An evaluation of the advantages and effectiveness of picric acid monitoring during solid phase peptide synthesis.

The effectiveness of picric acid for the monitoring of coupling completion during solid phase peptide synthesis was evaluated. Each coupling step was monitored during the synthesis of 124 peptides by the method of simultaneous multiple peptide synthesis. Of the peptides tested (1622 picrate determinations), approximately 10% underwent a single low yield step, another 10% had 2 or 3 consecutive low yield steps and about 20% contained "humps" 4 to 8 steps long. In virtually all instances, the low yield steps occurred only at or after coupling step 7. High picrate values obtained after incorporation of methionine appear to be caused by a sulfonium salt in the side chain of methionine. Differences in average reactivities of amino acids correlated with structural elements of their side chains. Picric acid, which is currently seldom used for monitoring of coupling completion during solid phase peptide synthesis, was found to be straightforward and nondestructive. While picric acid monitoring is not as sensitive as the more commonly used ninhydrin method, it yields information which is not detectable with the use of ninhydrin and is clearly superior for certain applications and investigations.