Inclusion complexes of norepinephrine with β-cyclodextrin, 18-crown-6 and cucurbit[7]uril: experimental and molecular dynamics study

The formation of binary and ternary inclusion complexes of norepinephrine, NP, with β-cyclodextrin (βCD), cucurbit[7]uril (CB7) and 18-crown-6 (18C6) have been investigated. Fluorescence spectroscopy revealed the enhancement of emission at around 312 nm from NP in the presence of βCD. However, in the presence of CB7 and 18C6 this band was quenched and the spectra exhibited an excimer emission at longer wavelength due to aggregation of guest molecules. The stoichiometry of the inclusion complexes has been investigated by electrospray mass spectrometry (ESI-MS) revealing the presence of binary and ternary complexes of NP with the hosts studied. On the other hand, the solid complexes prepared by lyophilization were characterized by FT-IR, Raman spectroscopy and powder X-ray crystallography. Our data, taken together, clearly suggest that the three hosts form stable complexes with NP. Additionally, we performed molecular dynamics simulations on these complexes in aqueous media for 15-20 ns. The results obtained indicated that hydrophobic forces together with extensive hydrogen bonding are responsible for the stability of these complexes.