Peptide Coupling Calculator

Understanding Amino Acid Coupling

What is Amino Acid Coupling?

Amino acid coupling is the fundamental reaction driving Solid Phase Peptide Synthesis (SPPS). It involves the formation of an amide bond between the free amine group of the resin-bound peptide chain and the activated carboxylic acid group of the incoming protected amino acid.

How the Calculations Work

The scale of the synthesis is determined by the starting mass of the solid-phase resin and its active site loading (mmol/g). To force the reaction to completion, an excess of amino acid and coupling reagents is used—typically 4 to 10 equivalents.

Standard Coupling Systems

• DIC / Oxyma: A modern, highly efficient standard. Forms an active ester while minimizing racemization. Often used with 4 equivalents of DIC and 4 equivalents of Oxyma.
• HATU / DIPEA: One of the most powerful uronium-based coupling reagents, ideal for difficult or hindered couplings. Typically requires 8 equivalents of a base like DIPEA.
• HBTU / DIPEA: A traditional, cost-effective uronium reagent, largely superseded by HATU for difficult couplings but still a workhorse in standard syntheses.

Step-by-step Example

If you set up a synthesis with 200 mg of resin loaded at 0.50 mmol/g, your scale is 0.100 mmol. Using 4 equivalents of Fmoc-Val-OH requires 0.400 mmol (135.7 mg). To match this, you need 4 equivalents of DIC (0.400 mmol, 62.6 µL) and 4 equivalents of Oxyma (0.400 mmol, 56.8 mg).