Hi there,

From the literature epsilon at 388nm for pyridoxal phosphate is 5300 to 5500 M-1.cm-1. The one for pyridoxal is far less documented. The only information I found is from the following paper (see the very beginning of the results section):

http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0041680

where a difference spectrum gives a delta epsilon of roughly 5000 M-1.cm-1 which means pyridoxal absorbs very little at 388nm...

So there is definitely something about the presence of phosphate...

Dear Prof. Liger,

Thank you very much for your response and for the link. Indeed, according to the paper, it seems that phosphate group does not shift the peaks for pyridoxal, but somehow enhance its ability to absorb light at 388 nm by 10 times.

According to your question, what a kind of pyridoxal will be used for comparison (e.g. pyridoxal HCl or another anion?) and what a pH are you using?

I send you a screenshot of pyridoxal (U-Vis) at various pH. (Source :

Chemical and Biological Aspects of Pyridoxal Catalysis: Proceedings ...

https://books.google.de/books?isbn=1483152383 - Diese Seite übersetzenE. E. Snell, ‎P. M. Fasella, ‎A. Braunstein - 2013 - ‎ScienceThe absorption spectrum of pyridoxal methochloride at pH values near 4 represents a mixture of the two forms, ... NS,” N Hç*H Hg "SH HC2 HO O -H" HOH,C 20

For the conversion millimicron to nm , please see www.convertunits.com

Good luck

JRG

Dear Dr Grezes,

Thank you for the information. We are going to use pyridoxal hydrochloride at pH 7.2. However, the difference between absorbance spectra of pyridoxal itself and its phosphate derivative seems to be around 390 nm. indeed, as can be seen from the attached figure, pyridoxal methochloride does not absorb much at this region.

But the key question now is WHY does pyridoxal phosphate have 10 times higher absorbance peak at 388 nm than pyridoxal itself?