Alloisoleucine isoleucine ratio

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. 

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). 

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. 

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 ... 

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... 

By measuring the L-isoleucine/D-alloisoleucine ratio in the protein isolated from the eggshells of an extinct Australian bird, it was determined that this bird lived approximately... 

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