Optimizing the removal of an Acm protecting group

Disulfide rich peptides have gained significant attention recently due to their incredible biological stability and tolerance to epitope grafting.  This class of peptides is often folded in solution, assuming the desired disulfide bond pattern correlates with the most thermodynamically stable structure.  Sometimes though, especially for chemically synthesized cysteine rich peptides, this is not the case.  The result is a complex mixture of peptides with varying disulfide bonding patterns and identical mass.

Using pairs of cysteine residues with matched orthogonal side chain protecting groups during chemical synthesis allows for precise regioselective control of the disulfide bond pattern on-resin, simplifying final purification steps.  In today’s post, I’ll explore conditions for removing acetamidomethyl (Acm) protecting groups with simultaneous disulfide bond formation.

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How to choose the right resin functionality for solid phase peptide synthesis

As a chemist new to the peptide community, there are many choices that have to be made.  Which coupling reagents to use? Heat or no heat to promote chemistry? And most importantly, which resin?  I have talked previously about resin choices, from loading levels to swelling capacity and how they affect the synthesis outcome.  But I haven’t addressed yet a fundamental feature of commercially available resins, and that’s the functional handle to which the peptide chain is conjugated.

In today’s post, I’ll describe some, and I mean only some, of the most commonly used chemical functionalities for Fmoc-based solid phase peptide synthesis and some scenarios in which you would choose one resin type over another.

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How to load the first amino acid onto Wang resin

While resins loaded with the natural 20 amino acids are commercially available these days, there may be times when loading the first amino acid onto the resin in house may be necessary.  And unlike loading the first amino acid onto amide-leaving resins, the first coupling reaction for C-terminal acids can be chemically more challenging.

There are several protocols published both in the literature as well as in technical notes from many peptide reagent and instrument suppliers, but they typically occur at room temperature over extended periods of time (3-24 hours and repeated).  In today’s post, I’ll evaluate several conditions suitable for efficiently loading the first amino acid onto Wang-type resin.

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Preventing aspartimide rearrangements during Fmoc-based solid phase peptide synthesis

Aspartimide rearrangements are a particularly nasty side reaction that can occur during fmoc-based solid phase peptide synthesis.  Not only is this a mass-neutral side reaction, chromatographically resolving the undesired, rearranged product can be particularly difficult.  To make matters worse, this side reaction can occur at any point during the synthesis after the Asp has been incorporated into the peptide.

In a prevous post, I described method that I have found useful for identifying whether or not an aspartimide rearrangment as occured during synthesis of a peptide that contains an aspartimide-susceptible sequence.  In today’s post, I’ll discuss some strategies that can be used to suppress, or even eliminate this side reaction. Continue reading Preventing aspartimide rearrangements during Fmoc-based solid phase peptide synthesis