Molecular aggregation in selected crystalline 1:1 complexes of hydrophobic d‐ and l‐amino acids. IV. The l‐phenylalanine series

The amino acid l ‐phenylalanine has been cocrystallized with d ‐2‐aminobutyric acid, C₉H₁₁NO₂·C₄H₉NO₂, d ‐norvaline, C₉H₁₁NO₂·C₅H₁₁NO₂, and d ‐methionine, C₉H₁₁NO₂·C₅H₁₁NO₂S, with linear side chains, as well as with d ‐leucine, C₉H₁₁NO₂·C₆H₁₃NO₂, d ‐isoleucine, C₉H₁₁NO₂·C₆H₁₃NO₂, and d ‐allo‐isoleucine, C₉H₁₁NO₂·C₆H₁₃NO₂, with branched side chains. The structures of these 1:1 complexes fall into two classes based on the observed hydrogen‐bonding pattern. From a comparison with other l :d complexes involving hydrophobic amino acids and regular racemates, it is shown that the structure‐directing properties of phenylalanine closely parallel those of valine and isoleucine but not those of leucine, which shares side‐chain branching at C^γ with phenylalanine and is normally considered to be the most closely related non‐aromatic amino acid.