Preparation method and operation guide of HEPES

The preparation of HEPES looks simple - weighing, dissolving, adjusting pH, and adjusting volume. However, in practical operation, the details of temperature, osmotic pressure, and sterilization method often determine whether this bottle of reserve solution can be used. This article explains the configuration process and error prone steps in one go.

1.First, figure out what form you have in your hand

There are two main forms of commercially available HEPES with different formulation logic:

HEPES free acid: The initial aqueous solution is slightly acidic and requires the use of a base (usually NaOH) to increase the pH

HEPES sodium salt: The initial pH is neutral or alkaline, usually adjusted downwards with HCl

The two cannot be confused. After receiving the bottle, first check the form and molecular weight on the label, and then decide on the direction to adjust the pH.

Key constant (free acid): molecular weight 238.3, CAS 7365-45-9, pKa approximately 7.5 (25 ℃).

2.Standard preparation process for 1M reserve solution

This is the most versatile reserve solution in the laboratory, which can be diluted to the working concentration as needed.

Materials: HEPES free acid powder, high-purity water (cell culture grade or Milli-Q), 10 N NaOH, 0.22 μ m filter, sterile storage bottle.

Steps:

Weigh 238.3 g of HEPES free acid powder and add it to approximately 800 mL of high-purity water.

Magnetic stirring until completely dissolved (the solution should be clear and free of undissolved particles).

Slowly add 10 N NaOH dropwise while stirring, adjust the pH to 7.2-7.4.

Add high-purity water to a volume of 1000mL.

Filter and sterilize using a 0.22 μ m filter membrane.

Transfer to sterile bottles and store at 4 ℃ in the dark.

Dilution conversion: Dilute 1M stock solution to 20 mM, adding 20 mL per liter of culture medium; dilute to 25 mM, adding 25 mL. This is also the most commonly used conversion step when preparing HEPES containing culture medium.

3.The four most prone to errors

3.1 The pH should be lowered at the operating temperature. The pKa of HEPES varies with temperature (approximately -0.014/℃), and when the pH is adjusted at room temperature, there will be a slight deviation at 37 ℃. If this buffer solution is to be used in a 37 ℃ system, it is recommended to calibrate the pH meter under conditions close to the operating temperature. This is a general principle: lower the pH at the temperature you are actually using.

3.2  Don't forget about osmotic pressure. Whether adding free acid to NaOH or directly using sodium salt, the addition of HEPES will increase the osmotic pressure of the solution. Most mammalian cells can tolerate 260-350 mOsm/kg, so when supplementing HEPES in the formula, it is usually necessary to downregulate the amount of NaCl or NaHCO3 accordingly. The commercially available pre prepared HEPES containing culture medium has generally been subjected to osmotic pressure equilibrium, and when self added, it must be tested and verified.

3.3  Use filter sterilization instead of high-pressure sterilization. The standard sterilization method for HEPES solution is 0.22 μ m filtration. There are different opinions on high-pressure sterilization in some materials. The safe approach is to follow the technical specifications of the product used and prioritize the use of filtration methods.

3.4  Avoid light throughout the entire process. This is often overlooked. HEPES can generate reactive oxygen species such as hydrogen peroxide under visible light irradiation, especially in the presence of riboflavin in the culture medium, which can affect sensitive cells. Therefore, the stock solution and culture medium containing HEPES should be stored away from light, and prolonged exposure to strong light should be avoided as much as possible during operation. There is a suggestion to consider using MOPS system for experiments that require long-term illumination imaging.

4.Storage and Quality Assurance

Powder: It has a certain degree of moisture absorption. It is recommended to weigh it in a dry environment, seal it immediately after use, and store it at room temperature away from light and moisture

Solution: Store at 4 ℃ in the dark and under sterile conditions; It is recommended to use the complete culture medium containing HEPES according to the conventional culture medium cycle to avoid long-term storage

A practical habit: Divide the reserve liquid into single use quantities and then freeze or refrigerate it to reduce the risk of contamination and moisture absorption caused by repeated opening of the lid.

5.Self inspection checklist

Before preparation, go through each item one by one:

Confirm the form of raw materials (free acid/sodium salt) and the direction of pH adjustment

Confirm target working concentration (conventional cultivation 10-25mM)

Calibrate pH close to the operating temperature, target 7.2-7.4

Verify that the total osmotic pressure of the formula falls between 260-350mOsm/kg

Filter and sterilize with 0.22 μ m

Avoid light throughout the process and store the solution at 4 ℃

Record batch number and preparation date

Even if the preparation process is standardized, the starting point is still the raw materials. The purity, moisture content, residual metal ions, and UV absorption of HEPES will be directly reflected in the clarity, pH stability, and repeatability of subsequent experiments of the reserve solution.

Hubei Xindesheng Material Technology Co., Ltd. (formerly known as Wuhan Desheng Biochemical Technology Co., Ltd. established in 2005) has long been dedicated to the research and production of biological buffering agents and related fine chemicals. HEPES and other products can provide supply and customized indicators from gram to ton levels. The company's headquarters is located in Guanggu United Science and Technology City, Gedian Development Zone, Ezhou, Hubei Province. It has two R&D and production bases in Gedian and Huanggang (70 acres), with an annual production capacity of 5000 tons for all categories. Its products are used in IVD in vitro diagnostics, biomedicine, and daily chemical industries.