A triple helix is a molecular structure in which three strand-like chains wind around each other to form a single, rope-like shape. The most well-known example is collagen, the protein that makes up about 30% of all protein in your body. But triple helices also appear in DNA and RNA, where they play surprisingly important roles. Unlike the famous double helix of DNA, a triple helix has three intertwined strands held together by hydrogen bonds, and its geometry and stability depend on the specific molecules involved.
The Collagen Triple Helix
Collagen is the classic triple helix and the one most scientists mean when they use the term. Three polypeptide chains (long strings of amino acids) coil around one another in a tight, elongated spiral. Each chain follows a repeating pattern of three amino acids: glycine, then one variable amino acid, then another variable amino acid. Scientists abbreviate this as the Gly-X-Y repeat. In type I collagen, the most abundant form, the X and Y positions are each filled by proline or hydroxyproline about 22% of the time.
Glycine is the smallest amino acid, and it has to appear at every third position because the interior of the triple helix is extremely tightly packed. Only glycine is small enough to fit there. When a genetic mutation swaps glycine for a bulkier amino acid, the helix destabilizes dramatically. Research on model peptides shows that replacing glycine drops the melting temperature from 45°C to around 10°C for the smallest substitutions (alanine or serine), and below 0°C for larger ones like valine or arginine. Five out of seven tested substitutions prevented the triple helix from forming at all, even at 0°C.
The geometry of the collagen triple helix is distinct from simpler helices like those found in most proteins. In a standard protein helix, the distance between each amino acid along the length of the helix is constant. In collagen’s triple helix, each residue within a Gly-X-Y triplet sits at a slightly different height and a different distance from the center. The overall structure completes one full turn roughly every 6.8 triplets, with each triplet-to-triplet rise of about 8.5 angstroms (less than a nanometer). This corresponds to what structural biologists call a 7/2 helix, meaning seven triplets span two full turns.
What Holds Collagen Together
Hydroxyproline, a modified form of the amino acid proline, is a key stabilizer. Collagen chains are initially assembled with regular proline, and an enzyme then converts some of those prolines into hydroxyproline. This conversion strengthens the hydrogen bonds between the three chains. Simulation studies have confirmed that higher hydroxyproline content directly correlates with greater thermal stability, and when collagen begins to unfold from heat, it starts in regions with less hydroxyproline.
This is where vitamin C enters the picture. Your body needs vitamin C (ascorbate) as a cofactor for the enzyme that converts proline to hydroxyproline. Without enough vitamin C, the conversion stalls, and the resulting collagen chains can’t fold into a stable triple helix or get properly exported from cells. This is the molecular basis of scurvy: not a lack of collagen production, but the production of collagen that can’t hold its shape.
When the Helix Goes Wrong
Osteogenesis imperfecta, commonly called brittle bone disease, is the most studied example of what happens when collagen’s triple helix is disrupted. The most common type of causative mutation replaces a glycine with a bulkier amino acid somewhere along the chain. Because the three strands are packed so tightly, even a single glycine substitution can break the direct hydrogen bonds between chains and cause local untwisting at the substitution site. The severity of the disease depends in part on which amino acid replaces glycine and where along the chain the substitution occurs. Smaller replacements like alanine cause less disruption than large, charged ones like aspartate or glutamate.
Triple Helices in DNA
DNA can also form triple helices, though these are far less common than the standard double helix. In a DNA triplex, a third strand winds into the major groove of a normal double-stranded DNA segment and binds using a different set of hydrogen bonds called Hoogsteen base pairs.
In standard DNA, the two strands pair through what are called Watson-Crick bonds. In a triplex, the third strand forms its hydrogen bonds at different positions on the bases. For example, in the first triplex discovered in the late 1950s, a strand of uracil (an RNA base) was shown to associate with an adenine-uracil duplex by bonding to different nitrogen and oxygen atoms than Watson-Crick pairing uses, specifically the N7 and N6 positions on adenine rather than N1. A cytosine-containing third strand can similarly bind to a cytosine-guanine duplex, though this requires mildly acidic conditions for the cytosine to pick up an extra proton.
These DNA triple helices, sometimes called H-DNA, tend to form at stretches where one strand is rich in purines (adenine and guanine) and the other in pyrimidines (cytosine and thymine). They are not just laboratory curiosities. H-DNA structures can form naturally in cells and may play roles in gene regulation and genome instability.
Triple Helices in RNA
RNA triple helices serve a practical function that scientists only recently appreciated: protecting the ends of RNA molecules from being chewed up by the cell’s recycling machinery. Most messenger RNAs are capped with a long tail of adenine bases (a poly(A) tail) that shields them from degradation. But certain long noncoding RNAs, which don’t code for proteins but perform regulatory tasks, lack this tail entirely. Instead, they fold their 3′ ends into triple-helical structures.
Two well-studied examples are MALAT1 and MEN β, both long noncoding RNAs involved in gene regulation. Their triple helices use U-A·U base triples at each end of the structure to block enzymes that would otherwise degrade the transcript from its exposed end. These structures don’t just keep the RNA stable. Research has shown they also support the RNA’s transport out of the nucleus and can even allow translation of a protein when spliced into a reporter gene, effectively doing everything a poly(A) tail does. A viral RNA called PAN, produced by Kaposi’s sarcoma-associated herpesvirus, uses the same trick: a triple helix at its 3′ end inhibits decay and helps retain the RNA in the nucleus.
Synthetic Triple Helices
Scientists are now building artificial collagen-like triple helices for medical applications. These collagen mimetic peptides (CMPs) are short, synthetic chains that fold into triple helices and can be customized in ways natural collagen cannot.
One promising direction is drug delivery. Researchers have attached CMPs to liposomes (tiny fat-based capsules) that include a recognition site for matrix metalloprotease enzymes. These enzymes are overproduced in certain cancers, so the liposomes could potentially release their drug payload specifically at tumor sites. Other groups have conjugated CMPs with fluorescent tags or therapeutic agents, or linked them to contrast agents for MRI or PET imaging, aiming to visualize damaged or diseased collagen in living tissue.
CMPs also show potential in tissue engineering. Because they bind naturally to collagen fibers, they can be used to modify collagen-based scaffolds. For instance, researchers have created PEG-coated CMPs that reduce unwanted cell adhesion on collagen films, and negatively charged CMPs that attract growth factors to promote blood vessel formation within 3D collagen gels. These applications are still in early stages, but the collagen triple helix has become a versatile building block for designing materials that mimic the structure and biological activity of natural tissue.