Amino acid in Deep Sea Drilling Project cores

Several amino acid diagenetic reactions, which take place in the deep-sea sedimentary environment, were investigated, using various Deep Sea Drilling Project (DSDP) cores. Initially it was found that essentially all the amino acids in sediments are bound in peptide linkages; but, with increasing age, the peptide bonds undergo slow hydrolysis that results in an increasingly larger fraction of amino acids in the free state. The hydrolysis half-life in calcareous sediments was estimated to be ~1-2 million years, while in non-carbonate sediment the hydrolysis rate may be considerably slower.
The amino acid compositions and the extent of racemization of several amino acids were determined in various fractions isolated from the sediments. These analyses demonstrated that the mechanism, kinetics, and rate of amino acid diagenesis are highly dependent upon the physical state (i.e., free, bound, etc.) in which the amino acids exist in the sedimentary environment. In the free state, serine and threonine were found to decompose primarily by a dehydration reaction, while in the bound state (residue or HCl-insoluble fraction) a reversible aldol-cleavage reaction is the main decomposition pathway of these amino acids. The change in amino acid composition of the residue fraction with time was suggested to be due to the hydrolysis of peptide bonds, while in foraminiferal tests the compositional changes over geological time are the result of various decomposition reactions. Reversible first-order racemization kinetics are not observed for free amino acids in sediments. The explanation for these anomalous kinetics involves a complex reaction series which includes the hydrolysis of peptide bonds and the very rapid racemization of free amino acids. The racemization rates of free amino acids in sediments were found to be many orders of magnitude faster than those predicted from elevated temperature experiments using free amino acids in aqueous solution. The racemization rate enhancement of free amino acids in sediments may be due to the catalysis of the reaction by trace metals. Reversible first-order kinetics are followed for amino acids in the residue fraction isolated from sediments; the rate of racemization in this fraction is slower than that predicted for protein-bound amino acids.
Various applications of amino acid diagenetic reactions are discussed. Racemization and the decomposition reaction of serine and threonine can both be used, with certain limitations, to make rough age estimates of deep-sea sediments back to several million years. The extent of racemization in foraminiferal tests which have been dated by some other independent technique can be used to estimate geothermal gradients, and thus heat flows, and to evaluate the bottom water temperature history in certain oceanic areas.

本研究以多份深海钻探计划（Deep Sea Drilling Project, DSDP）岩芯为研究对象，对深海沉积环境中发生的多种氨基酸成岩反应（amino acid diagenetic reactions）开展了系统调查。初始研究结果显示，沉积物内几乎所有氨基酸均以肽键（peptide linkages）结合的形式存在；随着沉积年代增长，肽键会发生缓慢水解，使得游离态氨基酸的占比逐步升高。研究估算得出，钙质沉积物（calcareous sediments）中的水解半衰期约为100万至200万年，而非碳酸盐沉积物（non-carbonate sediment）中的水解速率则明显更慢。

研究人员对沉积物中分离得到的不同组分进行了氨基酸组成分析，并测定了部分氨基酸的外消旋（racemization）程度。分析结果表明，氨基酸成岩作用的机制、动力学特征与反应速率，高度依赖于其在沉积环境中的赋存物理状态（如游离态、结合态等）。在游离态条件下，丝氨酸与苏氨酸主要通过脱水反应发生降解；而在结合态（即氨基酸残基或不溶于盐酸的组分）中，可逆羟醛裂解（aldol-cleavage）反应则是这两种氨基酸的主要降解途径。有研究指出，沉积物残基组分的氨基酸组成随时间产生的变化，源于肽键的水解过程；而有孔虫壳体（foraminiferal tests）的氨基酸组成随地质时间的演化，则由多种降解反应共同作用导致。沉积物中的游离氨基酸未表现出可逆一级外消旋反应动力学（first-order racemization kinetics）特征。针对这一异常动力学现象的解释涉及一系列复杂反应，包括肽键水解以及游离氨基酸极快的外消旋过程。研究发现，沉积物中游离氨基酸的外消旋速率，远高于通过水溶液中游离氨基酸的高温实验所预测的结果，差距可达数个数量级。沉积物中游离氨基酸的外消旋速率被显著提升，可能源于痕量金属（trace metals）对该反应的催化作用。从沉积物中分离得到的残基组分内的氨基酸，则遵循可逆一级反应动力学，该组分中的外消旋速率慢于蛋白质结合态氨基酸的理论预测值。

本研究还探讨了氨基酸成岩反应的多项应用场景。尽管存在一定局限性，但丝氨酸与苏氨酸的外消旋作用及降解反应，均可用于估算数百万年内深海沉积物的大致沉积年代。通过已通过其他独立定年技术确定年代的有孔虫壳体的外消旋程度，可以估算特定海域的地热梯度（geothermal gradients）、热流值（heat flows），并重建该区域的底层水温演化历史。