Crystalline Features of Chitosan-L- and D-Lactic Acid Salts

The crystal structures of chitosan-L- and D-lactate salts were studied by X-ray diffraction measurements on fiber diagrams. In each lactate, chitosan took on a different crystalline polymorph depending on the preparation temperature. At low temperature, they gave a similar fiber pattern to that of the type II salt which has been found to be one of the two forms of chitosan acid salts in which the backbone chitosan molecules take on an eight-fold helix. At high temperature, however, the fiber pattern was that of the type I salt, another form of chitosan salt in which the backbone chains apparently retain the 21 symmetry of chitosan itself. The high-temperature polymorph of the L-lactate was a monoclinic (pseudoorthorhombic) unit cell whose lattice parameters were a=10.51, b=10.85, c(fiber axis)=10.34 Å and γ=90°. That of the D-lactate was also a monoclinic cell having parameters a=11.20, b=11.60, c(fiber axis)=10.38 Å and γ=93.0°. Their unit cell volumes coupled with their observed density values indicate that two chains of chitosan lactate were accommodated in each unit cell, that the L-lactate was an anhydrous crystal, but that the D-lactate was hydrated. The preparation temperature at which the salt changed from type II to type I was different between the D- and L-lactate, suggesting that these acids had different affinity to the chitosan molecule. When chitosan powder was suspended in a racemic lactic acid solution, the resultant solution always showed a minus sign for the rotation angle, indicating that D-lactic acid had higher affinity to chitosan than the L-isomer.