In the earlier works, the task of charges in inclusion complexformation of cyclodextrin has been investigated in terms of electrostaticinteractions, hydrogen bonding and steric effects between the host and theguest molecules.
Chosen guest molecules for investigation L- and D enantiomersof amino acids. The latter is particularly interesting from point of view ofthe applications in biological systems. In case of amino acids the stability ofthe associates can be the result of various effects: the hydrophobic part canenter the cavity, whilst the hydrated amino acid moiety remains outside of thering and can take part in electrostatic and hydrogen bonding interactions,depending on the different ionization states according to the pH of solution.On the other hand, enantiomers of these compounds permit of the investigationof chiral recognition. Aim was to investigate the structure of some amino acid &b–cyclodextrincomplex, which has higher stability constant.
Investigation of complexes ofalpha-amino acids by diverse techniques like Potentiometric method and1H and 13C NMR spectroscopy concluded that aminoacids can be classified into three types. Theutmost stability constants have been obtained for phenylalanine, tyrosine and tryptophan, inaccordance with the expectations: the presenceof the aromatic side group is especially favourable for complex formation. Complexes of leucine have also aconsiderable stability in the middlerange. The lowest stabilities are set up for the smallest or most hydrophilic compounds as threonine, andaspartic acid, glutamic acid and histidine.The complexes are characterized by stability constants. Asignificant raise of the stability constants as compared to the nativecyclodextrin has been found only in the case of the anionic forms of tyrosine,L-aspartic acid and L-glutamic acid with quaternary ammonium cyclodextrin. Thiscan be understood considering that in these cases hydrogen bonding is possiblewith both rims of the cyclodextrin, in addition to the improved electrostaticattraction.
This effect can overcompensate steric hindrance of thesubstituents, resulting in deeper penetration of guest moiety. The resultsconfirm deeper insertion of phenolate ring of tyrosine in the cyclodextrincavity and the stronger interaction between the functional groups of the aminoacid anion and the primary alcoholic group(s) of the cyclodextrin 515. A probable structure of thecomplex of L-tyrosine with quatenary ammonium b-cyclodextrin is drawn in the review investigation as
