Identification of non-exchanging peptides

[werpuc]

In recent data, we encountered a problem with peptides, that are so stable they do not undergo exchange in both states. For our own use, we call them "płaszczaki". The issue is that the maximal exchange control is very similar to the minimal exchange control, and when calculating the fractional deuterium uptake, the uncertainty is very high and it influences the houde confidence limit very much.

Questions:

• how to correctly identify such peptides?
• what to do with them.

Example of deuterium uptake curve:

[werpuc]

My proposals:

• the absolute difference between minimal and maximal control should be bigger than 0.5 Da (it is rounded to 0, and that means no exchange)
• the absolute difference between minimal and maximal control should be bigger than 10% of the length of the peptide

[michbur]

Possible solutions:

1. Rank approach: płaszczak versus normal curve
   

2. Linear model: if a is larger than 0.

3. Theoretical mass versus measured mass.