Hydrogen exchange (H-T and NMR H-H) on DNA and RNA reveals two distinct base-pair opening modes: single-base microsecond openings and multi-base millisecond soliton-like loops.
0:00Welcome to Base by Base, the paper cast that brings genomics to you wherever you are. Thanks for listening, and don't forget to follow and rate us in your podcast app. glad to be here. So, uh, your genetic code is locked inside this microscopic vault, the famous DNA double helix.
0:15Right. It's incredibly secure structure. Yeah, super stable, but that structural security actually creates a massive logistical nightmare. It really does. I mean, if the physical letters of your code are buried deep in the center of a closed vault, how did the biological machines inside you actually read them?
0:34That is the big question. Right. Like, how do the proteins that replicate your DNA or transcribe it into the building blocks of life get access to the information if the door is firmly shut? It is basically the ultimate paradox of biology.
0:47Because we always picture DNA is this static, majestic ladder, you know, like in the textbooks. Right, is sitting there perfectly twisted? Exactly. But for that genetic code to be useful to a living organism, the vault actually has to open.
1:00It has to come apart. Right. The ladder has to physically separate. And it turns out DNA actually breathes. Yes, it does. It spontaneously pops open on its own, constantly exposing those internal letters to the surrounding cellular environment.
1:15And this happens even without a protein prying it apart. Which is pretty wild to think about. It is. But, uh, what really happens when our genetic code opens up, like, how big are these opening? And how fast do they have?
1:28Exactly. Answering those specific questions triggered one of the most intense 30-year-long scientific conflicts in the history of biophysics. Oh, it was a massive debate. We're talking about 2 camps of brilliant scientists using 2 highly accurate measuring methods and getting completely undeniable contradictory results.
1:48Okay, let's unpack this. To really understand how this monumental mystery was finally solved. We have to look at the remarkable research that managed to step back. to look at the entire landscape and reconcile decades of conflicting data.
2:01Today, we celebrate the work of S. Walter Englander and the Perilman School of Medicine at the University of Pennsylvania, who have advanced our understanding of nucleic acid-based pair open states. And the significance of this specific deep dive really cannot be overstated.
2:17It's a huge deal. It really is. We are looking at a paper that acts as a true Rosetta Stone for the biophysics field. Wow. Yeah, it takes a fundamental conflict that dates all the way back to the 1990s and brilliantly reveals that everyone involved in this 30-year argument was actually right.
2:35Wait, everyone was right. Yes. They were just looking at completely different parts of the same elephant. Right. The classic blind men in the elephant scenario. love that. Exactly. But to fully appreciate the drama here.
2:46I mean, we need to understand how scientists even know DNA is breeding in the 1st place. Because it's not like you can just look at it. Exactly. You can't just slide some DNA under his standard microscope, pull up a chair and watch it breathe.
2:59No, the molecules are far too small. Right. And the movements happen way too fast. So they have to use a trick. Yes. And it's a wonderfully elegant trick based on something called the Linderstrom Lang theory.
3:10Okay, lay that out for us. So the core premise is this. Your DNA is surrounded by water inside the cell. Water is made of hydrogen and oxygen. The DNA base pairs the rungs of the ladder, are held together by hydrogen bonds.
3:24The chemical glue. Exactly. The theory states that if you want a hydrogen atom that is locked deep inside the DNA double helix to swap places or exchange, with a hydrogen atom in the surrounding water, something has to happen first.
3:38A door has to open? Precisely. The protective hydrogen bonds of the DNA base pair must dynamically open. I was actually trying to visualize this earlier. It's sort of like having a brightly colored tennis ball locked inside a heavy steel safe.
3:52And you want to swap it with a tennis ball that's just, you know, bouncing around the room. The absolute only way that physical exchange happens is if the safe door swings open, even for a fraction of a split second.
4:05Precisely. The heavy steel safe is the closed double helix. So by mathematically tracking the rate at which these hydrogen atoms exchange with the surrounding solvent, which is the room in your analogy, scientists can work backward.
