It’s a paradox: Life needs water to survive, but a world full of water can’t create the biomolecules that would be necessary for early life. Or so the researchers thought.
Water is everywhere. Most of the human body is made up of it, much of planet Earth is covered in it, and people can’t 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 such as 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 main classes of biological molecules that make up your body. Proteins are, in essence, chains of amino acids linked together by chemical bonds. These bonds are formed through a condensation reaction that results in the loss of a water molecule. Essentially, amino acids must “dry out” to form a protein.
Considering that the Earth before life was covered in water, this was a big problem to make proteins necessary for life. Like trying to dry out in a swimming pool, two amino acids would struggle to lose water to join the primordial soup of the early Earth. And it wasn’t just proteins that faced this problem in the presence of water: Other biomolecules essential to life, including DNA and complex sugars, also rely on condensation reactions and water loss to form.
Over the years, researchers have proposed many solutions to this “water paradox.” Most of these are based on very specific scenarios on the early Earth that could have allowed for the removal of water. These include drying pits, metal surfaces, hot springs and hydrothermal vents, among others. These solutions, while plausible, require special geological and chemical conditions that may not have been common.
In our recent study, my colleagues and I found a simpler and more general solution to the water paradox. Ironically, it may be water itself – or rather, very small droplets of water – that allowed early biomolecules to form.
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 tiny droplets of water in sea spray and other aerosols would plausibly provide a simple and abundant place for the first biomolecules to gather.
Micro-water droplets – typically very small droplets around a millionth of a meter in diameter, much smaller than the diameter of spider silk – may not seem to solve the water paradox at first, until you consider the very specific chemical environments they create .
Microdroplets have a significant surface-to-volume ratio that increases the smaller the droplet. This means that there is a significant space where the solvent they are made of (in this case, water) and the medium they are surrounded by (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 these factors allow the microdroplets to accelerate the chemical reactions that occur 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 an entire day could now be completed in just a fraction of a second using these tiny droplets.
In our recent work, we 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 tiny water droplets toward a mass spectrometer, an instrument that can be used to analyze the products of a chemical reaction. We found that two amino acids can be successfully joined in the presence of water via microdroplets. When we added more amino acids and collided two sprays of this mixture, mimicking the crashing waves in the prebiotic world, we discovered that this can 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 on the early Earth. In other words, the droplets may have provided a chemical medium that allowed the basic molecules of life to form from the simple, small compounds that were dissolved in the vast primordial ocean that covered the planet.
Small droplets past and future
Microdroplet chemistry can be useful to address 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 incorporated with automation and new tools to accelerate synthesis rates to more than one reaction per second as well as biological analysis in less than one second per sample.
In this way, the same phenomenon that could have helped form 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 the deeds that move the wheels of the world: small hands do them because they must, while the eyes of the great are elsewhere.”
I believe that the importance of these little droplets is far greater than their tiny size.
Nicolás M. Morato, PhD candidate in Chemistry, Purdue University
This article is republished from The Conversation under a Creative Commons license. Read the original article.