COMMENT

Yesterday I received the following interesting comment from Emily on “How to dye your hair red with henna.”

Hello… I have been using pure Henna over natural dark blonde hair for over 25 years. I first started applying it when I visited Morrocco and was told to mix the henna powder with two raw eggs and warm water. I have been doing it this way for all my adult life. I love it!!! I don’t wait any time at all after mixing it up, I just apply and use the plastic bag over my head and leave for three hours… it makes me feel so alive… The raw eggs are excellent to keep the mixture from running and adds lustre to your hair. I have never heard of using the lemon juice. I wonder what that is for?
Emily

I started nerding out. Since my response wound up being blog post-size anyways, I gave in and decided to make it its own post.

And blogosphere: If you see anything in need of revision, please correct me.
Biochemical knowledge is always welcome here.

RESPONSE

Hi Emily,

It’s funny you should ask about lemon juice. A few weeks ago, I was curious about the chemistry behind henna’s dying process, so I went on a downright research spree (I even pinned an image of lawsone, the dying agent in henna).

Bear with me and I’ll do some nerdy explaining:

The molecule in henna responsible for dying hair is lawsone, otherwise known as hennotannic acid (or 2-hydroxy-1,4-naphthoquinone by IUPAC standards). Here’s a picture I snagged from Sigma Aldrich’s website:

Lawsone, a.k.a. hennotannic acid

In the world of hair dye, the lawsone of henna is considered a “deposit-only” dye since it doesn’t interact with the melanin of hair strands. As a naphthol molecule, it’s also termed a red coupler in the cosmetics industry. Red couplers produce red pigments on the specific surface they are dying, not in the prepared mixture (a.k.a. dye precursors) itself.

Most melanin is found in the innermost cortex layer of hair, along with keratin. Keratin is key. Lawsone has an affinity for keratin, and interacts with it directly in a non-oxidative process (i.e. a process directed towards the retention of electrons).

The anatomy of a hair strand. Image from Wikipedia.

Lemon juice is a very acidic substance with a pH generally cited between 2 and 2.5. (For comparison, the outer albumen sac of eggs is typically cited as slightly basic, around 7.6, and the inner yolk is cited as around 6.0, slightly acidic.) However, lemon juice is not so acidic that human skin and hair cells are negatively affected. (All things being relative.)

At the heart of it, the acidic functionality of lemon juice serves to destabilize the henna plant’s cellulose while allowing the lawsone molecules to remain intact. Both lawsone and lemon juice are acids, and as such follow the general rule of thumb of “like dissolves like.” The lemon juice attacks the lawsone directly to create a unified acidic substance.

This unified acidic substance comes in handy later when interacting with keratin (discussed below).

When mixing henna with egg, I would predict that the mixture turns brownish, indicating that the red pigment-causing lawsone is binding to or interacting with the protein-dense eggs and basic elements of the plant. (Please correct me if I’m wrong about the color— it is only a prediction. I have not observed it firsthand). It therefore does not act like a dye precursor. When mixing henna with acidic lemon juice, however, there is no color change. The mixture remains green, indicating that the lawsone molecules have not bound to the added reagent (lemon juice instead of egg).

Henna powder does not contain much lawsone (in most peer-reviewed articles it’s measured at less than 2% for pure henna) and the process of breaking down cellulose is lengthy, which is why it takes several hours. (Think of all the extra machinery ruminants need just to do it on a day-to-day basis.) Using eggs as a reagent, I would also predict it takes less time because the reagent is not targeted at degrading cellulose—instead, it is interacting with it at multiple points, creating a conditioning formula that has the ability to bind to multiple proteins within the hair strand. Some lawsone molecules may still be present to bind to keratin, but it is more likely to be inhibited and/or sterically hindered by the other molecules it has interacted with in the egg mixture. In short, eggs decrease the effectiveness of the dying agent, but perhaps not the mixture’s conditioning abilities.

On a slightly related note, this is why henna and placenta conditioner exists. Similar mixture, similar outcome.

When a lemon juice-henna mixture is applied to the hair after several hours, another important reaction occurs. The solution is still hopping with hydrogen atoms after hours; it’s still acidic. These abundant hydrogen molecules serve to reduce (i.e. add electrons to) the disulfide bridges of the composed keratin fibers, which breaks the bridges and “cuts” the keratin fibers up. In its reduced form, sans most bridges, keratin is more accessible. Ideally, each cystine disulfide bridge is broken into two cysteine residues.

The disulfide bridges of keratin can be converted into separate cysteine residues by adding acid (lemon juice). Image from Sigma Aldrich.

Lawsone, a relatively small molecule, then migrates to the cortex of the hair strand to coat the keratin via a Michaels’ reaction. You’ll note this reaction happens quite quickly, making it a somewhat annoying ordeal to remove henna stains from skin, even after only a few seconds of exposure.

Lawsone’s affinity for keratin is intense.

If you’re really curious, you can get a sense of the numerous reactions by drawing out a few of the amino acids (here’s an older article with a list) and lawsone. You’ll note that there are many locations at which a Michaels’ reaction can occur. For your reference, common attacking nucleophiles (a.k.a. Michael donors) include lithium dialkyl cuprates, enamines, ß-diketones, ß-keto esters, ß-keto nitriles, and alpha-nitro ketones, while common electrophiles (a.k.a. Michael acceptors, the alpha,ß-unsaturated carbonyl compounds) include conjugated aldehydes, conjugated ketones, conjugated esters, conjugated amides, conjugated nitriles, and nitroethylenes.

I’d be interested to know if the predictions I mentioned are correct, and how intense a red color you’re getting from using eggs.

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