Happy World Handwashing Day! The perfect excuse to review what’s the rub behind that sudsy scrub! The science of soaps and detergents (artificial soaps) – and how (despite all this attention on fancy new things) they remain one of our greatest weapons against the coronavirus – and other viruses, bacteria, & more! Also, how we use detergents in the lab – and which ones when.
Soap – like everything that “matters,” is made up of molecules. Molecules are made up of basic units of elements called atoms (think individual carbons, hydrogens, etc.) linked together by sharing electrons, which are negatively-charged subatomic particles that whizz around each atom’s core “atomic nucleus.” Inside the nucleus are positively-charged subatomic particles called protons that can counterbalance the negative charge of the electrons. If you have an even number of protons and neutrons, an atom or molecule will be neutral overall – but if the electrons aren’t shared fairly between the atoms of a molecule, some parts of the molecule can be partly charged while others are negatively charged in an uneven charge distribution referred to as “polarity”⠀
This is important because water is super polar because oxygen is really electron-hoggy. So water’s H’s are partly positive and its O’s are partly negative, and opposites attract, so water really likes to stick to copies of itself, forming extensive networks of water molecules. And if something else tries to get in, it needs to offer up something the water likes as much as or more than another water molecule (like polar parts or, even better, full charges). We call these water-loved molecules “hydrophilic”⠀
If a molecule doesn’t have anything to offer the water other than a bland neutral surface, as you get with nonpolar molecules, the water molecules are like “no way dude!” and don’t let those “hydrophobic” molecules into their clique. Instead, the water molecules gang up on the hydrophobes, forcing them into as small a space as possible so that as few as possible water molecules have to interact with them. This is called the hydrophobic exclusion effect. So even though we call them hydrophobes as if these water-excluded molecules are afraid of water, it’s really just that water doesn’t like them – so stop judging!⠀http://bit.ly/hydrophobiceffect
But it’s not like all molecules are hydrophobic OR hydrophilic. Some molecules are BOTH – one part is hydrophobic and one part is hydrophilic – we call such molecules AMPHIPHILIC (aka AMPHIPATHIC) and they include soaps and detergents⠀
They have a hydrophilic head that water’s cool with and a hydrophobic tail that water rejects. So these molecules orient themselves so that those hydrophobic tails huddle together with the heads sticking out to interact with the water. Their heads are “bulkier” so when they do this, they forms spheres called micelles with room inside for other hydrophobes to reside – if there’s greasy (hydrophobic) gunk present, micelles can form around it, coating that gunk with their hydrophobic tails facing the gunk & hydrophilic heads facing the water. So now, instead of stuck-on gunk, you have a soluble “packet” of gunk you can wash away ⠀
A lot of times we talk about this gunk in terms of things like greasy stuff stuck on a pan. But these days, we’re often talking about gunk in terms of virus particles stuck on your skin, hoping you’ll touch your nose, eyes, or mouth, so they can sneak in!⠀
But they can’t sneak in if you wash it off first! Viruses and other microbes sitting on your skin or surfaces, when confronted with soapy water, can get caught up in those micelles too, so you can wash them away. And it’s really important that you do that scrubbing action (I strongly encourage amino-acid-song-singing to get to that 20s of hand washing….) in order to dislodge and wash away that gunky junk.⠀
Simple “dislodging” isn’t the only way soaps can combat these wannabe invaders – “viral particles” like those of SARS-Cov-2 (the virus that causes the disease COVID-19) are “just” a sack of genetic info and proteins, surrounded by a membrane made up of phospholipids. Phospholipids are similar to soaps and detergents in that they are amphiphilic, but they have multiple tails and they can form phospholipid bilayers, which are like sandwiches with the tails as the “peanut butter” and the heads as the breads. This configuration helps them cordon off the watery inside of the particle from the (often watery) outside world.⠀
Soaps and detergents can break up this membrane, causing the virus to “fall apart” – because of their shape, soaps & detergents are more likely to form spherical micelles whereas phospholipids are more likely to form bilayers. But they’re similar enough that detergents can break up phospholipid bilayers, helping break open (lyse) viral particles. You might think that this would be dangerous because it leads to the virus releasing all its insides – but without the coat, the virus has no way to get into your cells to do any damage. And, without getting into cells, a virus can’t replicate itself – instead, its genome (genetic blueprint, which in the case of SARS-Cov-2 is a single strand of RNA) gets degraded. No harm done!⠀⠀
Our own cells, including those of our skin, are also surrounded by phospholipid membranes – so you might wonder why soap doesn’t just dissolve us in the process – thankfully, our skin is protected by a thick layer of dead skin cells called the stratum corneum.
