Lawsone

October 14, 2024
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Lawsone (2-hydroxy-1,4-naphthoquinone; aka hennotannic acid) is a dye found in the leaves of the henna plant (Lawsonia inermis). Solid lawsone is yellow; but when it or powdered henna leaves are dispersed in water or other aqueous materials to form a paste, it becomes a deep reddish-brown.

Henna has been used for thousands of years to dye skin and hair. A paste is prepared from powdered henna leaves, water, and often “sticky” materials such as sugar to make it stay on the skin. The paste must be stored for 2–6 days to allow the lawsone to leach from the powder. After the paste is applied, it must stay on the skin for 4 h to overnight to create the dark color the individual wants. Then the paste is removed with a nonaqueous liquid—not water, which interferes with stain development.

Lawsone has also been investigated for reasons other than henna use. In 1949, Harold E. Zaugg at Abbott Research Laboratories (then in North Chicago, IL) reported that the Michael addition of lawsone and benzalacetone1 produces an adduct that can be further cyclized to a ketal. Zaugg’s objective was to develop compounds for use in antimalarial studies; however, neither product was active to the malaria parasite Plasmodium lophurae.

Two much more recent studies had better outcomes for lawsone. In 2022, Alfonso T. García-Sosa at the University of Tartu (Estonia); Anne Robert and Michel Baltas at Laboratoire de Chimie de Coordination du CNRS-UPR 8241 (Toulouse, France); and collaborators at these and other institutions described anti-infectious agents synthesized via the reaction of lawsone with various aldehydes and isocyanides under microwave radiation. Many of the compounds exhibited moderate-to-good activity against the pathogens Plasmodium falciparum, Leishmania donovani, and Mycobacterium tuberculosis.

And last November, Patrick A. Ward at Savannah River National Laboratory (Aiken, SC), Joseph A. Teprovich, Jr., at California State University Northridge, and colleagues at both institutions reported that highly fluorescent lawsone polymer dots (LPDs) have excitation-independent emission with well-defined electronic transitions and a high quantum yield of 37%. The researchers also prepared a water-soluble LPD derivative and demonstrated its feasibility as a fluorescence microscopy dye to image breast cancer cells.

1. CAS Reg. No. 937-53-1; now called benzylideneacetone.

Lawsone hazard information*

Hazard class**GHS code and hazard statement
Acute toxicity, oral, category 4H302—Harmful if swallowedChemical Safety Warning
Skin corrosion/irritation, category 2H315—Causes skin irritationChemical Safety Warning
Serious eye damage/eye irritation,
category 2A
H319—Causes serious eye irritationChemical Safety Warning
Specific target organ toxicity,
single exposure, respiratory tract irritation, category 3
H335—May cause respiratory irritationChemical Safety Warning

*Compilation of multiple safety data sheets.
**Globally Harmonized System (GHS) of Classification and Labeling of Chemicals. Explanation of pictograms.

MOTW update

Methanol1 was the Molecule of the Week for March 11, 2013. The simplest alcohol, it is a widely used solvent, motor fuel, ethanol denaturant, and especially a feedstock for manufacturing other chemicals.

Methanol is toxic: During Prohibition, many people died from drinking methanol-laced liquor; and this type of poisoning is still widespread today. Recently, Douglas J. Lanska at the University of Wisconsin–Madison and others reported that methanol’s metabolites in the body are actually the true poisons. The principal culprits are formaldehyde and formic acid, which are produced by the action of liver enzymes on methanol. These metabolites can cause eye and brain damage and metabolic acidosis.

1. CAS Reg. No. 503612-47-3.

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Lawsone fast facts

CAS Reg. No.83-72-7
SciFindern name1,4-Naphthalenedione, 2-hydroxy-
Empirical
formula
C10H6O3
Molar mass174.15 g/mol
AppearanceYellow crystals
or powder
Melting
point
195 °C (dec.)
Water
solubility
1.8 g/L
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