see this:

https://www.rsc.org/suppdata/c5/mt/c5mt00003c/c5mt00003c1.pdf

Hi Nick,

Thank you for the link. If I want to reference this what paper does it come from.

Also, they used 15N isotopically enriched A (1-40) samples - I wanted to ask if you know why and if this variation has a different absorbance values and extinction coefficients from normal values.

Sorry I don’t know, just came avross that on the internet

Hi Abdullah,

It looks like this question has been addressed on Research Gate before:

https://www.researchgate.net/post/How-may-I-determine-the-concentration-of-b-amyloid-1-42/1

Regarding your subsequent question about 15N enriched samples, that is typically done for NMR experiments. Extinction coefficients of proteins and peptides are generally calculated from the number of Tyrosine and Tryptophan residues (Phenylalanine if you are measuring more in the 270's, and I've seen Cysteine used as well). If the 15N became part of the indole ring of Tryptophan, it would affect its vibrational transitions and therefore its "vibronic" transition (in physical chemistry we talk about the combined electronic and vibrational transitions in light absorption). I have no experience with measuring absorbance spectra of isotopically enriched samples, but my guess is the extinction coefficient would not change enough to be important to most experiments. I would guess that the reference Nick provided just used the standard calculators for extinction coefficients like the one at biomol: https://www.biomol.net/en/tools/proteinextinction.htm.