Green solvents for solid phase peptide synthesis

As peptide therapeutics continue to gain interest from the medical community and pharmaceutical companies, concerns regarding the cost of manufacturing also grow.  Cost  includes the expense of reagents and solvents, including DMF and NMP, used in the synthesis but also subsequent disposal.  

With growing motivations to improve the "green" characteristics of chemistry (reducing waste, reducing use of harmful solvents, etc), today's post will highlight some recent work evaluating alternative solvents for use in solid phase peptide synthesis.

The principles of green chemistry were originally outlined by John Warner and Paul Anastas in the late 1990s.  Generally the principles can be defined as strategies that decrease environmental impact and increase the overall safety profile of the chemistry being performed.  Both DMF and NMP are considered hazardous chemicals and extra precautions are recommended for their use and disposal.

During a recent peptide meeting presentation, 2-methyltetrahydrofuran (2-MeTHF) and cylcopentylmethyl ether (CPME), among others, were mentioned as alternative solvents for solid phase peptide synthesis, Figure 1.  The toxicological effects of both 2-MeTHF and CPME were evaluated by Merck and found to be substantially less toxic than DMF or NMP.  Before changing solvents based on the toxicological effects though, one has to consider the effects of an alternative solvent for general solubility of the amino acids and coupling reagents, swelling capacity of the resin(s) and finally the reaction profile for the deprotection and coupling reactions.  The Albercio group in Barcelona has published several reports evaluating these concerns.

Figure 1: Structure of 2-methyltetrahydrofuran (left) and cyclopentyl methylether (right).

The first report evaluates the solubility efficiencies of several solvents as potential replacements for DMF.  Ultimately they found that 2-MeTHF provided the highest crude purity in the synthesis of Aib-enkephalin as well as the lowest racemization potential for the dipeptide Z-Phg-Pro-NH2.  This is a great start!

The second report focuses more closely on protocol development with 2-MeTHF as a neat solvent or in conjunction with several green solvents during the wash steps.  Interestingly, they found that better crude purity was achieved using 2-MeTHF as the primary solvent for peptide synthesized on ChemMatrix® resin.  That observation did not hold true for peptides synthesized with polystyrene resin.

During the above evaluation, the authors noted that the Fmoc removal required the most optimization to achieve a high level of efficiency.  This is further evaluated in a third report.  The study included 2-MeTHF and CPME, but also several other solvents previously demonstrated to satisfy the criteria for a green solvent.  Unlike the coupling reactions, they found that the deprotection conditions are highly sensitive to the solvent choice when using a 20% piperidine solution.  In fact, neither 2-MeTHF nor CPME were the best performing solvents for the Fmoc step.  This study clearly shows the level of optimization required before new solvents are widely incorporated in standard solid phase peptide synthesis approaches.

Given these data, I’m now motivated to further evaluate these solvents in my own synthetic efforts, particularly for peptides synthesized with microwave heating.

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