What Is Liquid-Phase Peptide Synthesis (LPPS)?

Published October 2026 · Patriot Labs Research Team · ~19 min read

The short answer

Liquid-phase peptide synthesis (LPPS), also called solution-phase synthesis, builds a peptide one amino acid or one fragment at a time while everything stays dissolved in solvent, rather than anchored to an insoluble resin as in solid-phase synthesis. Classical LPPS couples small protected fragments and purifies them by crystallisation; modern LPPS anchors the growing chain to a soluble tag that is removed by precipitation. It is prized for large-scale, lower-waste manufacturing.

The contrast that defines it is where the peptide lives during synthesis. In solid-phase synthesis the chain is tethered to a resin bead, so after each coupling the excess reagents are simply rinsed away; in liquid-phase synthesis the chain is a dissolved molecule like any other, so each intermediate has to be separated from the reaction mixture by ordinary means — extraction, crystallisation or precipitation. That single difference drives everything else: the reagent quantities, the speed, the ease of automation, the amount of solvent, and the scale at which each method is practical.

The rest of this guide works through the two senses of the term, the chemistry of a coupling cycle, the difference between adding residues one at a time and joining pre-made fragments, the history that explains why the field moved to resins and is now partly moving back, and the sustainability case that has made LPPS a live topic in commercial peptide manufacturing.

Two things people mean by “LPPS”

“Liquid-phase peptide synthesis” is used today for two related but distinct approaches, and separating them prevents most of the confusion around the term. One is classical solution-phase synthesis; the other is soluble-tag, or anchored, synthesis.

Classical solution-phase synthesis is the original chemistry: protected amino acids or short protected fragments are dissolved, activated, coupled, and the product is isolated and purified before the next step. It is a monophasic system, so the choice of solvent is dictated by what dissolves, and it uses close to stoichiometric amounts of reagent because there is no resin to drive reactions with a large excess.^[1] Soluble-tag synthesis keeps that solution chemistry but attaches the growing chain to a large, specially designed molecule — a soluble anchor — that dissolves in one set of solvents and crashes out of solution in another, so the peptide can be purified by precipitation after every step rather than by crystallising a small fragment.^[1][2]

Because both happen in solution, both are called “liquid-phase.” A 2025 commentary in the peptide-science literature argues that this shared label, together with overlapping terms such as solution-phase, tag-assisted and hybrid synthesis, has become genuinely ambiguous, and calls for clearer naming conventions.^[3] In this guide, “classical LPPS” means the fragment-and-crystallisation approach and “soluble-tag LPPS” means the anchored approach; where a point applies to both, it is simply “LPPS.”

How solution-phase synthesis works

A liquid-phase coupling cycle is the same four chemical operations as any peptide synthesis — activate, couple, deprotect, isolate — carried out on dissolved molecules. What is distinctive is that the intermediate is a free compound that must be purified before the chain can grow again.

The sequence runs roughly as follows. The carboxyl group of the incoming protected amino acid is activated, usually by converting it into a reactive ester or anhydride so that it will readily form an amide bond; in classical solution work this is often done with a pre-formed active ester such as a hydroxysuccinimide (OSu) ester, which can be isolated and used without a separate coupling reagent.^[2] The activated residue then couples to the free amine of the growing chain to form the new peptide bond. Throughout, the groups that must not react — the other amine, the side chains — are kept masked by protecting groups, and the temporary one on the chain’s amine is removed after each coupling so the next residue can add. A classic solution-phase scheme pairs a Boc group (removed by acid) on the chain-end amine with benzyl-type (Bzl, Z) side-chain protection removed by catalytic hydrogenation, and often protects the C-terminus as a methyl ester that base can later cleave or convert to a hydrazide for azide coupling.^[2] Finally the intermediate is isolated, by extraction or crystallisation, before the cycle repeats.

Two features follow from working in solution. First, because reactions are run near stoichiometry and the product is purified each time, the impurity level of a solution-phase intermediate can be kept very low — there is no resin hiding partly-reacted chains.^[1][2] Second, solubility becomes the limiting factor: as a protected peptide grows it tends to fall out of solution or become hard to handle, which is why classical solution-phase synthesis has traditionally been practical only for short sequences, often cited as up to about ten residues.^[2] The methods that follow in this guide are, in large part, ways to get around that solubility ceiling.

