Conformations and lipophilicity profiles of some cyclic β-(1→3)- and β-(1→6)-linked oligogalactofuranosides

The conformational features of cyclooligosaccharides composed of β-(1→3)- and β-(1→6)-linked galactofuranose units, i.e., cyclo[d-Galfβ-(1→3)]n with n=4 (1) and 5 (2), and cyclo[d-Galf β-(1→6)]n with n=3 (3) and 4 (4), were investigated by means of Monte Carlo simulations. The flexibility of the macrocyclic backbone strongly favors bent and asymmetrical conformations over round geometries. Generation of the molecular surfaces of the global minimum-energy structures reveals disk-type shapes for 1–4 without through-going central cavities, yet distinct indentations close to the O-2/O-3 groups, respectively. The molecular lipophilicity patterns prove these surface dents to be hydrophobic for the β-(1→6)-linked cyclogalactans 3 and 4, whereas their β-(1→3)-linked counterparts display an inverse situation with a hydrophobic outer core structure.