Water was both a necessity and an obstacle to early life on Earth. Microdroplets are a potential solution to this paradox

It’s a paradox: life needs water to survive, but a world full of water cannot generate the biomolecules that would have been essential at the beginning of life. Or so the researchers thought.

Water is everywhere. Most of the human body is made of it, much of planet Earth is covered with it, and humans cannot survive more than a few days without drinking it. Water molecules have unique characteristics that allow them to dissolve and transport compounds through your body, provide structure to your cells, and regulate your temperature. In fact, the basic chemical reactions that enable life as we know it require water, photosynthesis being one example.

However, when the first biomolecules like proteins and DNA began to come together in the early stages of planet Earth, water was actually a barrier to life.

The reason is surprisingly simple: the presence of water prevents chemical compounds from losing water. Take, for example, proteins, which are one of the major classes of biological molecules that make up your body. Proteins are basically chains of amino acids linked together by chemical bonds. These bonds are formed by a condensation reaction that results in the loss of a water molecule. Essentially, amino acids must become “dry” to form a protein.

Considering that the Earth before life was covered in water, it was a big problem to make the proteins essential for life. Like trying to dry off inside a swimming pool, two amino acids would have struggled to lose water to come together in the primeval soup of early Earth. And it’s not just proteins that have faced this problem in the presence of water: other biomolecules essential to life, including DNA and complex sugars, also depend on condensation reactions and loss of water. water to form.

Over the years, researchers have proposed many solutions to this “water paradox”. Most of them are based on very specific scenarios on the early Earth that could have enabled the elimination of water. These include pools of water being dried up, mineral surfaces, hot springs and hydrothermal vents. These solutions, while plausible, require special geological and chemical conditions that might not have been commonplace.

In our recent study, my colleagues and I found a simpler and more general solution to the water paradox. Ironically enough, it could be water itself – or to be more precise, very small water droplets – that allowed the first biomolecules to form.

Why microdroplets?

Water droplets are everywhere, both in the modern world and especially during prebiotic (or pre-life) Earth. On a planet covered in crashing waves and raging tides, the small water droplets contained in sea spray and other aerosols would likely have provided a simple and abundant place for the assembly of the first biomolecules.

Micro-droplets of water – usually very small droplets with a diameter of about one millionth of a meter, much less than the diameter of spider silk – may not seem to solve the water paradox at first glance, until until you consider the very particular chemical environments they create.

Microdroplets have a substantial surface area to volume ratio which increases the smaller the droplet. This means that there is a significant space where the solvent that composes them (in this case, water) and the medium that surrounds them (in this case, air) meet.

Over the years, researchers have shown that the air-water interface is a unique chemical environment. The chemistry of these microdroplet interfaces is dominated by large electric fields, partial solvation where the molecules are partially surrounded by water, highly reactive molecules, and very high acidity. All of these factors allow microdroplets to accelerate the chemical reactions that take place in them.

Our lab has been studying microdroplets for a decade, and our previous work has shown how the rate of common chemical reactions can be accelerated up to a million times faster in microdroplets. Reactions that would have taken a full day could now be completed in just a fraction of a second using these tiny droplets.

In our recent work, we have proposed that microdroplets could be a solution to the water paradox because their air-water interface not only accelerates reactions but also acts as a “drying surface” that facilitates the reactions needed to create of biomolecules despite the presence of water.

We tested this theory by spraying amino acids dissolved in microdroplets of water towards a mass spectrometer, an instrument that can be used to analyze the products of a chemical reaction. We found that two amino acids can successfully join in the presence of water via microdroplets. When we added more amino acids and collided two sprays of this mixture, mimicking crashing waves in the prebiotic world, we discovered that it could form short peptide chains of up to six amino acids.

Our findings suggest that water microdroplets in environments such as sea spray or atmospheric aerosols were fundamental microreactors in early Earth. In other words, the microdroplets may have provided a chemical medium that allowed the basic molecules of life to form from just small compounds dissolved in the vast primordial ocean that covered the planet.

Past and future microdroplets

Microdroplet chemistry could be useful in addressing current challenges in many scientific fields.

Drug discovery, for example, requires synthesizing and testing hundreds of thousands of compounds to find a potential new drug. The power of microdroplet reactions can be integrated with automation and new tools to accelerate synthesis rates to more than one reaction per second as well as biological analysis to less than one second per sample.

In this way, the same phenomenon that might have contributed to the origin of the building blocks of life billions of years ago can now help scientists develop new drugs and materials faster and more efficiently.

Perhaps JRR Tolkien was right when he wrote, “Such is often the course of actions which move the wheels of the world: little hands do them because they must, while the eyes of great are elsewhere.”

I believe that the importance of these small droplets is much greater than their small size.

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