Stepwise versus convergent assembly

Peptides can be built either one residue at a time from one end, or by making several short fragments and then joining them; the second approach, convergent synthesis, is the natural partner of liquid-phase work. The distinction is about the shape of the synthetic route, not the individual coupling chemistry.

Solid-phase synthesis is inherently stepwise: the chain is anchored at its C-terminus and grown residue by residue toward the N-terminus.^[2] A convergent route instead prepares each fragment separately — often itself by solid-phase synthesis — purifies each one, and then condenses the finished fragments in solution.^[1][2] The advantages are real: the fragments can be made in parallel rather than in one long serial build, each fragment is isolated and quality-checked before it is committed to the final product, and the small per-step losses that accumulate over a long stepwise synthesis are broken into shorter runs. Bachem gives the HIV-entry inhibitor enfuvirtide, a 36-residue peptide, as an example that combines solid-phase synthesis of protected fragments with solution-phase coupling of those fragments.^[2]

The trade-off is planning. A convergent route has to be designed — where to cut the sequence into fragments, which couplings will be clean, how to keep the fragments soluble, and how to avoid racemising the residue at each junction — and that design work is one of the recognised costs of liquid-phase manufacturing compared with the near-universal protocol of stepwise SPPS.^[4] The chemistry of the junction coupling, and the reagents that keep it from scrambling stereochemistry, is covered in Peptide Coupling Reagents and What Is Racemization.

A short history: oxytocin, Merrifield, and the return of solution

Liquid-phase chemistry came first by decades, and the whole field of chemical peptide synthesis began in solution. The move to solid supports in the 1960s was a revolution in convenience, but it never fully displaced solution-phase manufacturing, which is now enjoying a renewed industrial interest.

Chemists assembled peptides in solution from the early twentieth century, but the landmark came in 1953, when Vincent du Vigneaud’s group completed the first synthesis of a biologically active peptide hormone, the nine-residue oxytocin, entirely in solution — work recognised with the 1955 Nobel Prize in Chemistry.^[5] For the next decade solution-phase synthesis was the only game, and it was laborious: every intermediate had to be isolated and purified by hand. In 1963 Bruce Merrifield published solid-phase peptide synthesis, anchoring the chain to an insoluble polymer so that excess reagents could be washed away instead of separated, which made assembly dramatically faster and, eventually, automatable.^[6] SPPS became the standard for research and for the first kilograms of most peptide drugs.

Solution-phase chemistry persisted, however, for established short-peptide drugs and for large-scale production, where its low solvent use and ordinary equipment were advantages rather than inconveniences. As recently as 2020, a market snapshot of contract peptide manufacturing put liquid-phase routes at roughly 39 percent and solid-phase at roughly 41 percent of production — a near-even split that reflects how much commercial volume still runs in solution.^[1] The surge in demand for larger peptide medicines, and the strain that demand has put on specialised solid-phase equipment, is now driving fresh investment in the soluble-tag methods described next.^[4]

Soluble-tag (anchored) LPPS

Soluble-tag LPPS is the modern answer to the solubility ceiling: instead of a solid resin, the chain is anchored to a large, soluble molecule that keeps it dissolved during reactions and lets it be precipitated out for purification. It aims to combine the easy purification of solid-phase work with the low solvent use and scalability of solution chemistry.

The idea is to attach the first residue to a bulky anchor — commonly a hydrophobic group — so the growing peptide dissolves in a less polar solvent such as tetrahydrofuran or dichloromethane during coupling, and then precipitates when a polar antisolvent such as acetonitrile or methanol is added.^[1] Excess reagents and small by-products stay dissolved and are filtered away, so each intermediate is purified by a solvent switch rather than by chromatography. Several proprietary platforms implement this idea, and a 2022 review in Green Chemistry catalogues the main ones:^[1]

Published examples give a sense of what these routes can reach. Chiba-type anchored synthesis has been reported for peptides including a 15-residue TNF-α antagonist, the 14-residue somatostatin, the 9-residue leuprolide, the 20-residue bivalirudin and a 28-residue ghrelin by a convergent route, and for icatibant acetate, a 10-residue peptide reported at 57 percent yield and greater than 99 percent purity in one 2019 account.^[1] The Green Chemistry review is careful to note that much of the detailed process data for these commercial platforms is not published, so independent comparison of their “greenness” is limited.^[1] The underlying anchor chemistry draws on the same protecting-group and coupling toolkit covered in What Is a Protecting Group and Fmoc vs Boc Peptide Synthesis.

