During cleavage of peptide after solid phase synthesis ... cleavage is done using 1-5% TFA in methylene chlordie containing 5% triisopropylsilane. What is the role of the triisopropylsilane in this step? How should I expect the results if I missed using this reagent? How would using higher concentrations of TFA (up to 50%) would affect the cleavage and the resin itself?
Thanks in advance
Triisopropylsilane is typically used in such cleavage reactions to scavenge cations, specifically trityl. It is analogous to using anisole while removing Boc groups but excess can be removed easily due to its volatility.
As for the TFA concentration- that would depend on the linker used on your solid support.
Amr Hefnaway,
I agree with Patrick Maloney. The silane is used to scavenge cations. Absence of this can lead to undesired alkylation of your peptide cleavage product, particulary tryptophan which is prone to react with electrophiles under acidic conditions.
As for the TFA concentration, if you are using a chlorotrityl resin, 1-5% TFA is sufficient to release your peptide while retaining any t-butyl protection groups (OtBu, NBoc). If you want both resin cleavage as well as global deprotection, you need to increase your TFA concentration to at least 50%, but usually 95% TFA (with 2.5% each of water and triisopropylsilane) is the commonly used procedure.
Hi, depending on your solid support/resin that you are using for your SPPS, you have to use a varying strength of TFA in the 'reagent cocktail'. Practically if you are using a support where your peptide chain is anchored to the support via an ester functionality (e.g. Barlos trityl chloride polystyrene resin) you can use 1-5% TFA in DCM to just cleave your peptide off the resin without touching any of the protecting groups. In case you want to have a global deprotection or if you are using a resin with an amide anchor like the Rink amide resins then you must use a much harsher TFA cocktail as amide hydrolysis under acidic condition requires harsher reagents.
TIPS act as a reducing agent (a hydride transfer reagent because of the weak Si-H bond) and it scavenges all the electrophilic species that generate from the acid catalysed deprotection of your protecting groups such as Trityl, Boc, t-Bu etc. If these electrophiles are not scavenged, they might start reacting to the nucleophilic side chain of amino acids such as cysteins, serins, arginines, lysines etc. For these reasons the use of triisopropylsilane (TIPS) with 1-5% TFA cocktails is usually not necessary as there are no carbocations to be scavenged. One can also consider adding thiol/dialkyl sulfide based scavengers like EDT, thioanisole etc to the TFA cocktail but only for some special group of peptides. Hope I could answer your query.
it is a scavenger, it prevents that the carbocations generates in the cleavage will re-attach to the peptide
I have used these two cocktails and they work pretty good. Sometimes you need to optimize the cleavage period at RT.
( TFA:TIPS:water, 95;2,5;2,5)
( TFA:DCM:anisole, 49:49:2).
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