In an article yesterday, I described how Rice University chemist James Tour elucidates, in his video series on the origin of life, the challenges associated with synthesizing life’s building blocks. Today, I will describe how he details the implausibility of the building blocks ever joining into large composite molecules (aka, macromolecules) such as proteins, RNA, and DNA.
Requirement of Exacting Protocols
The key challenge is that the building blocks could easily combine in multiple ways, but only one linkage is useful to life. To mitigate this hurdle, linking procedures must entail highly controlled steps including the following:
- Starting with only the desired building blocks in unrealistically high purity and concentrations.
- Adding molecules that protect exposed atoms and side chains in the building blocks and prevent them from interacting with other building blocks in a manner that would result in incorrect linkages.
- Adding molecules that force building blocks to link properly.
- Removing the protecting molecules.
This process must be repeated for the addition of each amino acid or nucleotide (monomer) until the chain reaches the desired length. As with the synthesis of the building blocks, the exquisite exactness in protocols and the use of molecules not available on the prebiotic earth make such experiments exceedingly unrealistic models for the origin of biologically relevant macromolecules.
Challenge of Homochirality
Another next to impossible hurdle is for any natural process to isolate only one version of molecules that can exist as mirror images, such as selecting only left-handed amino acids in the formation of proteins. Yet, using exclusively one version (homochiral) is essential for biological operations, such as those that require chiral-induced spin selectivity. The most highly skilled chemists can carefully design experiments starting with large quantities of homochiral molecules to yield amino acids with an enantiomeric excess (more left-handed than right-handed or the opposite), but the degree of excess is far too small to be useful for life.
Only one reaction has ever been observed that can lead to significant enantiomeric excess, which is the Soai autocatalytic reaction, but that reaction and the underlying physical process driving it are recognized by leading experts to have no relevance to any process that could promote homochirality in any biologically relevant molecule. Even if creating homochiral mixtures were plausible, natural processes would continuously deteriorate such mixtures toward equal quantities of left-handed and right-handed molecules (racemization). This barrier alone completely undermines all undirected origin-of-life proposals.
Problem of Degradation
Many have argued that a cell could have emerged over enormous periods of time. Problems that appear overly daunting in a laboratory setting might become tractable if allowed to progress over billions of years. This assertion is negated by the fact that macromolecules degrade far too quickly (here, here, here) to allow for their accumulation in a developing protocell. I have noted previously that a functional protein, RNA, or DNA molecule in only one liter of water containing a cell membrane would likely not even make contact with the membrane through diffusion before it would break apart. The main search mechanism would have to be diffusion since water sufficiently agitated to mix molecules at microscales would likely eviscerate any cell membrane. In other words, any progress toward life would be lost if a fully functional cell did not emerge within a reasonably short period of time.
Equally problematic, even if building blocks naturally joined together and never degraded, no natural process could order the monomers in the correct sequence to contain useful biological information. Each of these challenges represents an insurmountable barrier to any biologically useful macromolecule ever appearing and migrating into the staging ground for life’s origin.
Tomorrow, “James Tour Video Series on the Origin of Life — Assembly of Cellular Components.”