# The Solubility of Tyrosine

I find very little data on the solubility of tyrosine, other than that it has a very low solubility at room temperature for an AA. (1)

Does anyone know different ways to dissolve fair amounts of it in water (preferred) or an organic solvent?

I suspect I could increase its water solubility by decreasing the $$\mathrm{pH}$$ and increasing temperature, but I'm not sure what is the best way to do it, which would be the most effective, and by how much it would increase the solubility.

• Can either go low in pH or high, depending on what you plan to do with it. Sigma-Aldrich reports 100 mg/mL solubility in 1 M HCl (with heating) and ~4 mg/mL at pH 10. I used to make stocks in 1 N NaOH but I don't remember the max solubility. sigmaaldrich.com/content/dam/sigma-aldrich/docs/Sigma-Aldrich/… – Andrew Jul 11 '20 at 16:20
• Thank you Andrew! – Hans Jul 12 '20 at 8:03

According to Product Information of Sigma-Aldrich for L-Tyrosine, the solubility is $$\mathrm{pH}$$-dependent:

Preparation Instructions: This product is soluble in $$\pu{1 M} \ \ce{ HCl}$$ ($$\pu{100 mg/ml}$$), with heating. The solubility in water ($$\pu{25 ^\circ C}$$) is $$\pu{0.45 mg/ml}$$ in the pH range $$3.2-7.5$$ (Ref.1). At $$\mathrm{pH}$$ $$1.8$$, the solubility is $$\pu{2.0 mg/ml}$$; at $$\mathrm{pH}$$ $$9.5$$, the solubility is $$\pu{1.4 mg/ml}$$; and at $$\mathrm{pH}$$ $$10$$, the solubility is $$\pu{3.8 mg/ml}$$.

The above claims can be justified by the study of the solubility of tyrosine in the acidic and basic conditions (Ref.2), the summery of which states that:

Measurements have been made of the solubility at $$\pu{25 ^\circ C}$$ of tyrosine in hydrochloric acid and in sodium hydroxide solutions varying from $$0.001$$ to $$\pu{0.05 M}$$, and also in distilled water. The $$\mathrm{pH}$$ of the saturated solutions was measured with the hydrogen electrode. The following values for the ionization constants of tyrosine have been obtained from the measurements: $$k_\mathrm{b} = 1.57 \times 10^{–12}$$, $$k_\mathrm{a1} = 7.8 \times 10^{–10}$$, $$k_\mathrm{a2} = 8.5 \times 10^{–11}$$. The changes in solubility with $$\mathrm{pH}$$ can be satisfactorily explained by the use of these ionization constants.

According to plot of solubility versus $$\mathrm{pH}$$ of their results, it was concluded that the highest solubility of tyrosine can be achieved in the $$\mathrm{pH}$$ range of $$1-2$$ or in the $$\mathrm{pH}$$ range of $$9-10$$. For example, the concentration of tyrosine in $$\pu{0.05 M}$$ $$\ce{HCl}$$ is found to be $$\pu{0.0165 M}$$ with the $$\mathrm{pH}$$ of solution being $$1.450$$. And also, the concentration of tyrosine in $$\pu{0.0498 M}$$ $$\ce{NaOH}$$ is found to be $$\pu{0.0358 M}$$ with the $$\mathrm{pH}$$ of solution being $$9.953$$. The least solubility of tyrosine is shown to be with in the $$\mathrm{pH}$$ range of $$3-8.5$$ (Ref.2).

Note: The extrapolation of the acidic side of above graph has shown the solubility increases up to about $$\pu{0.04 M}$$ at $$\mathrm{pH} = 1$$ (Ref.2).

References:

1. T. A. Brown, In Molecular Biology: LabFax; 1st Edition; Bios Scientific Publishers: Oxford, England, 1991, p. 29 (ISBN: 1‐872748‐00‐7).
2. David I. Hitchcock, "The solubility of tyrosine in the acid and in alkali," Journal of General Physiology 1924, 6(6), 747–757 (https://doi.org/10.1085/jgp.6.6.747)(PDF).
• What about temperature? Do you know anything about the max solubility in HCl when increasing the temperature (to say 80C gor instance)? – Hans Jul 12 '20 at 8:07
• @Hans: I did not concentrate on solubility with temperature since it wasn't a concern according to the question. Nevertheless, solubility decreases with the decreasing temperature as rule of thumb. Since solubility of tyrosine is highly depend on its ionization (total positive or negative charge by acid or base, respectively), difference in solubility might not be too big of a deal (c.f., solubility of $\ce{NaCl}$). – Mathew Mahindaratne Jul 12 '20 at 16:13
• @Mathew_Mahimdaratne: Thank you! :) – Hans Jul 12 '20 at 19:38