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Low-barrier hydrogen bond

Low-barrier hydrogen bond is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Low-barrier hydrogen bond rather than just read about it. In short: A Low-barrier hydrogen bond (LBHB) is a special type of hydrogen bond. LBHBs can occur when the pKa of the two heteroatoms are closely matched, which allows the hydrogen to be more equally shared between them.

Low-barrier hydrogen bond — main illustration
Low-barrier hydrogen bond — illustration

Key takeaways

  • Low-barrier hydrogen bond belongs to chemistry; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Low-barrier hydrogen bond to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Low-barrier hydrogen bond from memory before moving on to harder problems.

Reference excerpt

A Low-barrier hydrogen bond (LBHB) is a special type of hydrogen bond. LBHBs can occur when the pKa of the two heteroatoms are closely matched, which allows the hydrogen to be more equally shared between them. This hydrogen-sharing causes the formation of especially short, strong hydrogen bonds.

Description

Standard hydrogen bonds are longer (e.g. 2.8 Å for an O···O h-bond), and the hydrogen ion clearly belongs to one of the heteroatoms. When pKa of the heteroatoms is closely matched, a LBHB becomes possible at a shorter distance (~2.55 Å). When the distance further decreases (< 2.29 Å) the bond is characterized as a single-well or short-strong hydrogen bond.

Proteins Low barrier hydrogen bonds occur in the water-excluding environments of proteins. Multiple residues act together in a charge-relay system to control the pKa values of the residues involved. LBHBs also occur on the surfaces of proteins, but are unstable due to their proximity to bulk water, and the conflicting requirements of strong salt-bridges in protein-protein interfaces.

Enzyme catalysis Low-barrier hydrogen bonds have been proposed to be relevant to enzyme catalysis in two types of circumstance. Firstly, a low-barrier hydrogen bond in a charge relay network within an active site could activate a catalytic residue (e.g. between acid and base within a catalytic triad). Secondly, an LBHB could form during catalysis to stabilise a transition state (e.g. with substrate transition state in an oxyanion hole). Both of these mechanisms are contentious, with theoretical and experimental evidence split on whether they occur. Since the 2000s, the general consensus has been that LBHBs are not used by enzymes to aid catalysis. However, in 2012, a low-barrier hydrogen bond has been proposed to be involved in phosphate-arsenate discrimination for a phosphate transport protein. This finding might indicate the possibility of low-barrier hydrogen bonds playing a catalytic role in ion size selection for some very rare cases.

References

Illustrations

Low-barrier hydrogen bond: Energy profiles for different hydrogen bond types between oxygen heteroatoms. Standard hydrogen bonds are asymmetrical, with the hydrogen being associated with one heteroatom. When the pKa between the heteroatoms is equal, a symmetrical hydrogen bond forms with the hydrogen in equilibrium between two locations. At shorter distances, the barrier between the two energy minima is low enough that the hydrogen is equally bound as a low-barrier, or single-well hydrogen bond.
Energy profiles for different hydrogen bond types between oxygen heteroatoms. Standard hydrogen bonds are asymmetrical, with the hydrogen being associated with one heteroatom. When the pKa between the heteroatoms is equal, a symmetrical hydrogen bond forms with the hydrogen in equilibrium between two locations. At shorter distances, the barrier between the two energy minima is low enough that the hydrogen is equally bound as a low-barrier, or single-well hydrogen bond.
Low-barrier hydrogen bond: In this aza crown-type, macrocyclic compound, a proton sits between two amide carbonyl oxygens separated by a distance of 2.45 Å.[2]
In this aza crown-type, macrocyclic compound, a proton sits between two amide carbonyl oxygens separated by a distance of 2.45 Å.[2]

Worked examples

Example 1 — a first encounter with Low-barrier hydrogen bond

Start with the simplest possible case. Write down what Low-barrier hydrogen bond claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Low-barrier hydrogen bond before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Low-barrier hydrogen bond ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Low-barrier hydrogen bond

In research
Low-barrier hydrogen bond appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Low-barrier hydrogen bond in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Low-barrier hydrogen bond is common in secondary-school and first-year university syllabi. It links to neighbouring topics Chemical bonding, so understanding it makes those chapters shorter.
In everyday life
Look for Low-barrier hydrogen bond outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Low-barrier hydrogen bond in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Low-barrier hydrogen bond means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Low-barrier hydrogen bond out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Low-barrier hydrogen bond in simple terms?

A Low-barrier hydrogen bond (LBHB) is a special type of hydrogen bond. LBHBs can occur when the pKa of the two heteroatoms are closely matched, which allows the hydrogen to be more equally shared between them.

Why does Low-barrier hydrogen bond matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Low-barrier hydrogen bond?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Low-barrier hydrogen bond.

Tags

  • Chemical bonding

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