What Is the Isoelectric Point (pI) of Lysine?

The isoelectric point of lysine, the pH at which the molecule carries no net electrical charge, is approximately 9.74. That number is substantially higher than the pI of most amino acids, which cluster between 5 and 6. The reason is lysine’s extra amino group on its side chain, which keeps the molecule positively charged across a wide pH range and pushes the balance point well into alkaline territory. While 9.74 is the standard value for free lysine in water, the effective pI shifts dramatically when lysine sits inside a folded protein or undergoes chemical modification after being incorporated into a polypeptide chain.

Why Lysine Has Such a High Isoelectric Point

Most amino acids have only two groups that gain or lose protons as pH changes: the amino group at one end and the carboxyl group at the other. Those two groups are enough to make any amino acid act as a tiny acid-base system, but they produce a pI that sits somewhere in the mildly acidic range for the majority of the twenty standard amino acids. Lysine is different because it carries a third ionizable group: an additional amino group at the tip of its four-carbon side chain, called the ε-amino group. This side chain amino group has a pKa around 10.5, meaning it holds onto its extra proton tenaciously and only surrenders it in strongly alkaline solutions.¹ Because two of lysine’s three ionizable groups are basic (both amino groups), the pH at which the positive and negative charges cancel out gets pulled far to the right on the pH scale.

The three pKa values for free lysine in aqueous solution are roughly 2.2 for the α-carboxyl group, 9.0 for the α-amino group, and 10.5 for the ε-amino group. At very low pH, all three groups are protonated, giving the molecule a net charge of about +2. As the pH rises past 2.2, the carboxyl group loses its proton first, dropping the net charge to +1. Lysine stays at +1 through the entire range of physiological and mildly alkaline pH. Only when the pH climbs past 9.0 does the α-amino group begin to lose its proton, and only above about 10.5 does the side chain amino group follow. The pI falls between the two pKa values that bracket the species with zero net charge, which in lysine’s case are the α-amino (9.0) and ε-amino (10.5) pKa values. Averaging those two gives 9.74, sometimes rounded to 9.7 in different textbooks.

Which pKa Values Go Into the Calculation

A common stumbling block when computing pI for any amino acid is figuring out which two pKa values to average. The rule is straightforward once you see the logic: you identify the form of the molecule that has zero net charge, then take the average of the pKa values on either side of that form. For lysine, the zero-charge form exists when the carboxyl group has already lost its proton (giving a −1 contribution) while both amino groups are still protonated (each contributing +1). That species sits between the deprotonation of the α-amino group (pKa 9.0) and the deprotonation of the ε-amino group (pKa 10.5). The α-carboxyl pKa of 2.2 is not used in the calculation because it flanks a different charge state entirely.

For a simple amino acid like glycine, which has only two ionizable groups, the pI is just the average of those two pKa values and lands near 6.0. For acidic amino acids like glutamate and aspartate, which carry an extra carboxyl group on their side chains, the pI drops below 4 because the two relevant pKa values are both low. Lysine and arginine represent the other extreme: their extra basic groups push the pI above 9. Histidine falls in between, with a side chain pKa around 6, giving it a pI near 7.6.

What Lysine’s High pI Means at Physiological pH

Your body maintains blood pH around 7.4, which is well below lysine’s pI. That means free lysine in your bloodstream is overwhelmingly positively charged. Both amino groups are protonated, the carboxyl group has released its proton, and the net charge sits at roughly +1. At physiological pH, the ε-amino group of free lysine is protonated to a degree of more than 99.9%.¹ This persistent positive charge is not just a chemical curiosity. It shapes how lysine-rich proteins behave in your cells, how they interact with DNA (which is negatively charged), and how researchers separate proteins in the lab.

In protein purification, the charge state of individual amino acids determines how proteins migrate in an electric field or bind to chromatography resins. Ion-exchange chromatography and isoelectric focusing both exploit differences in charge to pull proteins apart. A protein studded with lysine residues on its surface carries a net positive charge at neutral pH, causing it to bind to negatively charged (cation-exchange) resins. This principle is used routinely when isolating enzymes and other proteins from complex mixtures.² Adjusting the pH or salt concentration of the buffer running through the column then selectively releases different proteins based on how tightly they cling to the resin. Knowing the pI of the constituent amino acids gives a rough prediction of where a protein will elute, though the actual pI of a whole protein depends on the combined contributions of all its residues and any modifications they carry.

