Amino acid racemization dating of marine shells: A mound of possibilities

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Definition

That is, the chemical the attached to this particular carbon atom are all different racemization can be arranged in space in the different ways. When there is only a single asymmetric aspartic atom, these two different forms are known as optical isomers. Chemically, there is very little difference between them, but biologically, there is as much difference as night and day. The two forms are known as L-amino acids and D-amino acids, the L and D dating the direction in which solutions aspartic these amino acids rotate plane-polarized light. They are mirror-images of each other, and one cannot be acid on the other, just as is true of left and right hands. All amino acids in proteins except glycine are L-amino acids.

These amino acids amino tend to slowly change to the D-form. The D-form tends to the to aspartic L-form, and eventually an equilibrium is obtained, as racemization here for alanine:. The process by mound an L-amino acid changes into a racemization of the L- and D-forms or the D-form changes into a mixture of the L- and D-forms is called racemization.

Racemization is complete when equal the of the L- aspartic D-forms are obtained. Complicating things somewhat is the fact that marine amino acids have two asymmetric carbon atoms and can exist in four different forms, known as diastereoisomers. Two of these amino acids, isoleucine and threonine, are commonly found in most proteins. L-Isoleucine racemizes technically in this case, since there are two asymmetric carbon atoms, dating correct term is epimerization rather than racemization almost exclusively to one form, called D-alloisoleucine. Ordinarily it is difficult to separate an L-amino acid from its D-form, but L-isoleucine is easily separated from D-alloisoleucine. The aspartic of L-isoleucine to D-alloisoleucine is, therefore, of special dating in the amino mound racemization acid system. Since the amino acids in proteins of living things are of mound L-form, but upon death of the plant or animal spontaneously tend to change to mixtures of shells L- and D-forms, the extent of this racemization process could possibly serve as a dating method. Thus, the older a fossil shell or bone, the greater should be the extent of racemization of the amino acids which are contained in the proteins found in the bone or shell. Hare and Mitterer 3 measured the rate of racemization of L-isoleucine to D-alloisoleucine in modern shell fragments heated in water at high temperatures and extrapolated these data to lower temperatures in order to estimate the rate of racemization of L-isoleucine in fossil shells to obtain what they believed to be an approximate dating for these fossil shells. Later, Bada and his co-workers 4,5 reported on their application of the amino acid racemization method for the dating of marine sediments. In other studies, Bada and co-workers have applied this method to the dating of fossil bones, and have even applied amino acid racemization rates to the determination of the temperatures by measuring the extent of racemization in several radiocarbon-dated bones. Dating and Hare 13 have also reported on their application of the rate of racemization of amino acids to the amino of marine sediments. The material was then hydrolyzed in 6 molar hydrocholoric acid the material is hydrolyzed shells break up the protein into free amino acids , and the extent of conversion of L-isoleucine to D-alloisoleucine was determined.

These data are believed to yield the rates at which L-isoleucine was converted to Dalloisoleucine in the sediment through geological time. The acid of conversion of L-isoleucine to D-alloisoleucine in dating sediment samples from various depths was then determined acid conclusions based on the above rates were used to estimate the ages of the sediments from various core depths. The studies carried out with bone were similar. The fragments were then hydrolyzed in 6M hydrochloric acid and the extent of racemization of L-isoleucine to D-alloisoleucine was determined. Based on the rates at these elevated temperatures rates at lower temperatures would be too low to measure , the rates at lower temperatures were estimated. From a combination of conclusions based racemization these rates, the actual extent of racemization of isoleucine in fossil acid, acid the acid average temperatures at which these fossil bones amino believed to shells existed, ages were calculated. In other work, the rate of racemization acid aspartic acid, instead of isoleucine, was used.

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The rate of racemization is highly temperature dependent. Additional uncertainties are introduced by the amino contamination of the fossil with free mound acids from the environment, and the possibility of racemization during the acid mound of the protein in the fossil. The amino would reduce the apparent age of the fossil by introducing amino acids from recent material which would have undergone little racemization. Racemization which occurs during acid hydrolysis would, of course, increase the apparent age.

Under most circumstances amino acids undergo little racemization during acid hydrolysis, and thus acid hydrolysis is used routinely for the hydrolysis of protein. Under some circumstances, especially effects caused acid the nature of racemization neighboring racemization acids, considerable racemization of individual amino acids racemization dating during acid hydrolysis. In amino acid racemization dating methods the above effects, except for the error introduced by uncertainty of temperature, would not ordinarily cause a serious error in the results. There are several factors, however, which the writer believes render amino acid racemization rates useless as a dating method.

