Chapter 14
Amino Acids in the Amber Matrix and in Entombed Insects 1
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Xueyun S. Wang , Hendrik N. Poinar , George O. Poinar, Jr. , and Jeffrey L. Bada Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 22, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1995-0617.ch014
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Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093-0212 Institute of Zoology, Department of General Biology, University of Munich, D-88021 Munich, Germany Department of Entomological Sciences, University of California, Berkeley, CA 94720
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We have investigated the amino acids in both the bulk matrix and in insect inclusions in tree resins ranging in age from10 compared to other geochemical environments on the surface of the Earth. This is also apparently due to the anhydrous properties of the amber matrix. The excellent preservation of amino acids in amber insect inclusions suggests that other biomolecules would also be preserved much better than in other geochemical environments. This conclusion is consistent with the reported successful retrieval of DNA sequences from amber-entombed organisms. 4
We have recently shown that the amino acids present in a fly entombed in 40 million year old Baltic amber are remarkably well preserved [/]. Essentially no racemization, or decomposition of the unstable amino acid, serine, has taken place since the fly was encapsulated in the amber 40 million years ago. In contrast, in other geochemical systems on the surface of the Earth the L-amino acids initially present in the tissues of organisms are totally racemized (D/L = 1.0) on time scales of 10 to 10 years [2]. The more ancient the amino acids, the closer the D/L ratios should be to 1.0, provided there is no modem L-amino acid contaminates present [3]. 5
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4Corresponding author 0097-6156/95/0617-0255$12.00/0 © 1995 American Chemical Society
Anderson and Crelling; Amber, Resinite, and Fossil Resins ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 22, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1995-0617.ch014
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AMBER, RESINITE, AND FOSSIL RESINS
Abstraction of the amino acid α-hydrogen by water or OH", producing a planar carbanion intermediate, is the rate limiting step in the racemization reaction [4]. This is also the first step involved in serine dehydration [5]. Thus, environments where amino acid stereochemistry is preserved would also favor the preservation of serine. Sufficient water for α-hydrogen abstraction, and thus for racemization and serine dehydration to take place, is usually present in most natural geochemical environments. However, in desiccated samples [6], and in those from hydrophobic deposits such as tar pits [7], racemization is retarded. In the absence of water, racemization proceeds slowly, or is even quenched. Thus, the unusual preservation of amino acid stereochemistry, and the lack of extensive serine decomposition, in the fly entombed in amber is apparently the result of the anhydrous nature of the amber matrix. The importance of anhydrous media in molecular preservation is demonstrated by the observation that in anhydrous non-aqueous organic solvents, the stability of fragile biomolecules such as enzymes is greatly enhanced compared to aqueous solutions [8]. The anhydrous environment of amber has also been suggested to be important in the preservation of DNA in organisms encased in amber [9]. DNA has been successfully amplified from plant and insect tissues preserved in amber as old as 130 million years [10-14]. In most geochemical environments, DNA fragments containing several hundred base pairs should not be preserved for more than 10 years [75-77]. Depurination (hydrolysis of the deoxyribose/adenine or guanine bond), followed by rapid chain breakage, is thought to be the main reaction important in the fragmentation of DNA in the geologic environment [75,76]. We have recently shown that the rates of depurination and aspartic acid racemization at neutral pH are nearly identical over the temperature range of 45° to 120°C [7]. Also, the in vivo rate of aspartic acid racemization measured in human enamel, dentine and the eye lens nucleus [18] is similar to that estimated for in vivo depurination [79]. The retrieval of DNA sequences and the lack of any detectable racemization in amber-entombed insects suggests that DNA depurination and amino acid racemization are coincident with each other not only in aqueous solution, but apparently under anhydrous conditions as well. In order to further investigate the preservation of amino acids in both the bulk amber matrix and in amber-entombed insects we have performed amino acid analyses on various copal and amber samples [20] which span an age range from
