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Biotechnology Journal , 10 3 , Capability of PHSC equation of state for thermodynamic modeling of aqueous amino acid and peptide solutions. Solubility data of l -phenylalanine comes from four literature sources. The initial solubility measurement in water was conducted by Dalton and Schmidt , then in water and water—ethanol mixtures by Needham , Nozaki et al.
In this article, an additional set of data was collected in water—ethanol mixtures. All data was collected by the gravimetric method except for Dalton and Schmidt, who used the dissolution method. The data of Needham, Dalton and Schmidt, Nozaki et al. After an initial decrease in solubility, as ethanol mole fraction increases from 0 to 0. The solubility of l -phenylalanine decreases again at an ethanol mole fraction above 0.
The exception to this is Lu et al. Several authors have measured the solubility of glycine in water. Cao et al. Solubilities reported by Nozaki et al. At a solvent mole fraction of 1. These were 4. The solubility measured by Nozaki et al. Except for that in water, at all mole fractions of ethanol, the solubility measured by Nozaki et al. The solubility of l -tryptophan peaks between ethanol mole fractions of 0. The highest solubility of l -tryptophan was measured by Nozaki et al. It has been shown that l -glutamic acid can form l -pyroglutamic acid in solution.
Furthermore, the work of Mo et al. At pure water, the solubility reported by Dunn and Ross is approximately equal to the combined l -glutamic acid and l -pyroglutamic acid solubility collected experimentally in this work. At higher ethanol mole fraction, the difference between the data presented by Dunn and Ross and this work increases.
The solubilities reported by Dunn and Ross fall below the standard deviation of those in this report at ethanol mole fractions above 0. While the l -cysteine trials were kept in a low oxygen environment, there is still the possibility that oxidation to l -cystine took place, which in turn could affect the solubility of l -cysteine.
To accommodate for any build up on the detector of the mass spectrometer, the detector was cleaned before each measurement. In both measurements, only trace amounts of l -cystine were found, Figure S1 , leading to the conclusion that the trace amounts of l -cystine do not affect the solubility data presented of l -cysteine in this work.
Mole fraction solubility of l -cysteine and l -arginine. Previous work has published the solubility of dl -alanine but not l -alanine in various ethanol mole fractions. Furthermore, the reported solubilities of l -alanine in water vary widely. In water, more dl -alanine dissolved than l -alanine for all reported data. It is unclear from the literature what the individual fractions of d -alanine and l -alanine are in the dl -alanine mixture.
Measured as a mixture, the dl -alanine measurements are only slightly more soluble than l -alanine alone at 0. The solubility data of l -alanine in water, water—ethanol mixtures, and ethanol measured for this work were measured using the UPLC. Their solubilities are lower at higher ethanol mole fractions.
The l -proline solubilities in water reported by Held et al. The solubility of l -proline in water reported by El-Dossoki is much lower than the solubility of l -proline reported by Amend and Hegelson.
Furthermore, the solubility of l -proline in water reported by El-Dossoki is lower than the solubility of l -proline measured for this work at 0. Mole fraction solubility of l -proline and l -lysine. Also, both l -cysteine and l -arginine have similar solubility ranges, but the solubility of l -arginine decreases faster than that of l -cysteine as the ethanol mole fraction increases.
The solubility of l -cysteine in water that was reported by El-Dossoki and El-Damarany is higher than the solubility measured for this work. El-Dossoki and El-Damarany do not report that their measurements were taken in a sealed, oxygen-poor environment. This could account for elevated experimentally measured solubilty due to the formation of the dimer cystine.
The measurements of Zhang et al. The influence of ethanol on the solubility of amino acids is not the same for all amino acids. Most amino acids have a lower solubility when their solvent is at a higher ethanol mole fraction. All amino acids have a loss in solubility above a mole fraction of 0. The change in solubility is not the same for all amino acids in the range of 0—0. This difference between the amino acids is most pronounced at ethanol mole fractions around 0. The effect of ethanol on the solubility of amino acids can be characterized by the groups found in their side chains.
Five amino acids have a ring in the side chain. These amino acids are l -tryptophan, l -tyrosine, l -proline, l -phenylalanine, and l -histidine.
These rings include either phenyl, pyrrolidine, or imidazole. The amino acids with rings in the side chains had the least decrease in solubility as ethanol is added. The average decrease in solubility of these amino acids at an ethanol fraction of 0. In the case of l -tryptophan, the solubility was even increased by We hypothesize that the rings of these amino acids are ethanolphilic, while the amino and carboxylic groups on these amino acids are ethanolphobic.
Moderate ethanol fractions between 0. The water and ethanol molecules arrange themselves at the respective groups of the molecule, creating a lattice around the amino acids. Higher ethanol mole fractions lower the solubility of these amino acids, because the ethanol molecules surround the amino acid molecule and disrupt the water molecules surrounding the amino and carboxylic groups on the amino acid molecule.
The aliphatic amino acids, l -phenylalanine, l -isoleucine, l -leucine, l -alanine, l -methionine, and l -valine, show initially a low to medium decrease in solubility at an ethanol mole fraction of approximately 0. This decrease could be possibly mitigated by the phenyl ring. The hydroxyl containing amino acids, l -tyrosine, l -serine, and l -threonine, show a medium decrease in solubility.
Here, as in the case of l -phenylalanine, the decrease in solubility is mitigated by the phenyl ring. A high decrease in solubility is seen in the charged amino acids l -glutamic acid, l -aspartic acid, and l -lysine. The average decrease at an ethanol mole fraction of 0. It has the highest decrease in solubility at ethanol mole fraction of 0. Glycine, containing no side chain, had the largest decrease in solubility. Therefore, all amino acids that have a possibility to convert to other amino acids in solution should be analyzed by a technique that takes this into account.
Amino acids can also be basic, like lysine, or acidic, like glutamic acid. The sequence and interactions between the side chains of these different amino acids allow each protein to fold into a specific three-dimensional shape and perform biological functions.
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