Thermodynamic and structural studies on the complexation of guanidino-appended α-cyclodextrin derivatives with p-nitrophenolate ion

Abstract

Thermodynamic parameters (ΔG, ΔH, and TΔS) for the complexation of mono(3-deoxy-3-guanidino-altro)-α-cyclodextrin (1) or mono(3-deoxy-3-guanidino)-α-CD (2) with p-nitrophenolate ion were determined by 1H NMR titration experiments in D2O solution containing 0.10 mol dm−3 Na2CO3–NaHCO3 at 298, 308, 318, and 328 K, to investigate the effect of the macro ring flexibility of 1 on the complexation with the planar guest. Both ΔH and TΔS values for the inclusion complexation of 1 with the guest (−39.6 kJ mol−1 and −23.3 kJ mol−1, respectively) decreased more than those for 2 with the guest (−27.6 kJ mol−1 and −10.0 kJ mol−1, respectively). This fact suggested that the degree of freedom of the guest and the flexibility of the macro ring of 1 were decreased by the tight binding of the guest in the manner of induced fit. The existence of anisotropic ring current effect by the guest on the 1H NMR signals for C(3)- and C(5)-H's of 1 confirmed that the molecular rotation of the guest is retarded in the complex of 1 with p-nitrophenolate ion. The molecular orientation of the guest in the inclusion complex with 1 was determined by means of ROESY (rotating-frame overhauser effect spectroscopy) experiment.