Refractive Index: The Ultimate Tool for Real-Time Monitoring of Solid-Phase Peptide Synthesis. Greening the Process

Peptides are the basis of many drugs currently on the market and of more in advanced studies. Most peptides are synthesized using solid-phase peptide synthesis (SPPS). A characteristic of this strategy is that synthetic intermediates are not isolated. In this context, the development of a real-time monitoring method would assure an optimal synthetic process. The refractive index (RI) of a liquid provides key information about its physical properties and the composition of any solution. Here, we provide the first demonstration that the RI is a process analytical tool (PAT) suitable for the real-time monitoring of SPPS. This strategy comprises three steps: coupling, deprotection, and washes, which can be monitored online by refractometry. Given that monitoring capacity helps in the determination of the endpoint of the reactions and the optimization of all synthetic steps, it has a direct impact on the consumption of reagents, solvents, and time, thereby contributing to greener SPPS.