SEMICRYSTALLINE AND SUPERLATTICE STRUCTURES IN BINARY MIXTURE OF LONG CHAIN n-ALKANES C194H390 AND C294H590

Binary mixtures of long chain n-alkanes from C₁₂₂H₂₄₆ to C₂₉₄H₅₉₀ have been found to form solid solutions despite their large chain length differences [Zeng and Ungar. Novel Layered Superstructures in Mixed Ultralong n-Alkanes. Phys. Rev. Lett. 2001, 86, 4875–4978]. In this article we describe a study of the binary mixture of C₁₉₄H₃₉₀+C₂₉₄H₅₉₀ (50:50 w/w) using small angle x-ray scattering. The molecular chain length difference between the two components is 100 C-atoms, the largest so far studied. In accordance with the findings on some other binary mixtures, two types of lamellar structures are found: the semicrystalline form (SCF) at high temperatures (>105°C) and the triple-layer superlattice at low temperatures (<95°C). The SCF consists of alternating crystalline and amorphous layers: C₁₉₄H₃₉₀ molecules are fully crystallized in the crystalline layer while C₂₉₄H₅₉₀ molecules traverse the crystalline layer and are only partially crystalline; their protruding tails, or cilia, constitute the amorphous layer. The superlattice is a periodic 1-D array of triple-layer units: the two outer layers in the unit contain a mixture of C₁₉₄H₃₉₀ and C₂₉₄H₅₉₀ while the surplus tails of C₂₉₄H₅₉₀ coalesce and interdigitate in the center and form the third, thinner crystalline layer. In the superlattice form, the unusual diffraction order dependence of the linewidth is interpreted in terms of a particular type of stacking faults.     

Stereochemistry of charged nitrogen-aromatic interactions and its involvement in ligand-receptor binding

Summary Recently, new evidence was found for the involvement of charged nitrogen-aromatic interactions in ligand-receptor binding. In this study we report two favourable orientations of a phenyl ring with respect to a R-N⁺(CH₃)₃ group, based on crystal structure statistics from the Cambridge Structural Database. In the first orientation, the phenyl ring is situated in between the substituents at about 4.5 Å from the nitrogen atom, and the ring is approximately oriented on the sphere around the nitrogen atom. In the second orientation, the phenyl ring is situated in the same direction as one of the N-C bonds at about 6.0 Å from the nitrogen atom, and the ring is tilted with respect to the sphere around the nitrogen atom. The same two orientations were also found in the crystal structures of three ligand-receptor complexes, which implies that these orientations probably play a major role in molecular recognition mechanisms.