I am in the process of writing my thesis in peptide synthesis. My project deals with CGRP analogues synthesized by spps. In my synthesis of the CGRP analogues, I use following pseudoproline dipeptides on the following positions:

Fmoc-Val-Thr (psiMe,Mepro)-OH for Val (8) and Thr (9).

Fmoc-Leu-Ser(psiMe,Mepro)-OH Leu (16) and Ser (17).

After every 10 amino acid that has been coupled I would test the mass to ensure that the synthesis is going correct.

The theoretical mass was as following:

For the first 10 amino acids coupled - 1019.1 g/mol

For the next 10 aa (20 in total) - 2078.3 g/mol

For the next 10 aa (30 in total) - 3126.6 g/mol

I got a peak at around 1017 m/z for the first 10.

A peak at around 2075 m/z for the 20 in total.

But for the 30 total amino acids my m/z was about 3161.

So it was about 35 m/z higher than expected.

I continued with the synthesis coupling the last 7 amino acids.

When I did the mass analysis on my finale product I got a peak at about 3788 m/z. The mass of CGRP is 3790 m/z.

I talked with my supervisor and he told me that he had seen the increased m/z when coupling pseudo proline dipeptides before. He said it was probably them doing something. We didn't really have an answer to why the peak for the total 30 aa was +35 m/z but normal for the 10 total, 20 total and 37 total (finale product). My first thought was it may have been chloride but im not sure.

I've tried to look it up but I cant really find any answer to my question. There I was hoping someone could give me a possible answer.