Abstract:

Dilution enthalpies of some aromatic amino acids such as D/L-α-tryptophan, L-α-tryptophan, L-α-tyrosine and L-α-phenylalanine in aqueous solutions at 298.15 K were determined by sensitive mixing-flow microcalorimetry. A chemical interaction model for quasi-isodemic self-stacking was proposed and used to process the calorimetric data from which the model parameter K△Hm was calculated. The chemical interaction parameter (K△Hm) agrees well with and provides good insights into the pairwise enthalpic interaction coefficient (hxx) in the McMillan-Mayer approach for the existence of the equation K△Hm=hxx. Combined with results from literature we considered that aromatic π-πself-stacking is essentially a kind of special hydrophobic interaction manifesting commonly as an endothermic effect. Noteworthy effects arising from substituent hindrance, electrostatic interaction, hydrogen bonding and chiral recognition which are directed away from the aromatic core exert on aromatic π-πself-stacking. In nature, the composite parameter K△Hm describes a complex effect between the equilibrium and an enthalpic change of aromatic π-πself-stacking.

Key words: Aromatic amino acid, π-πself-stacking, Dilution enthalpy, Microcalorimetry, Chemical interaction model