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. 2022 Apr 22;12(1):6614.
doi: 10.1038/s41598-022-10546-9.

Melanin is a plenteous bioactive phenolic compound in date fruits (Phoenix dactylifera L.)

Muneeba Zubair Alam  1 , Tholkappiyan Ramachandran  2 , Asha Antony  3 , Fathalla Hamed  2 , Mutamed Ayyash  1 , Afaf Kamal-Eldin  4
Affiliations

Melanin is a plenteous bioactive phenolic compound in date fruits (Phoenix dactylifera L.)

Muneeba Zubair Alam et al. Sci Rep. .

Abstract

Date palm fruits (Phoenix dactylifera L.) were found to contain high levels of allomelanin (1.2-5.1%). The melanin is localized in the tanniferous cells between the inner and outer mesocarp tissues of the fruit. The melanin, extracted with 2 M sodium hydroxide, consisted of amorphous graphene-like granular structures of irregular shape and variable size. The date fruit melanin mainly comprises carbon (64.6%) and oxygen (30.6) but no nitrogen, and was thermally stable. It has radical scavenging (63.6-75.1 IC50, µg/mL), antimicrobial (250-1000 µg/mL), hypoglycemic (51.8-58.2%), and angiotensin-converting-enzyme inhibitory (65.8%) effects. The high level of melanin in date fruits highlights the importance of investigating its dietary intake and its impact on nutrition. This study also suggests that date fruit melanin can be a functional ingredient in foods, food packages, pharmaceuticals, and cosmetics.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1
Absorption spectra of date fruit melanin (a) UV–visible and (b) Fourier-transform infrared (FTIR).
Figure 2
Figure 2
Localization of melanin in mature date fruits, (a) Scanning electron microscopic (SEM) image showing the tanniferous cells between the outer and inner mesocarp, and (b) Light microscopic images stained with 4-dimethyl amino cinnnamaldehyde (DMACA, left) and ferric chloride/potassium ferric cyanide (right) showing the presence the localization of melanin/proanthocyanidins in the tanniferous cells layer.
Figure 3
Figure 3
Levels of melanin in twelve date fruit varieties (bars represent standard deviation).
Figure 4
Figure 4
Date fruit melanin particle characteristics, (a) Scanning electron microscopic (SEM) Images of melanin particles at different magnification, (b) Sorption isotherm, (c) Relative Barret-Joyner Halender (BJH) pore size calculated from the sorption isotherm, (d) X-ray diffraction (XRD) profile showing the amorphous nature of the melanin. The crystalline components are artifacts from the sample preparation.
Figure 5
Figure 5
The elemental composition of date fruit melanin, (a) X-ray photoelectron spectroscopy (XPS), and (b) Energy dispersive spectroscopy (EDS), both showing carbon and oxygen as main components and small amounts of sodium and chlorine.
Figure 6
Figure 6
Date fruit melanin thermogravimetric analysis (TGA) represented by dashed line and differential thermogravimetry (DTG) by solid line.
Figure 7
Figure 7
A typical HPLC separation of date fruit melanin on C18 reversed phase column using water: methanol; acetic acid (90: 10: 1, v/v/v) at o.2 mL/min as mobile phase and detection at 280 nm. Four peaks were detected with the last one eluting as a broad peak.
See this image and copyright information in PMC

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