Effects of cyclic oligosaccharide cyclodextrin (CD) on stability and conformation of lysozyme were clarified thermodynamically and rheologically by DSC, viscosity, and circular dichroism measurements. The modified beta-CD relatively destabilized the folded state of lysozyme by stabilizing the unfolded state due to inclusion of hydrophobic part into the hydrophobic interior of CD. The order of higher destabilization effect was acetyl-beta-CD > methyl-beta-CD > hydroxypropyl-beta-CD. Apparent number of bound CD to unfolded state for methyl-, hydroxypropyl-, and acetyl-beta-CD is 6.7 +/- 0.7, 4.2 +/- 1.1, and 18.6 +/- 4.3 and the binding constant is 5.5 +/- 0.8, 6.7 +/- 2.4, and 4.4 +/- 1.2 Lmol(-1), respectively. The viscosity for unfolded state was increased with an increase in the each modified beta-CD concentration, suggesting that the inclusion of CD on a part of hydrophobic core at unfolded state leads to break the hydrophobic core, then lysozyme would be more spread structure. The substituent of CD can accelerate instability by directly breaking hydrogen bond and/or can restrain instability by increase in hydrophobic interaction. The fact that the each modified CDs has different destabilization effect shows a possibility to control the stability of protein by the substitution of CD. (C) 2011 Elsevier B.V. All rights reserved.