Fmoc-Ala-Ser[psi(Me,Me)pro]-OH is an Fmoc-protected Ala-Ser pseudoproline building block for solid-phase peptide synthesis (SPPS). The Ser residue is incorporated in a reversible 2,2-dimethyloxazolidine pseudoproline form that temporarily changes local backbone hydrogen-bonding and conformation during chain elongation.

The preformed Ala-Ser unit introduces the sequence motif in one coupling operation. Under standard TFA-mediated final cleavage and deprotection, the acid-labile pseudoproline ring is opened and the native Ser residue is regenerated.

Product Information

Why Use a Preformed Pseudoproline Dipeptide

The Ala-Ser amide bond is already present before the reagent is coupled to the growing peptide. This matters because formation of the pseudoproline oxazolidine makes the downstream Ser nitrogen sterically constrained. Supplying the unit as a protected dipeptide avoids relying on direct acylation at that hindered nitrogen and installs two residues in a single step. For broader process context, see our difficult SPPS troubleshooting.

Sequence-Specific Use in Difficult SPPS

For a target containing a native Ala-Ser segment, this building block places the temporary backbone-disrupting element exactly at the Ser site. It is most relevant when synthesis around that region shows poor coupling, difficult Fmoc removal, deletion products, or other signs that chain association is limiting reagent access.

Coupling and Final Cleavage

The C-terminal carboxyl group remains available for incorporation through standard Fmoc-SPPS coupling chemistry. During final acid treatment, the pseudoproline ring is removed and the intended Ala-Ser sequence is recovered. Coupling reagent choice and cleavage cocktail should still be selected for the complete sequence rather than for this building block alone.

When to Choose This Building Block

We consider Ala-Ser pseudoproline substitution most useful when there is a sequence-specific reason to alter the physical behavior of the resin-bound chain. A routine Ala-Ser sequence that couples cleanly with conventional amino acids does not automatically require a pseudoproline strategy. Related motifs can be compared in the pseudoproline dipeptide range.

Experimental Planning

When a difficult sequence contains several possible Ser/Thr pseudoproline insertion sites, placement should be assessed together with chain length, hydrophobicity, resin loading, neighboring residues, and the stage at which coupling or deprotection performance begins to decline. The presence of an Ala-Ser motif alone does not establish that this building block is required.

For projects where building-block selection needs to be evaluated with the full target sequence, our sequence-specific peptide synthesis can support integrated planning, synthesis, purification, and QC requirements.

Product Documents

MSDS - Fmoc-Ala-Ser[psi(Me,Me)pro]-OH (AS4021)

The Safety Data Sheet provides product identification, hazard information, handling and storage guidance, exposure controls, transport information, and regulatory information for laboratory use.

Certificates of Analysis (COAs) are batch-specific. Please contact us to request the COA for your product, and we will provide it by email.

Frequently Asked Questions

What does psi(Me,Me)pro indicate?

It denotes a dimethyl-substituted pseudoproline formed from the downstream Ser residue. The five-membered oxazolidine temporarily changes backbone behavior during synthesis.

Why is the Ala-Ser unit supplied as a preformed building block?

The amide bond immediately before the pseudoproline nitrogen is already formed, avoiding direct acylation of a sterically hindered oxazolidine nitrogen and allowing the motif to be introduced in one coupling operation.

Does the pseudoproline remain in the final peptide?

No. Standard TFA-mediated final cleavage opens the pseudoproline ring and restores the downstream Ser residue.

Should this be used for every Ala-Ser sequence?

No. Pseudoproline use is most justified when the sequence shows aggregation, poor chain accessibility, or another reproducible assembly problem. Straightforward sequences may not require it.

Research Use Only.