HEPES

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HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid ) is an organic chemical buffer that is widely used in cell culture to maintain physiological pH. Its molecular weight is 238.3. It has a pKa of 7.55 at 20 °C.

HEPES is recommended for the protection of frozen solutions of enzymes from freezing-induced pH.

Lepe-Zuniga et al. reported a phototoxicity of HEPES when exposed to ambient light by the production of hydrogen peroxide. For best repeatability of results it is then strongly advised to keep any HEPES containing solution in darkness as much as possible.

Fears that Hepes may serve as a nutrient source for aerobic bacteria have been shown to be unfounded.

References

• Toxicity of light-exposed Hepes media. Lepe-Zuniga JL, Zigler JS Jr, Gery I. Journal of Immunological Methods. 1987 Oct 23;103(1):145.
• Analysis of the cytotoxic effects of light-exposed HEPES-containing culture medium.

Zigler JS Jr, Lepe-Zuniga JL, Vistica B, Gery I., In Vitro Cell Dev Biol. 1985 May;21(5):282-7.