The geometry of the cyclodextrins enable them to format inclusion complexes with a variety of substances. Compounds or compounds with groups that are hydrophobic can be included into the hydrophobic cavity of the cyclodextrins, with limitation to the cavity dimensions3,4. This complex binding ability of cyclodextrins can be used for paramedical applications, with advantages in enhanced stability, solubility, and elongated shelf life for proteins and peptides2,4. Cyclodextrins has in several studies shown stabilising effects on proteins1. The effects of cyclodextrins on human insulin stability, solubility and delivery behaviour have been studied previously5. The present study is aiming at investigating the nature of the cyclodextrin/insulin complex in order to clarify whether cyclodextrins are able to form complexes with the aromatic amino acid residues on the protein surface. NMR structures of the insulin monomer reveal that tyrosine residues are water exposed, which could explain the interaction of ß-cyclodextrin and insulin, at low concentrations and pH where insulin exists as monomer. Results from steady state fluorescence spectroscopy shows that ß-cyclodextrins increase the fluorescent yield of insulin, which suggests an interaction between the two compounds. Several other studies have shown increased fluorescent yields in the presence of cyclodextrins. A likely model for the interaction between ß-cyclodextrin and insulin involves encapsulation of surface positional tyrosines with the cyclodextrin. The resulting exclusion of water from the immediate tyrosine neighbourhood leads to the observed increase in fluorescence yield.

Figure 1 Likely model for the interaction between
ß-cyclodextrins and insulin tyrosine residude.

[1] Brewster M. E., Hora S. M., Simpkins J. W., and Bodor N., (1991), Use of 2-Hydroxypropyl-b-cyclodextrin as a Solubilising and Stabilising Excipient for Protein drugs. Parma. Res., Vol. 8, NO. 6

[2] Cooper A., (1992), Effect of Cyclodextrin on the Thermal Stability of Globular Proteins. J. Chem. Soc. 114, 9208-9209

[3] Starnes R. L., (1990), Industrial Potential of Cyclodextrin glycosyl transferases. Cereal and Food World Vol.35 NO. 11

[4] Szejtli J., (1994), Medical Application of Cyclodextrins. Med. Research Reviews, vol. 14, NO. 3, 353-386 [5] Tokihiro K., Irie T., and Uekama K., (1997), Varying Effects of Cyclodextrin Derivatives on Aggregation and Thermal Behaviour of Insulin in Aqueous Solution. Chemical Pharmaceutical bulletin 45(3) 525-531