When the Textbook pKa Values Stop Applying

The pKa of 10.5 for lysine’s side chain is measured for the free amino acid dissolved in water. Inside a folded protein, that number can change enormously. Lysine residues buried in the hydrophobic interior of a protein, away from water, often show pKa values that are dramatically lower than the textbook figure. A systematic study that engineered lysine residues into 25 internal positions of a model protein found that 19 of them had depressed pKa values, with some dropping as low as 5.3.³ A shift of more than five pH units is enormous in chemical terms. It means a lysine that would normally remain charged at neutral pH can become uncharged when surrounded by nonpolar protein interior.

These shifts happen because the protein interior is a fundamentally different environment from bulk water. Burying a charged group in a space that cannot stabilize the charge through hydrogen bonding or interactions with water molecules makes holding that charge energetically expensive. The lysine side chain responds by giving up its proton at a much lower pH than it otherwise would. The same study found that 20 of the internal lysine residues lost their proton without causing any detectable structural rearrangement of the protein, suggesting that the protein simply tolerated the uncharged lysine rather than unfolding to expose it to water.³

Nearby charged residues also matter. Surface carboxylate groups on aspartate or glutamate residues can interact with internal lysine residues, further altering their pKa values. So the local electrostatic neighborhood around a lysine in a real protein can either raise or lower its effective pKa relative to the free amino acid value. Enzymes exploit this extensively. Active-site lysine residues in certain enzymes have been found with pKa values of 8.5 and even 6.7, far below the expected 10.5, allowing them to participate in chemical reactions that would be impossible if they held the standard charge state.⁴ The unique microenvironments around these residues activate the amino group for chemistry that a normal lysine side chain could not perform.

How Post-Translational Modifications Alter Lysine’s Charge

Once lysine is incorporated into a protein, cells can chemically modify it in ways that change its charge behavior. The most common modifications are methylation and acetylation, and they have very different effects on the ε-amino group’s ability to hold a proton.

Methylation adds one, two, or three methyl groups to the nitrogen atom. Monomethylation and dimethylation leave the nitrogen still capable of being protonated. The pKa of monomethylated lysine is around 10.7, and dimethylated lysine sits at about 10.2, both close enough to the unmodified value of 10.5 that the charge state at physiological pH barely changes.¹ At pH 7.4, both forms are protonated well above 99% of the time, so they carry the same +1 charge as unmodified lysine. Trimethylation is different: it locks the nitrogen into a permanent positive charge because there is no proton left to lose. The nitrogen has four bonds to carbon or hydrogen and carries a fixed +1 regardless of pH. This makes trimethylated lysine a quaternary ammonium ion, permanently charged under all biological conditions.

Acetylation, on the other hand, neutralizes the charge entirely. Adding an acetyl group to the ε-amino nitrogen converts it into an amide bond, which does not pick up a proton at physiological pH. The result is that an acetylated lysine residue goes from +1 to effectively zero charge on its side chain. This is why histone acetylation, one of the best-studied epigenetic marks, weakens the grip of histone proteins on DNA. Histones are lysine-rich and positively charged, which lets them bind tightly to the negatively charged phosphate backbone of DNA. Acetylating those lysines strips away positive charges and loosens that binding, making the DNA more accessible for reading and copying.

Lysine Versus the Other Basic Amino Acids

Arginine and histidine are the other two amino acids commonly described as “basic,” but their side chains behave differently from lysine’s in ways that matter for biology and for lab work.

