Racemization dating

In most cases these monomers have chiral centers, so both the monomers and the repeating units in the polymers will contain asymmetric carbon atoms and will be capable of existing as different optical isomers. Also in most cases investigated to date, racemic mixtures of the monomers were polymerized to give optically inactive polymers, so tacticlty could not be determined by, or correlated with, optical activity. Nevertheless, stereoregularity in either the o- or 3-substituted polyesters and polyamides prepared from these monomers can be discussed in terms of tacticlty in the normal sense as illustrated in Figure 2 for the polymers with either isotactic or syndiotactic dyads in which the substituents are either on the o- or 3-carbon position (or both). 

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

Ammo acid racemization (Section 27 2) A method for dating archeological samples based on the rate at which the stereo chemistry at the a carbon of ammo acid components is ran domized It is useful for samples too old to be reliably dated by decay... 

The type of CSPs used have to fulfil the same requirements (resistance, loadabil-ity) as do classical chiral HPLC separations at preparative level [99], although different particle size silica supports are sometimes needed [10]. Again, to date the polysaccharide-derived CSPs have been the most studied in SMB systems, and a large number of racemic compounds have been successfully resolved in this way [95-98, 100-108]. Nevertheless, some applications can also be found with CSPs derived from polyacrylamides [11], Pirkle-type chiral selectors [10] and cyclodextrin derivatives [109]. A system to evaporate the collected fractions and to recover and recycle solvent is sometimes coupled to the SMB. In this context the application of the technique to gas can be advantageous in some cases because this part of the process can be omitted [109]. 

The purpose of the present chapter is to provide an up-to-date review of methods which may be applied for the synthesis of both achiral and chiral (racemic and optically active) sulphoxides as well as their derivatives. Since the synthesis of optically active sulphoxides is based on many special procedures, it was found necessary to separate the syntheses of achiral and racemic sulphoxides from those of optically active ones. 

However, the most common and important method of synthesis of chiral non-racemic hydroxy phosphoryl compounds has been the resolution of racemic substrates via a hydrolytic enzyme-promoted acylation of the hydroxy group or hydrolysis of the 0-acyl derivatives, both carried out under kinetic resolution conditions. The first attempts date from the early 1990s and have since been followed by a number of papers describing the use of a variety of enzymes and various types of organophosphorus substrates, differing both by the substituents at phosphorus and by the kind of hydroxy (acetoxy)-containing side chain. 

If the environmental temperature is constant, the racemization process takes place at a uniform rate, which is determined, at any time during the process, by the relative amounts of / and d forms of the amino acid can be measured. As the racemization proceeds and the concentration of the / form amino acid decreases, the rate of racemization gradually slows down. When there is a mixture of 50% of each of the d and / forms, the racemization process stops altogether. The half-life of the racemization of aspartic acid, for example, a common amino acid in proteins, at 20°C is about 20,000 years. This half-life makes it possible to date proteins as old as about 100,000 years. So far, however, the dates obtained with the technique have proved somewhat inconsistent, probably because of the difficulty in verifying whether the temperature of the amino acids has been constant. 

DATING ANCIENT PROTEINS - AMINO ACID RACEMIZATION DATING... 

There is, however, a drawback to this method of dating. The half-life (see Textbox 14) of the racemization process is greatly affected by temperature it is shorter at higher temperatures and slows down as the temperature decreases. An uncertainty of 2°C in the temperature history of dead remains can lead to an error of about 50% in the age determined. For the racemization of amino acids to be applicable for dating, therefore, it is crucial that the temperature history of the environment where the amino acids (that is, skin or bone) have been deposited should be well known. 

FIGURE 88 Dating methods. Shortly after the discovery of radioactivity, at the beginning of the twentieth century, it was found that the decay of radioactive elements could be used to keep track of time. Many of the dating techniques developed since then are, therefore, based on radioactive decay phenomena, but others, such as the hydration of obsidian, amino acid racemization, and dendrochronology, are based on other physical, chemical, or biological phenomena. 

AAR dating Abbreviation for amino acid racemization dating. 

Bada, J. L. (1985), Amino acid racemization dating of fossil bones, Ann. Rev. Earth Planet. Sci. 13, 241-268. 

Rutter, N. W. and B. Blackwell (1995), Amino acid racemization dating, in Rutter, N. W. and N. R. Catto (eds.), Dating Methods for Quaternary Deposits, Vol. 2, Geological Association of Canada, Geotext, St. John s, Newfoundland, pp. 125-164. 

In nearly two decades, challenging structured of phomactins coupled with the interesting biological activity has elicited an impressive amount of synthetic efforts [16-18]. (+)-Phomactin D was first synthesized by Yamada in 1996 [19], and Wulff [20] reported the synthesis of ( )-phomactin B2 in 2007. However, (+)-phomactin A has been the most popular target because of its unique topology. To date, two monumental total syntheses have been accomplished Pattenden s [21] racemic synthesis and Halcomb s [22] asymmetric synthesis in 2002 and 2003, respectively. Both syntheses are beautifully done but also mimicked Yamada s synthesis of D, thereby underscoring the remarkable influence of Yamada s earlier work on the phomactin chemistry. Upon completing their synthesis of ( )-phomactin A, Pattenden [23] completed ( )-phomactin G via a modified route used for A. 

D. Kaufman, Dating Deep Lake Sediments by Using Amino Acid Racemization in Fossil Ostracodes, Geology, 31, 1049 1052 (2003). 

J. Robins, M. Jones, E. Matisoo Smith, Amino Acid Racemization Dating in New Zealand An Overview, Auckland University (2001). 

J.L. Bada, The Dating of Fossil Bones using the Racemization of Isoleucine, Earth and Planetary Science Letters, 15, 223 231 (1972). 

Bada, J. L., Master, P. M., Hoopes, E., Darling, The Dating of Fossil Bones Using Amino Acid Racemization, In Radiocarbon Dating, pp. 740-756, Berkeley, University of California Press, 1979. 

Bender, M. L., Reliability of Amino Acid Racemization Dating and Paleotemperature Analysis on Bones, Nature, 1974, 252, 378-379. 

Amino Acid Dating Techniques depend on the "rates of hydrolysis reactions of proteins and racemization, epimerization, and decomposition reactions of amino acids [they have] been applied to the age-dating of fossil bone, teeth, and shell. Activation energies range from near 20 kcal per mole for hydrolysis reactions to around 30 kcal per mole for racemization... 

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