Abstract
Protocatechuic aldehyde (PCAL) and protocatechuic acid (PCAC) are catechol derivatives and have broad therapeutic effects associated with their antiradical activity. Their pharmacological and physicochemical properties have been improved via the cyclodextrin (CD) encapsulation. Because the characteristics of β-CD inclusion complexes with PCAL (1) and PCAC (2) are still equivocal, we get to the bottom of the inclusion complexation by an integrated study of single-crystal X-ray diffraction and DFT full-geometry optimization. X-ray analysis unveiled that PCAL and PCAC are nearly totally shielded in the β-CD wall. Their aromatic rings are vertically aligned in the β-CD cavity such that the functional groups on the opposite side of the ring (3,4-di(OH) and 1-CHO/1-COOH groups) are placed nearby the O6–H and O2–H/O3–H rims, respectively. The preferred inclusion modes in 1 and 2 help to establish crystal contacts of OH⋅⋅⋅O H-bonds with the adjacent β-CD OH groups and water molecules. By contrast, the DFT-optimized structures of both complexes in the gas phase are thermodynamically stable via the four newly formed host–guest OH⋯O H-bonds. The intermolecular OH⋅⋅⋅O H-bonds between PCAL/PCAC 3,4-di(OH) and β-CD O6–H groups, and the shielding of OH groups in the β-CD wall help to stabilize these antioxidants in the β-CD cavity, as observed in our earlier studies. Moreover, PCAL and PCAC in distinct lattice environments are compared for insights into their structural flexibility.
Highlights
Published: 11 June 2021Protocatechuic acid (3,4-dihydroxybenzoic acid; protocatechuic acid (PCAC), Scheme 1) is a phenolic acid present in various edible and medicinal plants, for example, green tea [1], acai fruit oil [2], roselle [3], and mushrooms [4]
Based on NMR data, Protocatechuic aldehyde (PCAL), caffeic acid (CFA), ferulic acid and vanillin form 1:1 inclusion complexes with β-CD and have a common inclusion geometry with the aromatic ring embedded in the β-CD cavity and the C6 H3 (OH)2 (CHO) and COOH moieties protruded from the O2/O3 end [29]
Β-CD is conventionally numbered as carbohydrates, viz., atoms C63–O63 denote the β-CD is conventionally numbered as carbohydrates, viz., atoms C63–O63 denote the methylene C6−H2 connected to hydroxyl O6−H groups of glucose unit 3 (G3) in the commethylene C6−H2 connected to hydroxyl O6−H groups of glucose unit 3 (G3) in the plex β-CD−PCAL (1), Figure 1
Summary
Protocatechuic acid (3,4-dihydroxybenzoic acid; PCAC, Scheme 1) is a phenolic acid present in various edible and medicinal plants, for example, green tea [1], acai fruit oil [2], roselle [3], and mushrooms [4]. PCAC, PCAL, and other polyphenols like gallic, vanillic, caffeic, and ferulic acids are found in cork [10]. Both catechol-containing compounds exhibit a wide range of pharmacological activities, for example, antioxidant, anti-inflammatory, and anticancer activities for PCAC [11,12,13,14,15,16] and for PCAL [17,18]. The physicochemical properties of polyphenols2(epicatechin, quercetin, CGA, PCAC, CFA, etc.) in plant extracts have been improved via CD inclusion complexation, for example, the thermal stability of H. sabdariffa L. anthocyanin extract β-CD [31], the water of that propolis extract by on β-CD theassay thermal in vitro by of PCAC is ~3 times moresolubility potent than of Trolox, based the [32], DPPH [22].
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