Protein denaturation is the loss of a protein's folded shape, its secondary, tertiary and quaternary structure, while the chain of amino acids stays joined. Heat, acids and bases, urea, detergents and disulfide-breaking reagents all cause it. The peptide bonds survive, which is why a denatured protein keeps its sequence and its mass.
- LabFirst Peptides certificates report identity by LC-MS and purity by RP-HPLC on a C18 column, most read at 214 nm, with net content as a separate line, each by lot number and with a link to the issuing laboratory's site.
- Protein denaturation removes secondary, tertiary and quaternary structure while the peptide bonds of the primary structure stay intact.
- Heat above about 50 °C denatures proteins by supplying enough kinetic energy to disrupt hydrogen bonding and dispersion forces.
- Anfinsen's reduced ribonuclease A, unfolded in 8 M urea with 2-mercaptoethanol, regained 100 percent of its activity when allowed to reoxidize with a catalytic amount of thiol present, and about 1 percent without it.
- Eight cysteines can form four disulfide bonds in 105 different pairings, and only one pairing is native ribonuclease.
- Urea and guanidine hydrochloride denature proteins through both direct contact with the backbone and changes to the surrounding water that weaken the hydrophobic effect.
What this guide covers
Fold a sheet of paper into a crane, then flatten it out again. The crane is gone. The paper is all still there, every fiber of it, and nothing was cut. That is protein denaturation: the folded shape comes apart while the chain itself stays in one piece.
This guide covers what denaturation takes away and what it leaves alone, the agents that cause it, the ribonuclease experiment that showed a protein can find its own way back, when the process reverses and when it does not, and why a short research peptide has far less to lose than a full-size protein.
- Heat: shakes hydrogen bonds and dispersion forces apart
- pH extremes: change side-chain charges and break ionic bonds
- Urea and guanidine: let water reach the hydrophobic core
- Detergents: bind the chain and pull it open
- Reducing agents: open the disulfide bridges
- Peptide bonds: untouched by all of the above
What is protein denaturation?
Protein denaturation is any change in a protein's three-dimensional structure that stops it from doing its job. The sequence of amino acids, the primary structure, is held together by peptide bonds and is sturdy. The levels above it are held by weaker forces: hydrogen bonds along the backbone in helices and sheets, and a mix of ionic bonds, hydrogen bonds, disulfide links and dispersion forces that pack the chain into its final shape. Those weaker forces are what denaturation attacks.
Our guide to the levels of protein structure walks through all four. In plain terms, denaturation can undo levels two, three and four and leaves level one alone. Breaking the peptide bonds themselves takes fairly vigorous conditions, and that is a separate reaction, hydrolysis, covered in our peptide degradation pathways guide.
Globular proteins, the compact folded kind, denature far more easily than tough fibrous proteins. A denatured protein has usually lost its activity. The word covers more than the precipitation or coagulation people picture, though; a chain can be fully unfolded and still be in solution.
What causes protein denaturation?
Anything that can outcompete the weak forces holding the fold. The usual agents sort into a few groups, and each group goes after a different kind of bond.
Heat. Heating above about 50 °C gives the chain enough kinetic energy to shake apart hydrogen bonds and dispersion forces. Ultraviolet light does the same thing by the same route.
pH extremes. Acidic and basic side chains carry charges that depend on pH. Push the pH far enough and those charges change, so the ionic bonds between them, one of the four interactions that hold tertiary structure, no longer line up.
Urea and guanidine hydrochloride. These are the classic chemical denaturants, used at high concentration; the Anfinsen experiment below used 8 M urea. Urea (CH4N2O, 60.06 g/mol) and guanidine hydrochloride (CH6ClN3, 95.53 g/mol) act by changing how water surrounds the protein and by contacting the backbone and polar side chains directly. A 2003 simulation study of chymotrypsin inhibitor 2 in 8 M urea found both routes at work. Urea weakened the hydrophobic effect so water could reach buried groups, and it bound the backbone and held nonnative shapes in place. A 2011 review of the field concluded that both denaturants most likely work by making the protein's hydrophobic regions easier to solvate.
Organic solvents. Ethanol and similar solvents form hydrogen bonds with the protein and in doing so break the protein's internal ones.
