LIMITATIONS OF HALOTRIALKYLSILANES
From our work, the best reagent mixture for cleavage of peptides from PAM resins is the TMSBr/thioanisole/TFA mixture at 25° C. For the deprotection and cleavage of MTS protected arginine peptides, the results indicate the best reagent mixtures and reaction conditions were the TMSBr/thioanisole/TFA mixture at 25° C and the TMSCl/thioanisole/TFA mixture using lithium bromide as an additive at 50° C. These reagent mixtures and conditions cleanly cleave the MTS protecting group from arginine, but were not effective in cleaving the tosyl group from the arginine residue. Since TMSCl requires the higher temperature of 50° C, we would judge the TMSBr reagent to be superior for most peptide work because it can be used at a lower temperature. To date we have not studied the possible dehydration of Asn or Gln to the corresponding nitriles by heating with TMSCl at 50° C. However, the TMSCl is cheaper and more readily available and should be useful for peptide resins derived from standard Merrifield resin (chloromethylated divinylbenzene cross-linked polystyrene beads) that do not contain MTS protected arginine residues.
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