4:20Oh, I see. Yeah, they can calculate exactly how often the DNA is opening, and precisely how long it stays open. That's genius. It is. In the literature, we call this hydrogen exchange or HX. Well, and this brings us right to the historical drama.
4:34The 30 year war. Right. For decades, the biophysics community was essentially split into 2 heavily entrenched factions. really were. They were trying to answer the exact same question, but they used 2 completely different methods to measure this hydrogen exchange.
4:50And they got totally different answers. For decades. These two sides got completely irreconcilably different pictures of how DNA works. It wasn't just a minor discrepancy in the decina point. No, was a massive puzzle.
5:02Yeah, it had scientists scratching their heads at conferences year after year. It was incredibly frustrating for the entire field. I mean, both methods were rigorous. Both were highly accurate. Right Nobody was doing bad science.
5:13Exactly. Neither side was making mathematical errors. Yet they painted totally opposing realities of how the DNA structure behaved. Well, wait, I have to push back on this for a second. We're talking about the exact same Watson Crick base pairs, right?
5:28Yes, an AT pair or a GC pair? Right. So physics is physics. If it's the exact same molecule governed by the same universal laws of thermodynamics, how can DNA act completely differently depending on who is looking at it?
5:43Or what machine they happen to be using. Yeah, it doesn't make any logical sense. What's fascinating here is that the contradiction wasn't caused by the equipment failing, or the laws of physics suddenly shifting.
5:55Okay. Well, what was it? The answer to your question lies entirely in the physical size of the DNA being studied. Wait, the size? Yes. The 2 camps weren't just using different machines. They were effectively forced by those specific machines to look at completely different sizes of DNA molecules.
6:10All right, so let's look at the 2 heavyweights in this fight. Looking at the notes here. Camp number two, the NMR camp, had a massive handicap right out of the gate. Because early nuclear magnetic resonance technology was revolutionary, like taking an MRI of a molecule, but it couldn't handle the giant DNA strands you actually find in living cells.
6:31Right, the technology had limitations. Yeah, for the machine to get a clear signal, the molecules had to tumble and rotate rapidly in the solution to average out magnetic interactions. And long DNA doesn't do that.
6:43Exactly. If the DNA was too long, it tumbled too slowly, and the machine just gave them a blurry, unreadable mess. It was just noise. So they were essentially forced to chop the DNA up into these tiny bite-sized pieces, usually under 15 base pairs in length, just to get a reading.
6:59That is exactly right. The NMR camp was geographically constrained, so to speak, to studying only these very short fragments. Meanwhile, you have method number one, HT exchange, or Tritium exchange. The other side of the aisle.
7:13Right. This method didn't have that size limitation. They could look at massive, long polynucleotides. How massive. We're talking strings that are 1000s of base pairs long, much closer to the native, naturally occurring DNA in your cells.
7:26Okay, so how did their method work? They basically introduced radioactive tritium into the surrounding water. This acts as a traceable tag. Got it. And then they tracked how fast that radioactive tritium swapped out with the normal hydrogen inside the long DNA.
7:42So we have 2 perfectly good machines giving us 2 completely different realities. Camp one is looking at giant 1000s of letters long DMA strands, and Camp 2 is looking at tiny 15 letter DNA snippets. Exactly.
7:58But to figure out who is being tricked and why the results were so wildly different, we have to look much closer at what these machines are actually measuring at the atomic level. They have to look at the protons.
8:08Right. Specifically how willing these hydrogen atoms are to jump into the surrounding water. And this deep dive gets into Eigen's proton transfer theory, which is remarkably elegant. It is a beautiful piece of physical chemistry.
8:21Yeah. Eigen's theory explains that not all hydrogen atoms or protons in this context are holding onto the DNA with the same strength. Okay, so they have different grip strengths, basically. Basically, yes.
8:30There are 2 different types of exchangeable protons inside these base pairs, and they behave very differently when the safe door finally opens. What's the first type? First, you have the imodoprotons. These are relatively strong acids with a PK of about nine.