Arginine’s side chain contains a guanidinium group with a pKa around 12.5, making it even harder to deprotonate than lysine’s ε-amino group. The pI of arginine is about 10.8, higher than lysine’s 9.74. At physiological pH, arginine’s side chain is protonated essentially 100% of the time, and unlike lysine, its charge is spread over multiple nitrogen atoms through resonance. That delocalized positive charge makes arginine exceptionally good at forming stable salt bridges with negatively charged groups in proteins. Both lysine and arginine contribute positive charges to proteins at neutral pH, but arginine holds its charge more stubbornly across a wider pH range.

Histidine is the outlier. Its imidazole side chain has a pKa near 6.0, which means it sits right at the threshold of being protonated or deprotonated at physiological pH. Around a third of histidine side chains are protonated at pH 7.4. This makes histidine uniquely useful in enzyme active sites, where a residue that can toggle between charged and uncharged states near neutral pH can shuttle protons during catalysis. Lysine cannot do this job under normal conditions because its side chain is locked into the protonated state at any biologically relevant pH. When enzymes do use lysine in catalytic roles, it is typically because the surrounding protein structure has artificially depressed its pKa into a useful range, as described above.

Why Whole-Protein pI Does Not Simply Average Amino Acid pI Values

A question that naturally follows from understanding lysine’s pI is how to predict the pI of a whole protein that contains many lysine residues. The short answer is that you cannot simply average the pI values of individual amino acids. A protein’s pI depends on the combined ionization behavior of every charged residue on its surface, modified by the three-dimensional structure, the local environment of each residue, and any post-translational modifications. Two proteins with identical amino acid compositions but different folds can have measurably different pI values because burial and neighbor effects shift individual pKa values in unpredictable ways.

In practice, researchers estimate protein pI using computational tools that sum the expected charge contributions of all ionizable residues at each pH value and find the pH where the total crosses zero. These estimates work reasonably well for soluble globular proteins but can be off by a full pH unit or more for membrane proteins or proteins with heavily modified residues. Experimental measurement through isoelectric focusing remains the gold standard when accuracy matters, particularly for designing purification protocols or interpreting two-dimensional gel electrophoresis results.

Lysine in Food Science and Nutrition

Lysine’s charge behavior at different pH values has consequences outside the biochemistry lab. In food science, the Maillard reaction, which produces browning and flavor development when you cook protein-rich foods, involves the ε-amino group of lysine reacting with reducing sugars. The reaction proceeds faster when that amino group is in its uncharged (deprotonated) form, which is why alkaline conditions accelerate browning. At the mildly acidic pH of most raw foods, lysine’s ε-amino group is overwhelmingly protonated and less reactive, which is one reason why browning accelerates with cooking (heat drives the reaction even when the equilibrium favors the protonated form).

This reactivity has nutritional implications. Lysine is an essential amino acid for humans, meaning it must come from the diet. When food processing exposes lysine to high heat in the presence of sugars, the Maillard reaction can destroy the bioavailable lysine, reducing the protein quality of the food. This is well documented in baked goods, dried milk powders, and other heat-processed foods where reducing sugars and proteins coexist. Understanding that lysine’s ε-amino group is the vulnerable target, and that its protonation state influences how fast the reaction proceeds, helps food scientists design processing conditions that preserve nutritional value while still achieving desirable color and flavor.

Solvent Effects on Lysine’s Ionization

The pI of 9.74 assumes pure water as the solvent. In biological reality, amino acids and proteins exist in solutions containing salts, small molecules, and sometimes organic co-solvents. Adding salt to a solution screens electrostatic interactions between charged groups, which can subtly alter the effective pKa values of ionizable residues in proteins. The effect on free lysine in dilute solution is small, but in concentrated protein solutions or in the crowded interior of a cell, ionic-strength effects become meaningful.

Organic solvents mixed with water can shift amino acid pKa values more dramatically. In water-ethanol mixtures, for example, the lower dielectric constant of the mixed solvent makes it harder to stabilize separated charges, generally raising the pKa of acidic groups and lowering the pKa of basic groups. This means the pI of lysine in a mixed aqueous-organic system could differ from the pure-water value by enough to affect separation experiments performed in nonstandard solvents. Researchers working with peptides in organic or mixed-solvent systems need to account for these shifts rather than relying on the standard aqueous pKa tables.