Peptide Coupling Reagents Selection Guide

The Novabiochem^® brand offers one of the most extensive ranges of high-quality coupling reagents for in situ activation on the market. However, with such a plethora of reagents available, the choice of the optimum coupling reagent for a particular application is not always straightforward. Condensation reagents vary in terms of coupling efficiency, stability, solubility, or reactivity of active species.

In-situ activating reagents

In situ activating reagents are widely accepted because they are easy to use, they give fast reactions, even between sterically hindered amino acids, and their use is generally free of side reactions. Most are based on phosphonium or aminium (formerly known as uronium) salts which, in the presence of a tertiary base, can smoothly convert protected amino acids to a variety of activated species (Figure 1). The properties of Novabiochem's coupling reagents are summarized in Table 1.

Figure 1. Active esters generated with most commonly used coupling reagents.

The most commonly employed reagents, BOP, PyBOP, and HBTU generate OBt esters, and these have found wide application in routine SPPS and solution synthesis for difficult couplings. Coupling reagents are also available which generate esters that are more reactive than OBt. The most important are HATU², PyAOP^1,3, and HCTU⁴, PyClocK, which in the presence of base convert carboxylic acids to the corresponding OAt and O-6-ClBt esters respectively. Such esters are more reactive than their OBt counterparts owing to the lower pKa of HOAt and HO-6-ClBt compared to HOBt. Furthermore, HOAt has the added benefit of the pyridine nitrogen which provides anchiomeric assistance to the coupling reaction, making HATU and PyAOP the most efficient coupling reagent of the OBt series.

Recently, coupling reagents based on the Oxyma Pure leaving group have been introduced, the most useful of which are COMU^5,6 and PyOxim⁷. The evidence on the relative reactivity of the Oxyma esters generated by these reagents is equivocal. The original reports of these Oxyma-based reagents suggest they are more efficient than those based on HOAt, whereas our in-house tests⁸ indicate the HOAt reagents to be superior. Nevertheless, Oxyma-based reagents always perform better than those based on HOBt (PyBOP, HBTU) and O-6-ClBt (PyClocK and HCTU). One particular advantage of oxyma-based coupling reagents is that they are not based on potentially explosive triazole reagents.

In our experiments, the efficency of a coupling reagent seems to be almost entirely related to the nature of the active ester it generates, with an order of reactivity OAt> Oxyma Pure > 2-ClOBt > OBt. The structure of the uronium or phosphonium component appeared to have little influence.

Phosphonium Versus Uronium Reagents

With the exception of COMU, solutions of uronium-based reagents in DMF are exceptionally stable, making them ideal for use on synthesizers which utilize pre-made solutions of coupling reagents. In contrast, solutions of phosphonium reagents in DMF have moderate stability, and should be kept in closed vials and only used for a maximum of 2 days. However, phosphonium reagents are significantly more soluble in DMF than uronium reagents. This has important practical implications, as it enables reactions to be performed at higher concentrations with concomitant improvements in efficiency.

Phosphonium coupling reagents generally give cleaner reactions than uronium reagents. The latter can cause chain termination by guanidinylating the N-terminal amino group⁹. This side reaction is particularly problematic when carboxyl activation is slow, for example in the case of fragment and cyclization reactions, or if the excess uronium reagent is used. Formation of such by-products also causes difficulties in the assembly of long peptides as these short positively charged peptides can mask the presence of the target ion in the ESI mass spectrum. In contrast to uronium reagents, phosphonium reagents can be used in excess and can even be used to "feed" a slow cyclization or fragment coupling reaction, to help drive them to completion.

Figure 2. Guanidinylation caused by uronium coupling reagents.

