Cleavage Cocktails and Scavengers: Choosing the Right Mixture

The last step of an Fmoc synthesis does two jobs at once: it detaches the peptide from the resin and strips the side-chain protecting groups. Both happen in strong acid, and both generate reactive carbocations. Where those cations end up decides whether you get your peptide or a mixture of alkylated by-products. Scavengers are what decide it.

Why the cocktail is not just acid

Trifluoroacetic acid removes tert-butyl, trityl, Boc and sulfonyl-type protection, and cleaves the linker holding the peptide to the resin. Each of those departures leaves behind a stabilized cation. The residues with nucleophilic side chains — tryptophan, methionine, cysteine and tyrosine — are the ones that trap them, and the result is an alkylated peptide with a mass that is too high by the mass of whatever attached itself. A tert-butyl adduct adds 56 Da; a trityl adduct adds 242 Da. These show up as clean, separate peaks in the mass spectrum, which is a small mercy: they are easy to recognize once you know to look.

Scavengers are nucleophiles added in excess so that the cations have something more attractive to react with than your peptide. Which scavengers you need follows from which residues are present.

The standard cocktails

Cocktail	Composition	When it is the right choice
TFA / TIS / water	95 : 2.5 : 2.5	The default for straightforward sequences. No thiols, so no smell. Adequate for most peptides if the protecting groups were chosen sensibly.
Reagent B	TFA 88 : phenol 5 : water 5 : TIPS 2	A general-purpose mixture, again without thiols.3
Reagent K	TFA 82.5 : phenol 5 : water 5 : thioanisole 5 : EDT 2.5	The workhorse for sensitive sequences. Established as maximally effective at suppressing a wide range of side reactions.1
Reagent R	TFA 90 : thioanisole 5 : EDT 3 : anisole 2	Peptides with several arginines, where sulfonyl protection is slow to come off.2
Reagent H	TFA 81 : phenol 5 : thioanisole 5 : EDT 2.5 : water 3 : DMS 2 : NH4I 1.5 (w/w)	Chosen when methionine oxidation is the dominant worry. The dimethyl sulfide and ammonium iodide act together to reduce methionine sulfoxide back to methionine.
Reagent L	TFA 88 : TIS 2 : water 5 : DTT 5	A general mixture using DTT in place of EDT, which some labs prefer on odour grounds.
Reagent I	TFA 92.5 : TIPS 2.5 : DMB 5	The 1,3-dimethoxybenzene prevents formation of a C-terminal N-alkylated amide by-product when cleaving from Rink amide resin.

What each scavenger is actually for

Scavenger	Protects	Notes
Water	Asp, Glu, Ser, Thr, Tyr	Traps tert-butyl cations. Matters most when several tert-butyl-protected residues are present, which is the usual case.
Triisopropylsilane (TIS/TIPS)	Trt- and Pbf-protected residues	A hydride donor rather than a nucleophile. Quenches highly stabilized cations and assists trityl removal from cysteine. Non-malodorous.
1,2-Ethanedithiol (EDT)	Cys, Trp	The most commonly used thiol scavenger. Also helps remove trityl from cysteine and suppresses acid-catalysed tryptophan modification. Strongly malodorous.
Thioanisole	Met, Arg deprotection	Accelerates removal of Mtr, Pmc and Pbf from arginine. It is incompatible with cysteine protected as Acm or tert-butylthio, so thioanisole-containing cocktails such as Reagent K should not be used where that protection must survive.4
Phenol	Tyr, Trp	Offers protection to the two aromatic side chains most prone to alkylation.
Anisole, DMS, DTT	Various	Alternatives used in specific cocktails, generally where odour or a particular residue dictates the choice.

Practical points that change the outcome

• Wash and dry the resin properly. DMF is a high-boiling polar aprotic solvent that is not removed by simple evaporation, and residual base carried over from the deprotection step — piperidine rather than the solvent itself — will neutralise part of the acid and blunt the acidolysis. Washing through DMF, then acid, then dichloromethane, then shrinking the resin is worth the time.
• Use enough cocktail. Published protocols suspend the resin in roughly 5–10 mL of cocktail per gram, followed by a TFA wash of similar volume. There must be enough scavenger for the quantity of protecting groups present, not merely enough liquid to cover the resin.
• Run a test cleavage first. Twenty to fifty milligrams of resin will tell you the right time and cocktail before you commit the batch.
• Longer is not always better. Long peptides with many protecting groups need extended times, but every extra hour is also more time for side reactions. Where deprotection is still incomplete after a few hours, precipitating and repeating with fresh reagent is generally better than extending a single exposure.
• Acid-labile resins are different. 2-chlorotrityl and similar supports release the peptide under much milder acid, and should not be washed with acetic acid beforehand.

Reading the result

Most cleavage problems announce themselves in the mass spectrum rather than the chromatogram. A satellite peak at +56 Da is consistent with tert-butylation, +242 Da with a trityl adduct and +16 Da on a methionine-containing peptide with oxidation; and a peptide that is several hundred daltons heavy with no obvious pattern usually means protecting groups that never came off. Each of those points at a specific change to the cocktail or the protection scheme, which is why it is worth looking at the crude spectrum before deciding anything about the synthesis.

What to send for a quote

• The sequence, including any cysteine, methionine, tryptophan or multiple arginine residues
• Any modifications that must survive cleavage, or protection that must deliberately remain in place
• Whether disulfide formation is required after cleavage, and which pairing
• Purity level, quantity, and the counter-ion form you need

Quotation

Please email your project details to sales@lifetein.com or use our online quotation form. Difficult sequences are routine work for us, and the cleavage strategy is chosen per sequence rather than from a standard recipe.

Further reading

• What is a protecting group
• Resins and linkers
• Solid-phase peptide synthesis
• Peptide stability and degradation
• Reading a certificate of analysis

References

1. King DS, Fields CG, Fields GB. A cleavage method which minimizes side reactions following Fmoc solid phase peptide synthesis. International Journal of Peptide and Protein Research 1990;36(3):255–266. doi:10.1111/j.1399-3011.1990.tb00976.x
2. Albericio F, et al. Preparation and application of the 5-(4-(9-fluorenylmethyloxycarbonyl)aminomethyl-3,5-dimethoxyphenoxy)valeric acid (PAL) handle for the solid-phase synthesis of C-terminal peptide amides under mild conditions. Journal of Organic Chemistry 1990;55:3730–3743. (Reagent R)
3. Solé NA, Barany G. Optimization of solid-phase synthesis of [Ala⁸]-dynorphin A. Journal of Organic Chemistry 1992;57(20):5399–5403. doi:10.1021/jo00046a022 (Reagent B)
4. Stawikowski M, Fields GB. Introduction to peptide synthesis. Current Protocols in Protein Science 2012;69(1):Unit 18.1. doi:10.1002/0471140864.ps1801s69

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