Structural insight into cocrystallization with zwitterionic co-formers: Cocrystals of S-naproxen

The screening of S-naproxen, S-oxiracetam, S-diprophylline, and levetiracetam with a series of essential and nonessential amino acid co-formers has yielded cocrystals only for S-naproxen, thus showing that amino acids seem to have a preference for forming cocrystals with compounds containing a carboxyl group. Herein, we report the crystal structures of four S-naproxen cocrystals: S-naproxen/l-alanine, S-naproxen/d-alanine, S-naproxen/d-tyrosine, and S-naproxen/d-tryptophan monohydrate. All of the described cocrystals show similar structural motifs, i.e., amino acids form head-to-tail chains with strong charge-assisted hydrogen bonding, which are similar to those found in the individual amino acids, with S-naproxen molecules grafted on them. According to the systematic search of the Cambridge Structural Database for other cocrystals that involve zwitterionic co-formers, charge-assisted hydrogen bonds between amino acid molecules play an essential role, being present in the majority of structures. The results of this work provide an insight into structural aspects of cocrystallization with zwitterionic co-formers, offer new possibilities for S-naproxen pharmaceutical formulations, and can serve as guidelines when developing new cocrystals involving zwitterionic co-formers. This journal is