Fluorometric and theoretical studies on inclusion complexes of β-cyclodextrin and D-, L-phenylalanine

Inclusion complexes of β-cyclodextrin (β-CD) with L- and D-phenylalanine (Phe) have been characterized in solution by fluorometry and in gas phase by semiempirical PM3 calculations. The unimolar stoichiometric ratio of both β-CD-L-Phe and β-CD-D-Phe complexes and the stability constants (K) were deduced from fluorometric titrations. The β-CD-L-Phe complex is more stable than the β-CD-D-Phe complex as indicated by the larger K values, 21.1 vs. 6.86 M(-1). This is consistent with the stabilization energies (ΔE(stb)) and inclusion geometries obtained from PM3 calculations. The β-CD-L-Phe complex with L-Phe residing in the central β-CD cavity and pointing its COOH group downwards to the O6 end has ΔE(stb)=-62.7 kJ mol(-1), whereas the β-CD-D-Phe complex with D-Phe placing at 3Å beneath the β-CD O4-plane and pointing its COOH group upwards to the O2/O3 end has ΔE(stb)=-53.3 kJ mol(-1). The unison of host-guest intermolecular hydrogen bonds, hydrophobic interactions and molecular deformations plays an essential role in forming and stabilizing the inclusion complexes. Our results show that the β-CD-L-Phe and β-CD-D-Phe inclusion complexes are relatively stable and differentiable, suggesting the applications of CDs in foods and drugs.