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1H and 13C NMR spectra of compounds 1, 3, 8, 9, 10, 11, and 14 ()

A range of cleavage reagents for peptides synthesized on 2-chlorotrityl resin has been described. TFE/AcOH/DCM (1:1:3) has been developed by Barbs [62]. Cleavage is also rapidly attained with 0.5% TFA/DCM as well as with HFIP/DCM (1:4 or 3:7) [63].

All peptides are treated with complete confidentially.

Filtrates containing sulfur scavengers have to be oxidized to eliminate the pervasive obnoxious smell. For that purpose the TFA filtrate is neutralized (NaOH/ice) before addition of an oxidant (e.g. bleach or a saturated solution of iodine in ethanol).

A growing proportion of scavengers means reduction of the amount of TFA, thus, cleavage with cocktails containing a considerable amount of scavenger or many different scavengers may take longer.


Available Custom Peptide Modifications

During the reaction highly reactive carbocations are generated and it is necessary to trap them to avoid undesired reactions with sensitive amino acids such as Cys, Met, Set, Thr, Trp, Tyr.

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Concentrated TFA (95% aqueous TFA) is the typical reagent to perform the final cleavage of the peptide from the resin together with the removal of the side chain protecting groups.

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One other possibility of protection of the αnitrogen is to introduce dipeptides possessing a pseudoproline residue, derived from Set or Thr since Pro is known to disrupt aggregation [55].

RECOMMENDED STANDARD PROCEDURE

The other approach is to introduce backbone protecting groups which will prevent the formation of hydrogen bonds. Such protection is made by the introduction of the Hmb group on the αnitrogen [53]. It has been shown that the presence of a Hmb unit every 6-7 residues is sufficient to disrupt the peptide aggregation [54]. The Hmb protected amino acid is introduced under the form of N,O-bis-Fmoc-N-(2hydroxy-4-methoxybenzyl) derivative, the O-Fmoc protection being cleaved during the following piperidine treatment. At the end of the synthesis the Hmb group is cleaved in the final TFA cleavage.

RECOMMENDED STANDARD PROCEDURE

Two approaches can be used to disrupt the aggregation; the first one consists in modifying the environment in which the synthesis is made and to introduce elements known to disrupt hydrogen bonds; among them we list:

RECOMMENDED STANDARD PROCEDURE

The use of sulfur containing scavengers is recommended in the cleavage of sequences containing Met, Cys and Arg. Care must be taken in the use of thioanisole as it has been suggested that it participates in the premature cleavage of Acm, StBu, or tBu from Cys residues [58].

Automated, multiple solid-phase peptide synthesis

Some leads have however been listed but they can just be looked at as informative. The aggregation has a tendency to occur in a sequence of hydrophobic residues; it has been described as starting from the 5/6th residue from the C-terminus but not occurring after the 21st one [47].