LPPS versus SPPS, compared

Solid-phase and liquid-phase synthesis are complementary rather than competing: SPPS wins on speed and convenience, LPPS on solvent economy and scale. The table sets out the practical differences; none of them is a verdict, because the right method depends on the sequence, the quantity and the stage of development.

In practice the two are often combined. A hybrid route makes protected fragments by fast solid-phase synthesis and then couples them in solution, capturing SPPS speed for the hard stretches and LPPS economy for the assembly — the convergent pattern described above, and the one used for enfuvirtide and for several larger modern peptides.^[2][4]

Why LPPS matters for green, large-scale manufacturing

The strongest argument for liquid-phase synthesis is environmental: peptides are unusually solvent-hungry to make, and most of that solvent comes from the wash steps of solid-phase synthesis. Reducing it is the central goal of green peptide chemistry, and liquid-phase routes are one of the main levers.

The scale of the problem is well documented. The Green Chemistry review estimates that the process mass intensity of peptide manufacturing — the total mass of materials used per unit mass of product — runs on the order of 3,000 to 15,000 kilograms per kilogram of a typical peptide, roughly a hundred times that of conventional small-molecule drugs, with solvent making up more than 80 percent of it and resin washes accounting for 80 to 90 percent of the waste in solid-phase routes.^[1] A 2024 analysis of real manufacturing processes, led by the ACS Green Chemistry Institute Pharmaceutical Roundtable, found that liquid-phase routes can have more than ten times lower process mass intensity per amino acid residue than solid-phase routes, chiefly because they avoid the repeated washes.^[7][8]

Three levers make LPPS greener: fewer reagent equivalents, far less solvent per step, and the option to run in more benign solvents. Where solid-phase synthesis leans heavily on dimethylformamide and dichloromethane — both under regulatory pressure — liquid-phase routes have been demonstrated in greener alternatives such as 2-methyltetrahydrofuran, ethyl acetate and cyclopentyl methyl ether, and with coupling reagents that give water-soluble by-products.^[1][7] This matters commercially as well as environmentally: the sharp rise in demand for metabolic peptide medicines has strained the specialised equipment that solid-phase synthesis needs, and solution-phase and hybrid routes that run in ordinary batch plants are one way the industry is adding capacity.^[7] Kilogram-scale GMP campaigns have been reported for larger therapeutic peptides using hybrid solid-then-liquid routes with continuous processing, and a 2025 account describes a kilogram-scale liquid-phase route to a complex cyclic peptide drug candidate.^[7][9] The quality side of that story — how a lower-waste process is held to the same identity and purity standards — is the subject of the next section and of Understanding Peptide COA Testing.

How intermediates and products are checked

A liquid-phase route is checked the same way any peptide is — by separating the molecules present and weighing them — but its structure gives it an extra advantage: because intermediates are isolated compounds, they can be analysed directly rather than inferred at the end. That makes quality control a series of checkpoints rather than a single final verdict.

Reversed-phase HPLC measures purity as the fraction of the material that is the target molecule, and mass spectrometry confirms identity from the molecule’s mass; the pairing is the backbone of peptide analysis, explained in What Is HPLC and What Is Mass Spectrometry. In a convergent liquid-phase route, each fragment can be purified and qualified before it is committed to the final coupling, which keeps the hardest-to-remove impurities — the near-isobaric deletion and epimer by-products described in Peptide Impurities Explained — out of the finished peptide.^[4] Liquid-phase processes are also compatible with real-time, in-line monitoring such as on-line HPLC, so a reaction can be followed as it happens rather than sampled afterward.^[4]

None of this is a property of the method alone; it is a property of the method as run and documented. For reference, Patriot Labs is a first-responder-owned, US-based company, and every batch it sells is tested by an independent US laboratory — HPLC for purity and mass spectrometry for identity — with the certificate of analysis published on the product page. The route a given material was made by, solid-phase or liquid-phase, is a manufacturing detail; what a certificate reports is the identity and purity of the specific lot in the vial.