Detergents. Sodium dodecyl sulfate (C12H25NaO4S, 288.38 g/mol) binds proteins cooperatively and nonspecifically. A 2009 study of a heat-stable pyrophosphatase found that at high SDS concentration the enzyme unfolded in a single two-state step with no detectable intermediate.
Reducing agents. Disulfide bonds are covalent, so they need their own reagent. A thiol such as 2-mercaptoethanol (C2H6OS, 78.14 g/mol) or dithiothreitol (C4H10O2S2, 154.30 g/mol) reduces each cystine bridge back to two free cysteines.
Heavy metal salts. Mercury, silver and lead salts disrupt ionic bonds and disulfide linkages.
| Agent | Bonds disrupted | Example |
|---|---|---|
| Heat above about 50 °C, UV light | Hydrogen bonds, dispersion forces | Heating a protein solution |
| pH extremes | Ionic bonds between charged side chains | Strong acid or base |
| Chaotropes | Hydrophobic packing, backbone hydrogen bonds | 8 M urea, guanidine hydrochloride |
| Organic solvents | Internal hydrogen bonds | Ethanol |
| Detergents | Hydrophobic core | Sodium dodecyl sulfate |
| Reducing agents | Disulfide bonds | 2-mercaptoethanol, dithiothreitol |
| Heavy metal salts | Ionic bonds, disulfide linkages | Mercury, silver, lead salts |
What did Anfinsen's ribonuclease experiment show?
Ribonuclease A is a small enzyme with four disulfide bonds inside one chain. In work published in the early 1960s, Christian Anfinsen and colleagues unfolded it with 8 M urea plus 2-mercaptoethanol, so the chain lost both its fold and all four cross-links, and then removed the reagents by dialysis to see whether the chain could recover on its own.
It could. When the reducing agent was removed first and the disulfides were allowed to reform with the urea still present, the refolded protein kept only about 1 percent of its original activity, because the eight cysteines had paired at random. When a catalytic trace of 2-mercaptoethanol was left in so that wrong pairs could open and try again, activity returned to 100 percent.
Chance alone would never do that. Eight cysteines can pair up in 7 × 5 × 3 × 1 = 105 different ways (our arithmetic), and only one of those is the native set. Getting the native set back means the information needed to fold the protein is carried in its sequence, a conclusion Anfinsen set out in a 1973 paper on the principles that govern the folding of protein chains. For denaturation that has a clean consequence. Because denaturing leaves the sequence intact, it leaves the folding instructions intact too.
Is protein denaturation reversible?
Sometimes. A protein unfolded under gentle enough conditions can refold and may recover its activity, and small single-domain proteins are the usual success stories; early folding studies were built on proteins chosen because they could be denatured and refolded at will. Large multidomain proteins and proteins made of several subunits do much worse, with only a few percent refolding in a test tube.
Two things get in the way. The first is the disulfide problem above: once cysteines pair wrongly, the chain is stuck in a misfolded shape until something opens those bonds again. The second is aggregation. An unfolded chain exposes nonpolar side chains that were buried in the native fold, and those patches stick to the same patches on neighboring chains. Our peptide aggregation guide covers why that happens and how a laboratory spots it.
Whether a given protein comes back is a property of that protein and that particular insult. It is tested, never assumed.
Why do short peptides have so little to lose?
An alpha helix is held by hydrogen bonds from each carbonyl oxygen to the amide hydrogen four residues up the chain, with 3.6 residues per turn. A chain has to be long enough to build that pattern before it can lose it. A dipeptide has one peptide bond and no backbone pattern at all. A research peptide of ten or twenty residues may form a turn or a short stretch of helix in the right solvent, and that is the whole of its higher structure. The polypeptide backbone guide covers the angles involved.
So for most peptides a laboratory handles, heat and solvent matter for a different reason. The risks to a short chain are chemical change to the molecule itself and aggregation between chains, which alter what is in the vial rather than how it is folded. Those are covered in peptide degradation pathways, and the reason a lot ships as a dry solid is covered in our lyophilized peptide guide.
How does denaturation show up in laboratory testing?
