I worked with a variety of amino acids as substrates in enzyme reactions for decades. They were dissolved in distilled water at 0.1 m concentration, dispensed in 1 ml aliquots (Eppendorf tubes) and frozen at -30°C.  Acidic amino acids were adjusted to pH 7 with NaOH or another suitable base (e.g. Tris-base). Keep in mind, that addition of NaOH leads to higher ionic strength than when the amino acid is dissolved without NaOH. I was always used to use the sodium salts of acidic amino acids when preparing solutions.

Aliphatic amino acids in solution can be stored for years. Armomatic amino acids (in particular tryptophan) can suffer from repeated thawing and freezing. Tryptophan solutions can become brownish. The reason is that - to get it into solution small amounts of NaOH have to be added during dissolving - the indol ring is deprotonated and therofore prone to attack by hydroxyl ions, finally light and heat do  the rest to degrade it . As far as I remember, the solubility of tryptophan does not exceed 60 mM (please look in the standard tables) Proline at higher conc. by standing at room temeperature can get a brownish colour, too. However, at  0.1 M proline solution kept at -30 ° is also stable for long time. 

In case of sterile amino acids, all these amino acid solution can be autoclaved at 121 °C except tryptophan. The latter has to be sterilized by sterile filtration. 

Be aware that Cysteine may oxidize with time at elevated pH (above pH 5-6) forming disulfide bonds, so you may always prepare this one fresh. I'm not sure whether it is stable over time when frozen.

As Ullrich suggested, storing at -30 °C should conserve the AA solutions for a year at least. You could run HPLC with time to check if the AA degrade or do whatever strange.

Thanks for you excellent answer and explanation. Regards

All amino acids are soluble in water at room temperature. Leucine, isoleucine, valine, phenylalanine, tryptophane, methionine, tyrosine and cysteine areamong the most hydrophobic AA.

All AA (except for cystine) are soluble in Krebs bicarbonate buffer (PH 7.4 at 25°C) at concentrations that are at least 10 times greater than those in animal plasma. The solubility of α-AA in water varies with side chains, with proline and cystine being the most and least soluble, respectively. 

The solubility of β-alanine and γ-aminobutyrate in water is higher than that of lysine. Salts affects the solubility of AA in water.

Thanks for you suggestions

Dear sir, what is your use with the amino acids? Will you derive amino acids with silylation reagent like MSTFA? 

Dear Sir, i want to study effects of different amino acids on targeted bacterial and fungal growth at varying concentrations.

how can to dissolve leucine or other hydrophobic amino acid in distill water,,?

Hi Rasool,

you could try to start with a high concentrated stock solution of these AA in a more hydrophobic solvent such as acetonitrile, methanol or ethanol (be aware that these solvents do evaporate rather easy and this will result in concentration errors). Add the concentrated solution to the distilled water, you'll have a small amount of organic solvent in you solution.

However, if you stir the solution thoroughly (eventually over night), you should be able to dissolve any standard AA in distilled water. There must be a database available with the max. concentration per AA you can go in aqueous solution.

Good luck, Marcus

Thank you sir Marcus Binner,

yes but it may can create some problems because i will 28g AA per liter,

1) these solvent may can effect yeast growth, because methanol and ethanol can break cell wall or cell lysis,

2) they can evaporate during 5 days incubation period in shaking incubator (rpm 200) at 30°C.????

and 

very confusing how to solve,?

I am going to use these AA for yeast growth along with other growth media.

And how to determine free AA in aqueous solution or aqueous media,? please Sir 

Please find the attached link FYI, HPLC is the only method to go. You need standard substances with given concentration for quantitative analysis of your sample.

http://www.sigmaaldrich.com/analytical-chromatography/analytical-products.html?TablePage=22224654

Dear Sir Marcus Binner, Thank you for your kind help, stay happy.

What should I do if my tryptophan does not completely dissolve in solution to allow sterilization by filtration?

Add little amount of NaOH.

anybody can suggest me how I will solubilize Aspartic acid (31.25 mg/mL), Glutamic acid (41.56 mg/mL), Isoleucine (20.94 mg/mL), Phenylalaline (20.94 mg/mL), Leucine (62.50 mg/mL)? I need to solubilze all amino acids at time to make a single stock.

Sudhansu Sekhar Choudhury,

You can use 4 Molar or so HCl solution or NaOH solution to make it soluble, add drop wise the HCl or NaOH solution and stirrer gently. This is an easy way to solubilize the amino acid.

How to dissolve L-tryptophan in normal saline solution? Please suggest.

how can we dissolve DL-alanine? i tried dissolving in ethanol. but its not dissolved

It is insoluble in ethanol

To add more input, I have made 50mM solution for hydrophobic amino acids (alanine, leucine and isoleucine) by dissolving in miniQ water and sterile with 0.22um of filter column. Initially, I tried to make 200mM, alanine was fine to be dissolved, leucine was really not dissolvable at this concentration.

Working on how to make aa stock solutions. Learning a lot about solubility of the different amino acids. Have found some interesting references.