8:46And just as a quick refresher on chemistry, The PK scale is logarithmic, right? Yes, it is logarithmic. So a PK of 9 means this proton is practically looking for an excuse to jump ship. I mean, it's ready and willing to give up its spot the moment it gets a viable chance.
9:00Exactly. The very moment the DNA base pair swings open and exposes that immunoproton to the solvent, the proton transfers immediately. Like instantly. It happens on the absolute 1st collision with a water catalyst.
9:13In the field we call this EX1 exchange. Because the chemical transfer is completely instantaneous. The only thing slowing the process down is the wait for the DNA to open. Right, the bottleneck is the safe door.
9:24Precisely. Therefore, measuring this specific exchange gives you the exact unadulterated rate of DNA opening. Okay, that makes sense. The DNA opens, the immenoproton instantly swacks, but then we have the 2nd type, the amino protons.
9:39And based on this paper, they are a completely different beast. Very different. The amino protons are much weaker acids. Their PK is around 20. Whoa. Yeah, and because the scale is logarithmic, the difference between 9 and 20 is astronomical.
9:53It's massive. These amino protons do not want to let go of their hydrogen. When the DNA opens and a solvent molecule bumps into the immunoproton, it almost always fails to make the swap. Really? Even when the door is open.
10:07Even when it's open, the physical conditions just aren't right, the DNA has to open, close, open, close, open, and close over and over again before a successful exchange finally happens. I was trying to come up with a good way to picture this EX2 exchange because it's so strange.
10:22It very counterintuitive. It's less like a standard chemical reaction and more like trying to throw a dart through a rapidly spinning windmill. Like, the gap between the windmill blades opens and closes constantly.
10:34But your timing has to be absolutely perfect to make the dart through. Exactly. Most of the time, the water molecule bumps into the proton, but the windmill snaps shut before the swap can actually finalize.
10:45It takes dozens, maybe 100s of these open closed cycles before the timing and angle align perfectly for the exchange. That analogy perfectly captures the mechanism of probability at play here. And because this process requires multiple openings and closings and relies on that fleeting window of opportunity, it's called EX2 exchange.
11:06Okay. EX1 is fast, EX2 is slow. Right. And while it sounds incredibly frustrating to measure, It's actually a massive gift to scientists. How so? Because this exchange is so agonizingly slow and dependent on multiple cycles.
11:20The mathematics allow researchers to calculate the exact fraction of time the DNA spends in the open state. Ah. It gives us what we call the equilibrium constant. Okay, let's take stock. We have the tools.
11:30We do. We knew how to measure the opening rate using the eager amino protons, and we know how to measure the fraction of time it stays open using the stubborn amino protons. But this brings us back to the Great Divide.
11:42Method one, studying huge native DNA, and method 2 studying tiny, chopped up DNA. The root of the whole conflict. This feels like trying to understand the fundamental physics of water. Well, if the HT tritium camp is out there studying a massive complex churning ocean.
12:02Yes. And the NMR camp is studying a single cup of water sitting on a desk. That's great way to put it. If you only ever look at a cup of water, you might logically conclude that large tidal waves simply don't exist in nature, because how could a tidal wave possibly fit inside a cup?
12:17That is the exact illusion that tricked the field for 30 years. Wow. The size constraint dictated what phenomena could physically occur. Here's where it gets really interesting. The specific numbers they found.
12:28and how massive the discrepancy actually was. The data is shocking. When the NMR camp looked at their tiny short DNA strings, they found that the DNA opened for just microseconds. A 1000000th of a second.
12:41Just a flicker. Right. It would snap shut at an incredibly fast rate a 1000000 times per second. And these openings were incredibly rare. Only about one in a 1000000 base pairs was open at any given time.
12:53Which paints a picture of DNA as this incredibly tight, rigid, almost impenetrable structure. Yeah, totally lockdown. Right, where a single bass pair occasionally flickers open and instantly slams shut because it requires so much localized energy to break the structural stack.
