Abiogenic Synthesis of Monomers

Introduction

1.Oparin considered that the early Earth was strongly reducing , so he suspected that the abiogenic synthesis of smaller molecular weight organic compounds would have formed naturally on Earth's surface. The presence of O₂ everywhere on Earth's surface today would interfer with such reactions today.

2. Stanley Miller's Experiment as a graduate student with Harold Urey in Chicago in 1953 is one of those simple yet great experiments of the 20^th century. They hypothesized the components of a primitive Earth atmosphere and built an aparatus to simulate the conditions on the early Earth. The discovery of simple amino acids in the gunk that was generated led to the beginnings of experimental work in Chemical Evolution.

Basic Monomers

[from Wald, 1964] The 30 molecules basic to biochemical processes:

• 20 amino acids
• glucose ( a basic 6-C sugar)
• fats (i.e. fatty acids, long chain hydrocarbons)
• phosphatides (Phospholipids - technically not monomers, but they do form the basic monomeric (i.e. repeating) unit in the cell membrane.)
• the 5 nitrogeneous bases
• Ribose (5-C sugar) and its derivative, deoxyribose
• phosphoric acid

Amino acids are formed in the test sparking chambers as first begun by Miller. The reaction mechanism, via the Strecker synthesis, in the sparking chamber is,

RCHO + HCN + NH₃ � RCH(NH₂)CN + H₂O � RCH(NH₂)CO-NH₂ + H₂O � RCH(NH₂)COOH

aldehyde + hydrogen cyanide + ammonia � amino nitrile + water � intermediate + water � amino acid

Clifford Mathews at Illinois has also proposed a mechanism for the production of proteins directly from HCN, bypassing the intermediate stage of amino acid production.

The four most readily produced amino acids are, Glycine, alanine, aspartic acid and glutamic acid

B. Simple sugars � carbohydrates

• starch, a plant and algal food storage product
• cellulose, a plant structural component of cell walls
• glycogen, an animal food storage product

CO₂ + 2H₂ � H₂CO + H₂O.

Formaldehyde is very soluble in water, so it is likely to have dissolved in rainwater after synthesis in the upper atmosphere. Calculations have shown that within 10⁶ yrs enough H₂CO would have formed to be actively reacting with UV light in solution.

Formaldehyde is also found in comets and has been detected in interstellar space.

C. Nucleotides� nucleic acids, DNA and RNAs

Oró 1961: purine (adenine) synthesis from HCN

Pyrimidines in 1978 by Ferris et al. involving HCN at pH 8.5 using dilute solutions

D. fatty acids + glycerol � lipids, needed for cell membranes

Methods of Synthesis

• Spark discharge (to simulate lightening)
• Ultraviolet (UV) radiation
• Pressure waves and shock waves
• Heat, high temperatures

• Unknown chemical reactions � yellowish brown sludge. Insoluble tars & polymers.
• Fischer-Tropsch Reaction. Gaseous carbon passed over a hot catalyst such ae powdered Fe produces hydrocarbon chains, precursors to fatty acids
• Photochemical reactions, esp. UV
• Ferrous (iron-rich) clays react with CO₂ and H₂O to produce organic acids which then adsorb onto clays. These complexes can then react to produce stable macromolecular precursors.

The Problem of a Neutral Atmosphere

There is a fundamental problem with the synthesis of monomers on the Early Earth if the atmosphere did not contain a strong reducing agent like H₂ or NH₃. For example, many amino acids are unstable in neutral atmospheres. Miller has observed that without a reduced carbon source, like methane (CH₄), amino acids other than glycine are hard to produce in high yields. Current theory is not now in favor of a strongly reducing atmosphere for the secondary (stage II) atmosphere. One possible solution to this is that reducing agents associated with various mineral sources, especially clays, could have played a role in these reactions. Another possibility is that locally reduced environments could have played a role in the synthesis of different amino acids monomers.

Web Resources

• An interview with Stanley Miller.
• A primer on amino acids by Michael W. King, Ph.D / Medical Biochemistry / Terre Haute Center for Medical Education.
• All about carbohydrates from Dennison University.

update 2 October 2002