Because denaturation leaves every covalent bond in place, the intact mass does not change. A mass spectrometer reads the same molecular weight from a folded protein and an unfolded one, which is why identity by LC-MS speaks to what the molecule is and says nothing about its fold. The one covalent event in the list above is disulfide reduction, and that one does show: each bridge opened adds two hydrogen atoms, 2.016 Da by our arithmetic, to the mass.
Reversed-phase HPLC is more sensitive to shape, since it separates by how strongly each molecule sticks to a C18 surface, and a chain that exposes buried nonpolar groups can shift its retention time or spread into more than one peak. For a small peptide with no fold to lose, a changed chromatogram points to chemical change or aggregation instead. The purity versus identity guide explains why the two measurements have to be read together.
Every certificate LabFirst Peptides publishes reports identity by LC-MS, purity by RP-HPLC on a C18 column, and net content on its own line, each tied to a lot number, and the certificate index is where to find the record for a given lot. None of those lines measures folding, because the molecules we document are short peptides. A short peptide's quality questions are the ones on that certificate: is it the right molecule, how much of the sample is that molecule, and how much is in the vial.
Bottom line
Protein denaturation strips a protein of its secondary, tertiary and quaternary structure and leaves the primary structure, the peptide-bonded chain, in place. Heat, pH, urea, guanidine, organic solvents and detergents do it by competing with weak bonds; disulfides need a reducing agent. Anfinsen's ribonuclease came back to full activity from 8 M urea, which showed that the sequence carries the folding instructions. Short peptides have little fold to lose, so their failure modes are chemical, and a certificate's mass and purity lines are the measurements that catch them.
FOR LABORATORY AND IN-VITRO RESEARCH USE ONLY. NOT FOR HUMAN OR ANIMAL CONSUMPTION.
Common questions
What is the difference between denaturation and hydrolysis of a protein?
Does protein denaturation change the amino acid sequence?
What causes protein denaturation in a laboratory?
Is denaturation of protein reversible?
Can a short peptide be denatured?
More science guides
Sources
- Ball, Hill and Scott, The Basics of General, Organic, and Biological Chemistry, 18.4 Proteins (Chemistry LibreTexts). Four levels of protein structure, the interactions that hold each, the sturdiness of peptide bonds, and the table of denaturing agents including heat above 50 °C, pH, organic compounds and heavy metal salts.
- Jakubowski and Flatt, Fundamentals of Biochemistry, 4.5 Protein Denaturation and Folding (Biology LibreTexts). The Anfinsen ribonuclease experiment: 8 M urea with 2-mercaptoethanol, four intrachain disulfides, 1 percent versus 100 percent activity recovery, the 1/105 pairing probability, and reversibility of small versus large proteins.
- Anfinsen, Haber, Sela and White, The kinetics of formation of native ribonuclease during oxidation of the reduced polypeptide chain, PNAS 47:1309 (1961) doi:10.1073/pnas.47.9.1309. The original refolding experiment on reduced ribonuclease; record opened through Europe PMC and PubMed.
- Anfinsen, Principles that govern the folding of protein chains, Science 181:223 (1973) doi:10.1126/science.181.4096.223. The statement that the native conformation is determined by the amino acid sequence; record opened through PubMed.
- Bennion and Daggett, The molecular basis for the chemical denaturation of proteins by urea, PNAS 100:5142 (2003) doi:10.1073/pnas.0930122100. Simulation of chymotrypsin inhibitor 2 in 8 M urea; urea acts by both direct and indirect mechanisms.
- England and Haran, Role of solvation effects in protein denaturation, Annual Review of Physical Chemistry 62:257 (2011) doi:10.1146/annurev-physchem-032210-103531. Review concluding that urea and guanidinium most likely act by facilitating solvation of hydrophobic regions.
- Mu et al., Inactivation and unfolding of the hyperthermophilic inorganic pyrophosphatase from Thermus thermophilus by sodium dodecyl sulfate, Int J Mol Sci 10:2849 (2009) doi:10.3390/ijms10062849. SDS unfolding through cooperative nonspecific binding as a two-state process with no intermediate.
- PubChem PUG REST: formulas and molecular weights for urea, guanidine hydrochloride, sodium dodecyl sulfate, dithiothreitol and 2-mercaptoethanol. CIDs 1176, 5742, 3423265, 446094 and 1567, queried by formula and weight.