13:09Right. But then you look at the HHT tritium camps data on the massive long DNA. The ocean. The ocean. They found that DNA stays open for milliseconds. That is 1000s of times longer than the NMR camp claimed.
13:21A huge difference. They found it closes relatively slowly, only about 10 times per 2nd compared to a million. And most shockingly, they found that these openings were incredibly common. About one in every 100 base pairs was open at any given time compared to one in a million.
13:36You're looking at a 1000 fold difference in the raw data. That's insane. You can easily see why the 2 sides thought the other must be doing the math wrong or contaminating their samples. Obviously. You'd think, you guys are crazy.
13:49Exactly. But the elegant resolution presented in this paper is that both camps are perfectly right. Their data is flawless. Flawless. They're just measuring 2 entirely different modes of DNA opening. Because of the size of the DNA, the ocean versus the cup?
14:06Exactly. When you have a tiny string of DNA under 15 base pairs, it physically cannot support a large multi-base opening. Why not? Well, if it tried to open 10 base pairs at once, there wouldn't be enough impact structure left to hold it together, the whole molecule would just fall apart and melt into single strands.
14:24Oh, it would just disintegrate. Right. So the short DNA is constrained. It can only pop open a single, lonely base period of time. Okay. Breaking a single base out of the stack takes a massive amount of energy, which is exactly why it's snap shut in a microsecond and why it's so exceedingly rare.
14:39And that tiny flickering anomaly is what NMR was measuring. Yes. But the long DNA, the native like DNA that the Tribune camp was looking at, is a completely different physical environment. Because it has the structural support.
14:52Exactly. It's so long, so massive, and so stable that it can easily host massive multi-base openings without falling apart. And here's the real kicker of this deep dive. The tiny single base popping that NMR saw is absolutely still happening in the long DNA.
15:08Yes, he hasn't vanished. It's just that the large multi-base openings are a 1000 times more common because they are so large and frequent. They completely drown out the tiny openings in the macro data.
15:20The massive tidal waves hide the tiny ripples. That is the synthesis that resolves the 30-year conflict. Native DNA utilizes a completely different larger mode of opening that simply cannot exist in the artificial short snippets used for NMR.
15:36So we've definitively established that real native length DNA experiences these massive multi-base opening, but what does that actually look like inside the cell? Are we talking about a static bubble of 10 base pairs just sitting there completely open waiting for a protein to happen to bump into it?
15:52No, and this is where the biophysics gets truly beautiful. Okay, I'm ready. The paper introduces the concept of the soliton hypothesis to explain these large openings. Yes. A solitin isn't a static stationary bubble.
16:07It's a trapped migrating wave packet. A migrating wave packet, meaning the opening itself physically moves down the strand. Yes. Imagine an open loop of about 10 base pairs. Okay, I'm picturing it. And this is vital.
16:22Even though the horizontal hydrogen bonds connecting the 2 strands are broken in this loop, the structure doesn't collapse. Wait, why doesn't it collapse? I mean, if you break 10 rums on a ladder, that section of the ladder usually falls apart.
16:35You'd think so, but it's because of Vanderval's forces. Ah, Vanderval. All right. Even though the horizontal hydrogen bonds break, the flat chemical bases are still stacked vertically on top of each other.
16:45Almost like a tightly packed stack of coins. The physical Vandervall's forces, holding that vertical stack together, keep the energy state relatively low and prevent the structure from just melting. That's fascinating.
16:58But because of the constant thermal energy jostling the molecule in the water, this open stacked loop doesn't stay in one place. It travels back and forth along the DNA strand in a random walk. Okay, wait, let me clarify how this moves.
17:14Because if a physical bubble of DNA is moving down the line, wouldn't it tangle the entire double helix into microscopic knots? Is the DNA material itself sliding down the line? That is a crucial distinction to make.
17:28No, the DI material itself is not sliding or tangling. Okay. The actual nucleotides stay exactly where... I get it. It like a stadium wave at a baseball game. A skadium wave. The people in the stands representing the base pairs aren't physically running around the circumference of the stadium.
