This is an updated question:
I don't have a very strong background in biophysics, but I want to understand the theory behind NAD and NADH absorbance and fluorescence.
Background: I understand that the additional hydrogen that NAD gains to form the reduced NADH results in the molecule being able to absorb light at 340nm. And that only the reduced NADH is able to produce fluorescence. This is also the property which is explored when studying e.g. enzyme kinetics - if the enzyme binds NADH.
Question: The oxidized form, NAD, still has an aromatic ring - why is it that it is not able to fluoresce? Why, in a nutshell, is the transfer of the hydrogens causing fluorescent in NADH and such a big change in absorbance spectrum?
Also, perhaps a bit naive question, but why is it fluorescence better to measure than absorbance?
I have tried to look for articles (the old and original) that explores these properties, but I can't seem to find any that purely speak about the spectroscopic properties.