Thermal phase transitions of solid chiral N,N′-carbonyl-bis-(l-amino acids) and their methyl and benzyl esters

Compounds derived from different N,N′-carbonyl-bis-(l-amino acids) and their methyl and benzyl esters were synthesized and characterized by elemental analysis, infrared and nuclear magnetic resonance. The amino acids used were valine, leucine, phenylglycine and phenylalanine. All compounds revealed complex thermal behaviour as proved by differential scanning calorimetry, X-ray powder diffractometry and optical birefringence observation by polarizing microscope. Above isotropization temperature N,N′-carbonyl-bis-(l-amino acids) decomposed. The number and kinds of thermal phase transitions of investigated esters vary from a simple phase transition and melting to a complex polymorphism, and strongly depends on molecular structure. One to four phase transitions have been observed upon heating. Phase transition temperatures showed considerable variation with choice of the supstituent on symmetric carbons and therminal carboxylic groups. The results are discussed in terms of the architecture of investigated molecules that hinder mesomorphism.