17:44They are just standing up, throwing their arms in the air to break the connection with the person next to them, and then sitting back down in a specific sequence. That is a perfect analogy. The wave travels around the entire stadium, but the people stay in their exact assigned seats.
17:58The void moves, not the material. Exactly. opening just passes through them. Wow. As the leeing edge of the solitude wave approaches. A closed base pair opens up and joins the loop. As the trailing edge passes, it snaps back shut.
18:13It's so dynamic. It is. The information is exposed temporarily, and then safely secured again. So why does a migrating stadium wave of open DNA matter to you and me. Like, why does the cell go through all this trouble?
18:26If we connect this to the bigger picture, It solves the logistical problem we talked about at the very beginning of this deep dive. And the proteins, read the locked code. Exactly. This traveling solitin wave acts like a biological scanner is dynamically zipping back and forth along the DNA strand, spontaneously exposing stretches of letters.
18:44Just scanning along. It exposes these sequences just long enough for reading proteins. The complex biological machines that trigger gene expression, replication, or transcription to recognize their specific target docking sites.
18:56It's essentially a self-operating zipper that runs up and down the genetic code, whispering the secrets of the DNA to the rest of the cell, and then rapidly zipping itself back up before any chemical damage or mutations can occur to the exposed letters.
19:10That is incredibly efficient. It is a masterpiece of evolutionary engineering. But we do have to note that in the spirit of rigorous science, the solitan hypothesis is still just that a hypothesis. Oh, really?
19:24Yeah. While the mathematics align beautifully with the hydrogen exchange data we've discussed, It is still considered unproven by some theoretical physicists. What's their argument against it? I mean, if the math works so well, Why the skepticism?
19:38It primarily comes down to solvent viscosity and molecular inertia. Okay, meaning the water gets in the way. Pretty much. Some theorists argue that the sheer friction of the surrounding water, combined with the physical inertia of the molecules having to open and close so rapidly, would prevent a wave like this from traveling efficiently without dissipating its energy.
19:57Oh, I see. They argue the physical environment inside a cell is too chaotic to support a sustained orderly wave at that microscopic scale. So how do we prove it once and for all? If we can't use NMR because the DNA has to be too small, and we can't use tritium exchange because it only gives us the mathematical footprint and not an actual picture.
20:16What's the next step? How do we see the unseeable? The paper suggests that the future lies in cryolectron microscopy, or cryo EM. Cryo EM, right? This technology has advanced incredibly rapidly in recent years.
20:31It involves flash freezing molecules mid-action, at cryogenic temperatures, and taking unbelievably high resolution images. So we literally freeze the wave. Exactly. The profound hope of the biophysics community is as soon a cryo EM scan might finally catch one of these solitin waves right in the act, frozen in time, proving their physical existence visually, once and for all.
20:52So what does this all mean? It means that a bitter 30-year conflict in biophysics was essentially a trick of the light. An illusion caused entirely by the size of the DNA being studied in different machines.
21:03Perfectly stated. By finally stepping back and looking at the whole picture, this research reveals that native long DNA doesn't just flicker open one lonely base at a time. No, it doesn't. It relies on large traveling loops of open-based pairs, these majestic solitin waves to dynamically expose our genetic code to the machinery of life.
21:25The vault isn't just opening. It's actively scanning its contents for the proteins that keep us alive. And that realization opens up entirely new frontiers for science. What does this mean for the future of medicine?
21:37If we can understand exactly how these solitin waves travel, could we design drugs that predict where the DNA will open next, stopping a rogue gene from being read before the reading protein even arrives?
21:49That would change everything. It really shifts our understanding of genetics from a static library to a highly dynamic predictable highway. This episode was based on an open access article under the CCBY 4.0 license.
22:01You can find a direct link to the paper and the license in our episode description. If you enjoyed this, follow or subscribe in your podcast app and leave a five-star rating. If you'd like to support our work, use the donation link in the description.
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