Water-Based Amino Acid Coupling Technology for Sustainable Solid-Phase Peptide Synthesis

Addressing the environmental unsustainability of solid-phase peptide synthesis (SPPS) due to its heavy reliance on toxic organic solvents, a breakthrough study titled "Water-based coupling of amino acids for sustainable solid-phase peptide synthesis" was published in Nature Sustainability. This research, for the first time, developed a complete and efficient all-aqueous Fmoc-SPPS protocol. The core innovation lies in a simple amine salt formation strategy that confers high water solubility to all 20 common Nα-Fmoc-amino acids, complemented by a designed water-compatible activation system, a biodegradable hydrophilic solid support, and a green deprotection cycle. This method successfully synthesized various model therapeutic peptides ranging from pentapeptides to icosapeptides, yielding crude peptides with high purity. It reduced the total consumption of organic solvents by approximately 94%, offering a practical technological pathway for the large-scale green chemical production of peptide drugs.

1. Background: The Environmental Challenge Behind the Peptide Drug Boom

Peptide drugs, owing to their high target selectivity and favorable safety profile, have become one of the fastest-growing sectors in the pharmaceutical market, with a projected global market size of $75 billion by 2028. Chemical synthesis, especially 9-fluorenylmethoxycarbonyl (Fmoc)-based solid-phase peptide synthesis (Fmoc-SPPS), is the mainstream technology for preparing these peptide molecules.

However, traditional Fmoc-SPPS is a typical resource-intensive process. It heavily relies on large volumes of polar aprotic solvents (such as N,N-dimethylformamide, DMF) for resin washing and amino acid coupling, and uses piperidine for Fmoc deprotection. These solvents are classified as substances of very high concern (SVHCs) under the EU REACH regulation. Notably, producing 1 kg of a GLP-1 class peptide drug can generate up to 14 metric tons of organic solvent waste, with solvents constituting 80%-90% of the total synthetic waste. Developing environmentally friendly alternative methods is therefore urgently needed.

Ideally, water is the best green reaction medium. However, the strong hydrophobicity of the Fmoc protecting group has long rendered Nα-Fmoc-amino acids virtually insoluble in water, forming the fundamental technical barrier to achieving aqueous SPPS. Previous attempts, such as developing novel water-soluble protecting groups or preparing nano-dispersions, failed to gain traction due to issues of cost, stability, or practicality.

2. Breakthrough: Constructing a Cohesive All-Aqueous Synthetic System

This study did not circumvent the mature and inexpensive Fmoc chemistry. Instead, it systematically addressed each key aspect of aqueous synthesis through a series of innovations:

Solubility Breakthrough: The Amine Salt Formation Strategy

The study unexpectedly discovered that mixing an Nα-Fmoc-amino acid with at least an equimolar amount of an amine (such as the industrial-grade chemical N-methylmorpholine, NMM) forms a highly water-soluble amine salt. All 20 common Fmoc-amino acids (including those with side-chain protecting groups) dissolved in water at high concentrations ranging from 0.4 M to over 1.0 M. Crystallographic and NMR analyses confirmed that the formation of an ion pair (amine salt) is key to the enhanced solubility. The complex remained stable in aqueous solution (>16 hours), providing a reliable operational window for subsequent reactions.

Activation and Immobilization: Efficient Coupling in Water

The study employed the water-soluble carbodiimide EDCI and HOBt, which pre-react in water for 30 minutes to form an O-heteroaryl isourea adduct. This adduct exhibits milder and more selective reactivity, effectively suppressing side reactions with other nucleophiles.

Mixing the water-soluble Fmoc-amino acid amine salt with the solid support containing free amino groups allows the amino acid anion to preferentially bind to the support surface via ion exchange. Subsequent addition of the pre-formed activator adduct enables, through proximity effects, efficient active ester formation and amide bond coupling. This system effectively controlled amino acid racemization, with racemization levels below 0.5% even for susceptible amino acids like histidine and cysteine.

Reaction Support: Hydrophilic and Biodegradable Solid-Phase Carrier

The study utilized a novel solid support, SpheriTide Aq (and its macroporous version, Aq-M). This carrier is based on food-grade poly-ε-lysine cross-linked with a hydrophilic crosslinker (3,6,9-trioxaundecanedioic acid), making it fully biodegradable. It swells well in water, DMF, and methanol, and its hydrophilicity is highly compatible with the aqueous reaction environment. The macroporous structure further optimizes mass transfer efficiency.

Deprotection Cycle: Green Base and Byproduct Control

The study employed environmentally friendly neat morpholine instead of the regulated piperidine for Fmoc deprotection. The hydrophobic dibenzofulvene (DBF) produced during deprotection tends to precipitate and clog the system. Adding 1% (w/v) ferric chloride (FeCl₃) as a Lewis acid catalyst dramatically accelerated the formation of a water-soluble adduct between DBF and morpholine, thereby preventing precipitation issues. The morpholine/FeCl₃ system induced significantly less aspartimide formation (only 0.78%) compared to the traditional 20% piperidine/DMF system (15.04%), which is more favorable for peptide chain integrity.

3. Validation: Successful Synthesis from Model to Therapeutic Peptides

The efficacy of this aqueous SPPS process was fully validated by synthesizing peptides of varying complexity:

• Leucine-enkephalin (pentapeptide):​ A classic test peptide; crude purity 90.4%, cleavage yield 86.4%.

• ACP(65-74) (decapeptide, a "difficult sequence"):​ This sequence is prone to aggregation during synthesis. Using 50% aqueous urea solution as a chaotropic agent and solvent, the study successfully suppressed aggregation, obtaining crude peptide with >85% purity, demonstrating the method's potential for handling "difficult sequences".

• Bivalirudin (icosapeptide):​ This is a clinical anticoagulant drug containing D-amino acids. Synthesis proceeded successfully entirely in an aqueous medium. Intermediate monitoring showed high efficiency at each step, and the final crude product purity was ~83%, proving the process's capability for synthesizing longer therapeutic peptides.

4. Green Value and Industrial Prospects

This study demonstrates significant green value and broad industrial application potential. Regarding environmental benefits, the process makes water the primary reaction medium, drastically reducing the use of toxic organic solvent DMF by approximately 94% (used only for the initial esterification step), thereby cutting harmful waste generation at the source. Furthermore, the post-synthesis waste primarily consists of acids and bases, which can be rapidly purified using inexpensive mixed-bed ion exchange resins (e.g., Purolite MB400). The treatment cost is significantly lower than the complex processing required for organic solvents, and the treated water can be recycled, establishing a preliminary circular economy model.

However, two key challenges remain for the technology to achieve complete greenness: 1) the initial attachment of the first amino acid to the resin still requires organic solvents; 2) the final side-chain deprotection still relies on potentially regulated trifluoroacetic acid (TFA). Looking ahead, future research will focus on developing completely green side-chain deprotection protocols, optimizing the DBF byproduct capture system, and integrating this aqueous process with automated, continuous-flow production equipment, ultimately aiming for a completely green, scalable industrial production of peptide drugs.

5. Summary

This study, through ingenious "amine salt formation" chemistry, overcame the decades-long technical bottleneck of Fmoc-amino acid water-insolubility in one stroke. Building on this core innovation, a systematic all-aqueous Fmoc-SPPS methodology was developed. It is not merely a simple solvent substitution but represents integrated innovation in raw material processing, activation chemistry, support material, and cycle design. This work marks a crucial step forward for the peptide synthesis field towards green chemistry and sustainable manufacturing, offering a scientifically sound, practical, and economically viable pathway to address increasingly stringent environmental regulations and meet the enormous market demand for peptide drugs.