The binding properties of cucurbit[6]uril towards various peptides have been investigated in acidic aqueous solution. Stability constants and thermodynamic values of the complex formation between following peptides: glycyl-L-alanine, L-leucyl-L-valine, glycyl-L-asparagine, L-leucyl-L-phenylalanine, L-leucyl-L-tryptophan, glycyl-L-histidine, L-glutathione reduced (gamma-L-glutamyl-L-cysteinyl-glycine, GSH), and DL-leucyl-glycyl-DL-phenylalanine) with cucurbit[6]uril in aqueous formic acid (50%, v/v) have been calculated from calorimetric titrations. From these results it can be seen that the peptides form exclusion complexes with cucurbit[6]uril. Due to the polar peptide bond they are not included within the hydrophobic cavity of cucurbit[6]uril. The complex formation is favoured by entropic contributions. The release of water molecules from the polar amino groups of the peptides and the carbonyl groups of cucurbituril are responsible. (c) 2005 Elsevier B.V. All rights reserved.