What this guide does not establish

This guide is an educational account of a manufacturing method and its chemistry. Several limits are worth stating plainly.

• It describes methods, not any specific product. Whether a given peptide is made by solid-phase, liquid-phase or a hybrid route is a manufacturing decision; it cannot be inferred from the compound’s name, and it is not stated for any product sold here.
• The figures are process metrics, not product claims. Process mass intensity values, yields and purities cited from the literature describe the published processes they come from; they are not measurements of any particular vial or batch.
• The named peptides appear as synthesis examples only. Oxytocin, enfuvirtide, icatibant, bivalirudin and the rest are mentioned to illustrate which molecules have been made by which route. Naming them is not a statement about their use, and this guide does not describe how to use any substance.
• Nothing here concerns use in a person or animal. The chemistry is general, and the guide makes no medical, therapeutic or dosing claim of any kind.
• Nothing here describes where any Patriot Labs peptide is synthesised. The methods are described in general industry terms; the discussion is not a claim about the origin or process of any specific material.

Frequently asked questions

What is liquid-phase peptide synthesis?

Liquid-phase peptide synthesis (LPPS), also called solution-phase synthesis, is a method of building a peptide one amino acid or one fragment at a time while everything stays dissolved in a solvent, rather than anchored to an insoluble resin as in solid-phase synthesis. Protected amino acids are activated and coupled in solution, intermediates are isolated between steps, and the protecting groups are removed at the end. It is the oldest way peptides were made chemically, and modern versions attach the chain to a soluble tag to make purification easier.

What is the difference between SPPS and LPPS?

In solid-phase peptide synthesis (SPPS) the growing chain is attached to an insoluble resin bead, so excess reagents are simply washed away after each step; in liquid-phase peptide synthesis (LPPS) the chain stays dissolved and is purified by isolating the intermediate, often by precipitation or extraction. SPPS is faster, easier to automate and dominant for research and early-phase quantities; LPPS uses closer to stoichiometric reagents and far less solvent, runs in ordinary batch reactors, and scales to large commercial batches.

Is liquid-phase peptide synthesis the same as solution-phase synthesis?

They refer to the same idea: assembling a peptide while it is dissolved in solution. Classical solution-phase synthesis works with small protected fragments that are purified by crystallisation. Modern liquid-phase synthesis usually means tag-assisted or anchored synthesis, in which the chain is bound to a soluble support that is removed by precipitation. A 2025 commentary in the peptide literature notes that the overlapping terms cause confusion and argues for clearer naming.

What is a soluble tag in peptide synthesis?

A soluble tag, or soluble anchor, is a large molecule attached to the end of the growing peptide so that the whole assembly dissolves in one set of solvents and precipitates out in another. It plays the role the resin plays in solid-phase synthesis, but because the peptide stays in solution the reactions behave like ordinary solution chemistry, while a simple change of solvent crashes the product out of the reaction mixture and leaves excess reagents behind. Named examples include Molecular Hiving and AJIPHASE.

What is convergent peptide synthesis?

Convergent synthesis builds several short peptide fragments separately and then joins the finished fragments together, rather than adding one residue at a time from end to end. Because the fragments can be made in parallel and each is purified and quality-checked before assembly, convergent routes are well suited to liquid-phase manufacturing of longer peptides, and they keep the hardest-to-remove impurities out of the final coupling.

Why is LPPS considered greener than SPPS?

Solid-phase synthesis relies on large reagent excesses and repeated solvent washes, so most of its waste is solvent; published analyses put the process mass intensity of peptide manufacturing far above that of ordinary small molecules. Liquid-phase routes use closer to stoichiometric reagents, isolate intermediates instead of washing a resin, and can run in greener solvents, which a 2024 analysis links to substantially lower process mass intensity per residue. Lower solvent use is the main reason LPPS is described as a more sustainable route.

What peptides are made by liquid-phase synthesis?

