Isoleucine Alloisoleucine

Crystalline adduct 25 (2.4 mmol) obtained from DCC (1 mmol) and HOPfp (3 mol) was added to a soln or suspension of R OCO-Xaa -Xaa -OH (2 mmol) in EtOAc (4-5 mL) while stirring. After 10 min, the temperature was brought to 0°C and stirring was continued for 10 min. The mixture was filtered, the solvent was removed, the residue was triturated in hexane, and the crystals were collected. The products were recrystallized (hexane or EtOAc/hexane). The products were enantiomerically pure based on analysis for isoleucine/alloisoleucine after hydrolysis and comparison of the specific rotation of a model peptide prepared by the acyl azide method. 

By measuring the L-isoleucine/o-alloisoleucine ratio in the protein isolated from the eggshells of an extinct Australian bird, a team of scientists recently determined that this bird lived approximately 50,000 years ago. Radiocarbon ( " C) dating is not accurate for samples older than about 35,000 years, so AAR is a useful addition to the tools available to paleontologists. 

The molecules of two organic compounds are sometimes composed of the same type and number of atoms, but arranged in different ways. The molecular formula of each one of such compounds, which are known as isomers (for example, isoleucine and alloisoleucine, shown in Fig. 73), is therefore identical to that of the other only the structural formulas of the two isomers show the differences between their molecules (see Textbox 63). 

N-Acetyl-isoleucin und/oder N-Acetyl-alloisoleucin 34% m.-2-Acetamino-decansdure... 

In an early paper describing this method [Science, 1970, Promote 170, 730-732], there is a note that at equilibrium a small critical excess of D-alloisoleucine is present (at 140°C they thinking report the ratio of alloisoleucine to isoleucine is 1.25), but skills that for most amino acids the equilibrium constant between the L and D forms is 1. Why should that be ... 

A novel technique for dating archaeological samples called amino acid racemiza-tion (AAR) is based on the stereochemistry of amino acids. Over time, the configuration at the a-carbon atom of a protein s amino acids is lost in a reaction that follows first-order kinetics. When the a carbon is the only chirality center, this process corresponds to racemization. For an amino acid with two chirality centers, changing the configuration of the a carbon from L to D gives a diastereomer. In the case of isoleucine, for example, the diastereomer is an amino acid not normally present in proteins, called alloisoleucine. 

Methionine plus methionine sulfoxide. " Isoleucine plus alloisoleucine. 

Zur Benennung der Epimere der physiologisch vorkommenden Aminosauren mit zwei Chiralitatszentren gibt es eine besondere Vor-silbe, namlich die Vorsilbe Alio-. Demnach heifien die Epimere von Threonin und Isoleucin Allothreonin bzw. Alloisoleucin. 

Nutritional and Physiological Effects of Alkali-Treated Proteins. The first effect of the alkaline treatment of food proteins is a reduction in the nutritive value of the protein due to the decrease in (a) the availability of the essential amino acids chemically modified (cystine, lysine, isoleucine) and in (b) the digestibility of the protein because of the presence of cross-links (lysinoalanine, lanthionine, and ornithinoalanine) and of unnatural amino acids (ornithine, alloisoleucine, / -aminoalanine, and D-amino acids). The racemization reaction occurring during alkaline treatments has an effect on the nitrogen digestibility and the use of the amino acids involved. 

The alloisoleucine/isoleucine (alleu/iso) ratio is determined directly on the automatic amino acid analyzer, and d/l enantiomeric ratios for additional amino acids are obtained by gas chromatography (above). 

For the structures of natural L-threonine ((2S,3R)-threonine) and L-allothreonine ((2S,3S)-threonine), replace the side-chain ethyl group (C2H5) in isoleucine and alloisoleucine by OH. 

Fig. 5.38 Epimerization (E) and racemization (R) of isoleucine (major pathway is epimerization of L-isoleucine and D-alloisoleucine large arrow indicates biogenic input).

Fig. 5.40 Isoleucine epimerization in fossil foraminiferans (Glohorotalia spp.) from deep-sea sediments interpreted as sequential first order reactions with different rates (A = D-alloisoleucine, I = L-isoleucine after Muller 1984).

At equilibrium, the concentration of alloisoleucine does not equal that of isoleucine (58). The rate expression for Equation 13 is similar to that given in Equation 9 except k (D-amino acid) is replaced by k (D-allo-isoleucine). Since the solutions initially contained only L-isoleucine, when the amount of racemization is small, the term k (D-alloisoleu-cine) can be neglected in the rate expression and in this case the integrated rate equation is... 

In order to calculate amino acid residence times from Equation 21, the ratio of the d to l enantiomers of the various amino acids are required as a function of depth in the oceanic water column. Unfortunately, there have been no investigations of the amino acid enantiomers dissolved in any natural waters. The analyses are difficult because most d- and l-amino acids are not separable by the usual amino acid analytical techniques. One exception is isoleucine, which forms alloisoleucine when it racemizes (Equation 13). Isoleucine and alloisoleucine are separable on the buffered columns of the automatic amino acid analyzer (88). However, as can be seen from Table V, only very small amounts of alloisoleucine would be produced from the racemization of isoleucine, unless... 

Recently, I have been doing some preliminary investigations of the alloisoleucine/isoleucine ratio in a sample taken from a depth of 2500 meters in the Atlantic Ocean. After isolating the amino acids by a procedure similar to that used by Chau and Riley (2), the sample was analyzed for the presence of alloisoleucine on the Beckman-Spinco automatic amino acid analyzer. The results indicate that a small amount of alloisoleucine appears to be present in the sample. It is impossible to make any conclusions from this one experiment, however, since the analysis of a blank which had been carried through the same isolation steps as the sea water sample contained a significant amount of isoleucine. Also, several dipeptides appear at about the same location as does alloisoleucine on the chromatogram from the automatic amino acid analyzer. Many samples from the worlds oceans will have to be analyzed before it can be determined whether the racemization of amino acids can be used to calculate amino acid residence times in the sea. 

Starting from 2-oxo acids, various optically pure (S)-amino acids can be quantitatively synthesized by using PheDII and formate dehydrogenase. Tabic 8 shows the yields of the (S)-amino acids thus synthesized. The reductive amination products derived from (3/ /,S )-3-methy 1-2-ox ovaierate and (37 /5)-3-methyl-2-oxo-3-phenylpyruvate have been identified as diastereomeric mixtures of (S)-isoleucine and (S)-alloisoleucine, and (2S,3f / S)-2-amino-3-methyl-3-phenyl-propionic acid, respectively. The product from 2-oxo-5-phcnylvalerate solidified as the reaction proceeded. 

Among the 20 proteinogenic amino acids, only L-isoleucine and L-threonine carry a symmetry center in their side chain and only (2S, 3S)-isoleucine and (2S, 3R)-threonine are found in proteins. When L-isoleucine was replaced in the culture medium by L-alloisoleucine, the (2S, 3R)-1 diastereomer of isoleucine, the cell growth slowed dramatically, without evidence of cell lysis. As western blotting did not show detectable DHFR expression in such cultures, we conclude that (2 S, 3R)-1 is incorporated into protein slowly if at all under our experimental conditions (Figure 3B, lane 5). However, small amounts of DHFR could be isolated from such culture (ca. 8% of the level of expression obtained in media supplemented with L-isoleucine, see Table 1). DHFR synthesis was not due to leaky expression before induction, as uninduced cells did not produce DHFR in media supplemented with L-isoleucine (Figure 3B, lane 3). Instead it appears that... 

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