disadvantages of raising the number of different intermolecular contacts. The experiments of Wheeler with carboxylic acids and amides (Wheeler et al, 2008;Breen et al, 2008;Lineberry et al, 2008, and references therein), and those of Gö rbitz with amino acids (Dalhus & Gö rbitz, 1999a,b,c, 2000Gö rbitz et al, 2009) suggest that single crystals of quasiracemates are not difficult to grow, at least as long as the constituent molecules form good hydrogen bonds. Some inorganic examples have been characterized by Englert (Englert et al, 2002;Calmuschi et al, 2004, and references therein).…”

Structure Elucidation of Host–Guest Complexes of Tartaric and Malic Acids by Quasi‐Racemic Crystallography

“…L ‐malic acid ( L ‐ 4 ) differs from L ‐tartaric acid ( L ‐ 2 ) by the lack of one hydroxy group and by the absolute configuration of its only stereogenic center. The two acids are thus quasi‐enantiomers and their successful combination in quasi‐racemic crystals was demonstrated by Pasteur long ago9a and recently fully characterized 9b. We thus endeavored to crystallize both acids in complex with 3 .…”

“…As observed in solution, 4 ‐ l exists in two disordered positions in the solid state, arising from its dissymmetric structure. This disorder relates to the disorder of malic acid in its quasi‐racemic crystals with tartaric acid 9b…”