Abstract
The crystal structures of the inclusion complexes of naringenin in dimethylated and permethylated β-cyclodextrin (DM-β-CD and TM-β-CD) were determined and extensively analyzed. Naringenin is found with its 4-hydroxyphenyl residue fully immersed in the DM-β-CD cavity and its chromone group protruding from the narrow rim of the open-cone shaped host. The naringenin/DM-β-CD complex units are packed in a ‘herring bone’ fashion. In the case of naringenin/TM-β-CD, the complex units are arranged in a cage-type mode, the guest naringenin is partially encapsulated in the cavity of the closed-cone shaped host, with its chromone group laying equatorially and its 4-hydroxyphenyl protruding extensively from the wide rim of the host. Furthermore, the crystallographically-determined coordinates of both complexes were employed for Molecular Dynaimics simulations in explicit water solvent and in the absence of crystal contacts. The trajectories showed that naringenin rapidly penetrates the open narrow rim of DM-β-CD but not the closed narrow rim of TM-β-CD. Thus, in the latter case, the chromone group of naringenin is accommodated shallowly in the wide rim of the host, tethered via hydrogen bonds to the secondary methoxy groups of the host. Finally, a significantly higher binding affinity for naringenin in DM-β-CD than TM-β-CD was estimated by Molecular Mechanics/Generalized Born Surface Area calculations.
Highlights
Naringenin ((±)-2,3-dihydro-5,7-dihydroxy-2-(4-hydroxyphenyl)-4H-1-benzopyran-4-one) or (5,7-dihydroxy-2-(4-hydroxyphenyl)chroman-4-one) (Figure 1a) is the aglycone part of naringin, which is a flavonoid glycoside mainly found in citrus fruits [1]
The naringenin/DM-β-CD inclusion complex crystallizes in the P21 21 21 space group, its asymmetric unit containing one DM-β-CD, one naringenin, and 4.6 water molecules disordered over 9 sites
The guest molecule was inserting from the primary rim of the host with its 4-hydroxyphenyl residue accommodated in the host cavity and its chromone group protruding from the narrow rim of the rigid, open-cone-shaped host
Summary
Naringenin ((±)-2,3-dihydro-5,7-dihydroxy-2-(4-hydroxyphenyl)-4H-1-benzopyran-4-one) or (5,7-dihydroxy-2-(4-hydroxyphenyl)chroman-4-one) (Figure 1a) is the aglycone part of naringin, which is a flavonoid glycoside mainly found in citrus fruits [1]. The shape and size of the hydrophobic β-CD cavity enables the formation of inclusion complexes with a variety of suitable guest molecules of food, agricultural, and pharmaceutical interest [9,10,11,12]. The low Crystals 2020, 10, 10; doi:10.3390/cryst10010010 www.mdpi.com/journal/crystals offers unique information about the relative atomic positions of the guest and host molecules, revealing the inclusion geometry, the intra- and inter-molecular interactions, and the molecular arrangement of the complex units in the crystalline state. The simulations were analyzed with the Crystals 2020, 10, 10 aim to (a) gain insight into the dynamic behavior and the stability of the inclusion complexes in aqueous ambient and in the absence of crystal contacts, (b) monitor the main host-guest interactions β-CDare solubility is significantly increased the(c)methylation two or three hydroxyl groups of that responsible for the binding modes,by and observe anyofinterconversion of inclusion modes each glucopyranose unit
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