D-Alloisoleucine

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 ... 

By measuring the L-isoleucine/D-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 (l4C) dating is not accurate for samples older than about 35,000 years, so AAR is a useful addition to the tools available to paleontologists. 

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).

The spectral properties of a series of Boc-protected LD-alternating cooligopeptide methyl esters derived from the diastereomeric amino acids l-isoleucine and D-alloisoleucine with D-alloisoleucine as the COOH-terminal residue have been investigated (Lorenzi and Paganetti, 1977). The experimental data suggest that, in cyclohexane, the cooligopeptides (hexamer and octamer) assume helical conformations of the p type, with the same prevailing sense of twist. 

Isoleucine and threonine both have two chiral centers. According to official nomenclature the names L-isoleucine and L-threonine both designate only one optically active diastereoisomer. The mirror images are named D-isoleucine and o-threonine, whereas the diastereoisomers are named l-and D-alloisoleucine and l- and D-allothreonine. The configurations of L-isoleucine and L-threonine are shown in the formulas in Table I. The table also contains notation of the configuration at the two centers according to the R,S system and according to traditional nomenclature for amino acids (lUPAC-IUB, 1975). 

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... 

Formation of D-amino acids occurs through abstraction of a proton via a C2-carbanion. The reaction with L-isoleucine is particularly interesting. L-Isoleucine is isomerized to D-alloisoleucine which, unlike other D-amino acids, is a diastereoisomer and so has a retention time different from L-isoleucine, mtddng its determination possible directly from an amino acid chromatogram ... 

Soon after, Brockmann and Grubhofer (1949) isolated from a culture of 5. chrysomallus another actinomycin which they designated as C. It appeared to be different from actinomycin A or B on the basis of crystal form, melting point, solubihty, infrared spectrum, analytical composition and the nature of the amino acids present (Brockmann and Grubhofer, 1950,1951). Hydrolysates of actinomycin C contained the six amino acids D-valine, sarcosine, L-threonine, L-proline, N-methyl-L-valine and D-alloisoleucine. 

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