Table 1 The properties of Novabiochem's coupling reagents summarized

Coupling reagent	Prod. No.	Structure	Solubility (M)	Stability in DMF closed vial	Reactivity of active species	Comments
BOP	851004		>1.5	n/a	4	Water soluble by-products make it useful for solution phase synthesis
COMU	851085		1.5	Low	1/2	High reactivity, possibly superior to HATU for hindered couplings. Non-explosive. Limited solution stability. Causes guanidylation.
HATU	851013		0.45	Excellent	1	Gold standard for hindered couplings, but expensive compared to other reagents
HBTU	851006		0.5	Excellent	4	Excellent reagent for routine synthesis. May cause guanidinylation.
HCTU	851012		0.75	Excellent	3	More reactive and expensive than HBTU. May cause guanidinylation.
PyBOP	851009		>1.5	Moderate. Solutions need making fresh daily	4	Excellent reagent for routine synthesis. Ideal of in situ activation as it does not cause guanidinylation. Clean.
PyAOP	851221		>1.5	Low - Moderate. Solutions need making fresh daily. Best kept under N2	1	Excellent reagent for hindered couplings, fragment condensation and peptide cyclization. Ideal of in situ activation as it does not cause guanidinylation. Clean.
PyOxim	851095		>1.5	Moderate. Solutions need making fresh daily	1/2	Similar advantages to PyBOP but generates a more reactive active species. Clean.
TSTU	851206		n/a	Excellent	4	Useful for the conversion of dyes/PEG reagents to water compatible OSu esters

Summary of Uronium and Phosphonum Coupling Reagents

Uronium	Phosphonium
Synthesizers using stock solutions, particularly if solutions are open to atmosphere	Synthesizers where solutions held in closed containers and reagents consumed rapidly
Scale-up if cost of reagent is a consideration	Scale-up, if reagent concentration is a consideration
Cannot be used in excess during fragment condensation and cyclizations	Can be used in excess in fragment condensations and cyclizations

References

1. Albericio F, Cases M, Alsina J, Triolo SA, Carpino LA, Kates SA. 1997. On the use of PyAOP, a phosphonium salt derived from HOAt, in solid-phase peptide synthesis. Tetrahedron Letters. 38(27):4853-4856. https://doi.org/10.1016/s0040-4039(97)01011-3

2. Carpino LA. 1993. 1-Hydroxy-7-azabenzotriazole. An efficient peptide coupling additive. J. Am. Chem. Soc.. 115(10):4397-4398. https://doi.org/10.1021/ja00063a082

3. Albericio F, Bofill JM, El-Faham A, Kates SA. 1998. Use of Onium Salt-Based Coupling Reagents in Peptide Synthesis1. J. Org. Chem.. 63(26):9678-9683. https://doi.org/10.1021/jo980807y

4. Marder O, Shvo Y, Albericio F. 2003. HCTU and TCTU: New Coupling Reagents ? Development and Industrial Aspects. ChemInform. 34(32): https://doi.org/10.1002/chin.200332258

5. El-Faham A, Funosas RS, Prohens R, Albericio F. 2009. COMU: A Safer and More Effective Replacement for Benzotriazole-Based Uronium Coupling Reagents. Chem. Eur. J.. 15(37):9404-9416. https://doi.org/10.1002/chem.200900615

6. Subirós-Funosas R, Nieto-Rodriguez L, Jensen KJ, Albericio F. 2013. COMU: scope and limitations of the latest innovation in peptide acyl transfer reagents. J. Pept. Sci.. 19(7):408-414. https://doi.org/10.1002/psc.2517

7. Subirós-Funosas R, El-Faham A, Albericio F. 2010. PyOxP and PyOxB: the Oxyma-based novel family of phosphonium salts. Org. Biomol. Chem.. 8(16):3665. https://doi.org/10.1039/c003719b

8. Novabiochem Innovations 03/10.. [Internet]. Available from: https://www.merckmillipore.com/IN/en/reagents-chemicals-and-labware/novabiochem-learning-center/novabiochem-publications/novabiochem-innovations/J2ib.qB.rDYAAAFLHaQp.xHd,nav?ReferrerURL=https%3A%2F%2Fwww.google.com%2F&bd=1

9. Gausepohl H, Pieles U, Frank R. 1992. Peptides: chemistry and biology. ESCOM. Leiden.523.