A closer look at soaps and detergents at the molecular level…⠀
First – the terminology – detergents are just “artificial” versions of soaps. This doesn’t mean they’re bad, it just means they’ve been synthesized, which offers opportunities for molecular “enhancement.”⠀
Both have a hydrophobic tail made up of a long hydrocarbon chain – the linked-together hydrogens (H’s) and carbons (C’s) & C share electrons equally, giving you a long nonpolar part (how long depends on how many Cs & Hs, and the length helps determine “latherability”)⠀
Where soaps and detergents diverge is in the hydrophilic heads. SOAPS are derived from natural fatty acids, so they all have same head (carboxylate, which is ➖). But DETERGENTS can be synthesized to have different heads, which can be ➖ or➕, or even neutral but polar (nonionic detergents)⠀
Different detergents form micelles with different numbers of detergent molecules – the aggregation number – and in order for micelles to form you have to have enough detergent molecules for them to be able to find one another and team up – the concentration at which this becomes likely is the Critical Micelle Concentration (CMC) – it depends on the molecular makeup of the detergent (e.g. less lipophilic detergents are more less desperate to avoid water by hanging out together and minimizing their exposure so you have to get more of them together to “convince them” to -> higher CMC.)⠀
Detergents are used frequently in the lab, and lab-by detergents often have cool names, like “Tween” and “Triton.” More on those in a second, but I’ve gotta start by talking about the most “famous” detergent of them all – Sodium DodecylSulfate – the SDS behind SDS-PAGE!
Similarly to how soaps and detergents can sneak into cell membranes, some can slither into proteins & denature (unfold them) and others can break up protein-protein interactions but leave the proteins folded. SDS is a really harsh detergent – and that harshness is great for SDS-PAGE because it unfolds proteins so that you can separate proteins based only on their “length” instead of their shape. By “harsh” I mean that SDS actually denatures (unfolds) proteins, whereas milder detergents like Tween-20 leave them be, just disrupt proteins’ interactions with one another. Mild detergents like Tween-20 are often used at low concentrations designed to break up weak, non-specific interactions, but not denature (unfold) proteins and not “outcompete” specific interactions, whereas we use harsh detergents like SDS when we want to denature them.
SDS is negatively charged (anionic) – which is one of the primary reasons we use it for SDS-PAGE, a technique to separate proteins by size by sending them traveling through a PolyAcrylamide Gel mesh using Electrophoresis (bigger proteins get tangled up more so travel slower). The SDS is crucial to this technique for a couple of reasons: when SDS unfolds & coats proteins, it coats them with negative charge. And that negative charge gives us a way to direct them by putting a positive charge to “bribe them” to go where we want them to go (towards the bottom of the gel in SDS-PAGE. The negative charge also makes the proteins stay unfolded because negative charges repel each other. Once the protein’s unfolded the hydrophobic parts of the SDS can glob onto the hydrophobic parts of the protein (which were probably hidden in the center of the protein). And then the negative heads of the SDS repel each other so the protein doesn’t try to refold.
Tween-20 is a non-ionic (uncharged) detergent. And it’s considered “mild” because they don’t unfold proteins. Basically, proteins get their shape largely through interactions between the amino acids’ (protein letters)’ unique parts (side chains) and/or generic parts (backbone). Unlike the strong, covalent peptide bonds linking the letters together to form the polypeptide chain that folds up, these other interactions are non-covalent and reversible. Individually, they’re weak, but collectively they’re strong. If you think of these interactions as a sort of glue, you typically have more glue holding a protein’s shape together (intrAmolecular interactions) than you do holding proteins to other proteins or molecules (intERmolecular interactions). So those intermolecular interactions are easier to break up and milder detergents like Tween can break them up.
Detergents like Tween-20 are great for things like preventing non-specific binding during Western blots, etc. http://bit.ly/westernblotworkflow
In case you’re curious, the “20” in Tween-20 indicates that it has 20 ethylene oxide subunits – these are hydrophilic but NOT charged (NON-IONIC) and they’re attached to a sorbitol (a type of sugar) backbone ring. The hydrophobic part is a lauric acid (a type of fatty acid). Its relative, Tween 80 has an oleic acid tail instead.
In addition to ionic detergents like SDS (and deoxycholate, chelate, sarkosyl, etc.) and non-ionic detergents like Tween-20 (and triton x-100, DDM, digitoxin, tween 80, etc.) there are zwitterionic detergents – these have positive and negative parts that balance each other out so there’s no net charge. An example of this is CHAPS. more good info on lab detergents: https://www.labome.com/method/Detergents-Triton-X-100-Tween-20-and-More.html
Another common use for detergents in biology is in cell lysis (breaking open cells). Sound familiar?
If you want to learn more about all sorts of things: #365DaysOfScience All (with topics listed) 👉 http://bit.ly/2OllAB0 ⠀