Historically, oxytocin was first made in solution in the early 1950s, and many short peptide drugs have been produced by classical solution-phase routes ever since. Soluble-tag liquid-phase routes have been reported for peptides such as icatibant, bivalirudin, degarelix and leuprolide, and liquid-phase and hybrid solid-then-liquid routes have been used at kilogram scale for larger therapeutic peptides. Which method a given material is made by is a manufacturing decision, not something that can be read off the molecule.

Which came first, solid-phase or liquid-phase peptide synthesis?

Liquid-phase, by decades. Chemists built peptides in solution from the early twentieth century, and the first biologically active peptide hormone was assembled in solution in 1953. Solid-phase peptide synthesis was introduced by Bruce Merrifield in 1963 and quickly became the standard for laboratory and research-scale work, but solution-phase chemistry never went away and remains important for large-scale manufacturing.

References

1. Ferrazzano, L., Catani, M., Cavazzini, A., Martelli, G., Corbisiero, D., Cantelmi, P., Fantoni, T., Mattellone, A., De Luca, C., Felletti, S., Cabri, W., & Tolomelli, A. (2022). Sustainability in peptide chemistry: current synthesis and purification technologies and future challenges. Green Chemistry, 24. doi:10.1039/D1GC04387K
2. Bachem. Introduction to Peptide Synthesis Methods. Bachem Knowledge Center. https://www.bachem.com/knowledge-center/introduction-to-peptide-synthesis-methods/
3. Sharma, A., et al. (2025). Controversial Nomenclature in Peptide Synthesis: A Call for Clarity. Journal of Peptide Science. doi:10.1002/psc.70044
4. Bio, M. M. (2024). Liquid-Phase Peptide Synthesis (LPPS): A Sustainable Future for Peptide Therapeutics. Chemistry Today, 42(6). https://www.chemistry-today.com/articles/liquid-phase-peptide-synthesis-lpps-a-sustainable-future-for-peptide-therapeutics/
5. The Nobel Prize in Chemistry 1955: Vincent du Vigneaud — for his work on biochemically important sulphur compounds, especially the first synthesis of a polypeptide hormone. NobelPrize.org. https://www.nobelprize.org/prizes/chemistry/1955/vigneaud/facts/
6. Merrifield, R. B. (1963). Solid Phase Peptide Synthesis. I. The Synthesis of a Tetrapeptide. Journal of the American Chemical Society, 85(14), 2149–2154. doi:10.1021/ja00897a025
7. Kekessie, I., et al. (2024). Process Mass Intensity (PMI): A Holistic Analysis of Current Peptide Manufacturing Processes Informs Sustainability in Peptide Synthesis. The Journal of Organic Chemistry, 89(7), 4261–4282. doi:10.1021/acs.joc.3c01494
8. Bachem. What Is Liquid Phase Peptide Synthesis? Bachem Knowledge Center. https://www.bachem.com/knowledge-center/what-is-liquid-phase-peptide-synthesis/
9. Major Progress in the Liquid-Phase Synthesis of N-Alkyl-Rich Cyclic Peptides: Kilogram-Scale GMP Production of the KRAS Inhibitor LUNA18 (2025). Organic Process Research & Development. doi:10.1021/acs.oprd.5c00260
10. Frederick, M. O., et al. (2021). Kilogram-scale GMP manufacture of a therapeutic peptide by a hybrid solid-/liquid-phase route with continuous processing. Organic Process Research & Development, 25(7), 1628–1636. (as described in ref. 4)
11. BACHEM. Solid-Phase Peptide Synthesis vs. tag-assisted and solution-phase methods: comparison of scale, solvent use and length limits (Knowledge Center overview tables). https://www.bachem.com/knowledge-center/introduction-to-peptide-synthesis-methods/

For the chemistry these routes assemble, see What Is Solid-Phase Peptide Synthesis, How Peptides Are Made and Peptide Coupling Reagents; for the quality side, see Peptide Impurities Explained and Understanding Peptide COA Testing.

All Patriot Labs products are sold strictly for in-vitro research and laboratory use only. Not for human or veterinary consumption. This guide is educational and describes peptide-manufacturing chemistry in general terms; it is not medical advice, does not describe how to use any product, and makes no claim about the manufacturing method or origin of any specific product.