Amino Acid Structures

Abstract

Although this book deals with salts of amino acids, this chapter discusses the structures of pristine amino acids, since these molecular structures are also found in salts, and factors as solubility and conditions of crystal growth are important for the synthesis of amino acid salts as well as the pure form. In this context, the impact of minimal changes in conditions (such as impurities) on the growth of amino acid crystals is noted. The structural variation is very high, as not only amino acids differ from each other, but many amino acids exist in more than one form. This refers to the fact that enantiopure crystals can be grown as well as racemates (so-called dl-amino acids). Moreover, often more than one hydration state is found: Anhydrous forms are common, but many amino acids form hydrated crystals. For some amino acids (e.g., glycine, proline, methionine), more than one polymorph of the same hydration state is found. Some of these polymorphs form at ambient condition, often due to minuscule changes in conditions. For others, variation in temperature and pressure has been found to be the cause of the formation of different polymorphs. High-pressure data are available for some amino acids (e.g., alanine, serine, cysteine). Most amino acids are found to form a so-called head-to-tail motif, where acid and amino groups connect to form infinite chains. In some of the larger, nonpolar amino acids, a bilayer pattern is found, where polar groups of opposing molecules face each other, forming a layer, with the hydrophobic side chain facing outward (this motif is found in phenylalanine, methionine, leucine, or isoleucine). Not all amino acids crystallize readily, as is proved by the crystal structure of l-arginine, which could be determined only very recently, and that of lysine, which could not be solved at all to date. In addition to the standard 20, some nonstandard amino acids are discussed. Finally, notes on remarkable physical effects (such as piezoelectric data) or possible applications (as for instance the interactions of amino acids with carbon nanotubes) are presented.