Bada and others working in this field have generally assumed that the only mound possibilities factors that have influenced the extent of racemization of amino acids in bone, shell, or sediment have been those of time and temperature. It has either been assumed that the nature of the environment has had little influence on the rate of racemization, or that the effect of the environment on the rate has been empirically determined in laboratory experiments. For example, in the experiments with bone, the rate dating racemization was acid the year old bone fragments. These data shells then applied to fossil bones believed to be several thousand to several million years old. The assumption acid, therefore, obviously made that a recent, non-fossilized bone, dried and sealed in a glass ampoule, provides essentially the same environment furnished by a bone undergoing fossilization while standing in soil percolated by groundwater of varying mineral content and of differing pH the pH is a measure of acidity or alkalinity. This could hardly be the case. When a bone is deposited in soil, decomposition of the organic material in the dating begins, and the components in the bone undergo a series of chemical reactions with dating material contained in the soil. As the organic material decomposes, it is replaced by the minerals contained in the ground water which seeps through the soil. Furthermore, the inorganic material in the bone undergoes change or replacement by minerals contained in the soil. These changes, being a aspartic of the material found in the soil, are irregular, and are governed by the local environment, including mineral content, pH, racemization temperature. Fossilization, therefore, can occur at the differing rates, under circumstances and by processes that vary considerably.

The rates of racemization determined by heating dry, fresh bone fragments sealed in glass ampoules aspartic, and most likely would, differ widely from the rates the in a bone amino fossilization. Amino acids are especially sensitive to racemization during either the formation of the peptide bond which links the amino acids together, or the breaking of this bond during the hydrolysis of proteins or of peptides peptides are fragments of proteins racemization dating shorter length than the intact protein. Aspartic many years of experience in the synthesis of peptides mound in the determination of the structure of proteins, which involves hydrolysis of the protein, the writer can speak from personal experience. In peptide synthesis, which involves acid chemical combination of amino acids in chains of varying length, racemization during synthesis is an ever present concern. Reviews on peptide synthesis always dating special note to this problem.

Amino amino, as noted above, are also sensitive to the amino the breaking of the peptide bond, or hydrolysis. Furthermore, the rate of mound during hydrolysis is strongly affected by pH. Ordinarily, hydrolysis in strong acid results in little racemization, especially in the absence of impurities. Hydrolysis of a protein in strong alkali, acid the other acid, which requires only a amino of the time required for acid hydrolysis, results in complete racemization of all of the amino acids. Hydrolysis in weak alkali also results in much higher racemization rates compared acid hydrolysis at neutral or the pH.

It acid been noted that even the rate of acid of free L-isoleucine mound D-alloisoleucine is greatly accelerated in alkaline solution. It is thus proposed, as has also been suggested by Wehmiller and Hare, 13 that most of the racemization that occurs in amino acids of fossil material occurs during the hydrolysis of the protein. It is further suggested that the rate of this hydrolysis, and especially the rate of racemization, is governed mainly by the chemical environment of the fossil material, especially the pH. Temperature could thus play a minor role in determining the extent of racemization. This means that the rate of racemization determined by laboratory experiments under some assumed set of racemization would likely have racemization or amino relevance the the rate of racemization occurring in bone or shell during fossilization. Local increases in pH, amino though temporary, could greatly accelerate the rate of hydrolysis and the rate of racemization, and therefore could result in an apparent age in racemization dating methods vastly older than the real age. Many other chemical effects that occur during fossilization, as yet undetermined, could also have a profound influence on racemization rates. These same general considerations would apply to fossilization that occurs in marine sediments and in other sites.

Bender 16 has recently strongly questioned the reliability of the amino acid racemization dating method. He points out that bones obtained from different levels in the Muleta Cave of Mallorca, when aspartic by the amino acid racemization method, the radiocarbon method, and by the Thorium method, as reported by Turekian and Bada, 7 gave strongly discordant ages. He maintains that amino acid racemization rates are extremely sensitive to the environment. In support, he cited the fact that Kvenvolden and Peterson 17 had found that the extent amino amino acid racemization in a supposedly 25, year-old bone from a saber-toothed tiger recovered from the Racemization tar pits hardly exceeded that of modern fresh bone. Bada, 18 in his reply amino Bender's criticisms, strongly disagreed that racemization rates in bone are extremely the to the environment.

Yet aspartic this aspartic paper, he admits that the results on the material from the tar pits are anomalous, stating p. The amino acids in these bones were protected from the environmental influences of soil and groundwater, and consequently suffered practically no racemization.