Solvent-free crystallizations of amino acids: The effects of the hydrophilicity/hydrophobicity of side-chains

This work studied the self-assembling (crystallizing) behaviour of amino acids in the absence of solvent and additives (by sublimation and deposition in vacuum), instead of from aqueous solution. It is found that the hydrophilicity/hydrophobicity of side-chains can significantly affect the crystallization of amino acids in the absence of solvent. Crystal structures of amino acids having hydrophobic side-chains (l-valine, l-leucine, l-isoleucine and l-methionine) obtained from sublimation are the same with those obtained from aqueous solution. New polymorphs for six amino acids are thought to have been obtained, based on X-ray diffraction and IR data for three of them (l-tyrosine, l-Phyenylalanine and l-tryptophan), and just IR data for the other three (l-alanine, l-proline and l-threonine).