Since your peptide has 2 Phe residues, it should absorb in the UV at 258 nm. Even if it were degraded, the absorbance would still be there. However, this absorbance is quite weak with extinction coefficient of only 200 M-1cm-1, so you will need about 0.25 mM to get an absorbance of 0.1 at 258 in a 1-cm cuvette.

http://www.life.illinois.edu/biochem/455/Lab%20exercises/2Photometry/spectrophotometry.pdf

In which nm range are you trying to record the spectrum? Peptides have absorbance in 200-300 nm. And what`s your reference solution?

I tried in the 200-500 range and nothing. My reference solution is PBS 10mM. 

Since your peptide has 2 Phe residues, it should absorb in the UV at 258 nm. Even if it were degraded, the absorbance would still be there. However, this absorbance is quite weak with extinction coefficient of only 200 M-1cm-1, so you will need about 0.25 mM to get an absorbance of 0.1 at 258 in a 1-cm cuvette.

http://www.life.illinois.edu/biochem/455/Lab%20exercises/2Photometry/spectrophotometry.pdf

Hi Andre,

just out of curiosity ... you are using a quartz cuvette, right?

Best regards,

Christian

What about solubility of this peptide at pH7, it seems poorly soluble. pH arround 10-10,5 in soft alkaline solutions (carbonate or tris) could improbe solubility and give some positive absorbance spectrum. The expeted result is 220 nm   and 255-260 nm peaks. Easy to confirm. If you can use nanodrop technology, with low volumes range of ul...... this option could be tested with nanodrop with relative high peptide concentrations (0,5-1 mg/ml) and low peptide waste.

@Christian Wechselberger, no, i'm not. I'm using a 96 wells plate, UV-transparent made of clear acrylic copolymer plate in an Biotek Synergy 2 reader. 

@Adam B Shapiro, I wasn using a concentration of 0.1 mM, I'm going to try with a concentration of 0.25 and see if it works. 

Andre, since you are using a plate reader, fill the well as much as possible to maximize the pathlength for maximal sensitivity.