Nanosized titanium dioxide (TiO) particles are commonly present in TiO food additives (E171) and have been associated with potential adverse effects on health. However, little knowledge is available regarding the interactions between TiO nanoparticles (NPs) and other food components, such as flavonoids. In this study, we aim to study the molecular interactions between TiO anatase NPs and three structurally closely related polymethoxyflavones (PMFs, flavonoids found in citrus fruits), namely, 3',4'-didemethylnobiletin (DDN), 5-demethylnobiletin (5DN), and 5,3',4'-tridemethylnobiletin (TDN), using ultraviolet-visible (UV-vis) spectrometry and surface-enhanced Raman spectroscopy (SERS). In the UV-vis absorption spectra, bathochromic effects were observed after DDN and TDN conjugated with TiO NPs. The results from SERS analysis clearly demonstrated that DDN and TDN could bind TiO NPs with the functional groups 3'-OH and 4'-OH on ring B and formed charge-transfer complexes. However, 5DN with functional groups C═O on ring C and 5-OH on ring A could not bind TiO NPs. Knowledge on the molecular interactions between TiO NPs and food components, such as flavonoids, will facilitate the understanding of the fate of TiO NPs during food processing and in the gastrointestinal tract after oral consumption.

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Polymethoxyflavones (PMFs) belong to a unique class of flavonoids mainly found in citrus fruits. Characterization of different PMFs is important to further understand and apply these compounds as functional ingredients in food. The objective of this study is to characterize three monohydroxylated PMFs using surface-enhanced Raman spectroscopy (SERS) and to determine the role of hydroxylation in their SERS behaviors. Serial concentrations of 3′-hydroxylnobiletin (3HN), 4′-hydroxylnobiletin (4HN), and 5-hydroxylnobiletin (5HN) were incubated with silver dendrites for SERS analysis. Results demonstrated that three PMFs exhibited significantly different SERS behaviors. 5HN produced saturation peak intensity at relative low concentration (0.05 mM), while 3HN and 4HN produced saturation peak intensity at much higher concentrations (0.5 and 1 mM, respectively) according to principal component analysis. Below saturation, 5HN had the highest peak intensity, while 3HN had the lowest peak intensity. After reaching saturation, 4HN and 5HN had similar relative peak intensities that were much greater than 3HN. The HPLC analysis revealed that 36.13 ± 1.06% of 5HN, 18.40 ± 3.31% of 4HN, and 9.66 ± 0.94% of 3HN were bound to silver. Based on these results, we speculated that different positions of hydroxylation of PMFs were critical for determining spatial conformation of PMFs on binding sites, resulting in different binding affinities and saturation points, therefore their SERS behaviors. This study first reported that the position of hydroxylation in the monohydroxylated PMFs was crucial for their interactions with silver dendrites and provided valued information for further applying SERS for molecular characterization of flavonoids.

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Polymethoxyflavones (PMFs) are dietary flavonoid found in citrus fruit and have been shown to have various health promoting bioactivities. Our previous research has demonstrated that biotransformation of PMFs by gut microbiome play important role in their bioactivities. For example, glucuronide and sulfate conjugates of PMFs can be deconjugated to produce metabolites with much stronger bioactivities. Herein, we further investigated the biotransformation of PMFs by gut microbiome. Major PMFs, i.e. Nobiletin (NBT) and 5‐hydroxylnobiletin (5HN), and their six metabolites were anaerobically fermented with colonic microbiome freshly isolated from human volunteers. HPLC analysis was utilized to monitor the levels of these PMFs and the production of new metabolites. The results showed that 3’ and 4’‐hydroxylnobiletin can be demethylated to 3’, 4’‐ dihydroxylnobiletin (DHN); while 5, 3’ and 5, 4’‐ dihydroxylnobiletin can be demethylated to 5, 3’, 4’‐ trihydroxylnobiletin (THN) by human colonic microbiome. However, in term of demethylation, colonic microbiome had no effect on NBT, 5HN, DHN and THN. Our results further demonstrated that the extent of demethylation varied considerably among the microbiome isolated from different human subjects. This study provided important leads for further investigation on the bioactivities of PMFs.