Spectrophotometry - why can't some products be read?

[keetner]

Hi folks,

I'm reading about this coupled reaction between hexokinase/glucose-6-phosphate dehydrogenase in regards to spectrophotometry. The reaction I'm given is:

glucose + ATP ----(hexokinase)---> G-6-P + ADP

G-6-P + NADP+ <---(G-6-P dehydrogenase)---> 6-phosphogluconolactone + NADPH

The above is just for your information. Any way, I'm told that the reason why we need to do coupled reactions is because with spectrophotometry, not all reactions will give viable products that can be read. In this case, it would be G-6-P. My question though is, why? What properties of G-6-P make it unreadable? The only thing I've really come across is that a lot of these products might not absorb light or fall under that UV spectrum. But I'm wondering if you guys could give me a slightly more in-depth reason or other examples/characteristics that make something unreadable.

Just something I'm a bit curious about.

Thanks.

[JackBean]

you got the reason, nothing to add

[jonmoulton]

The structure of a molecule determines what energies of electromagnatic radiation it will absorb. Lower-energy photons like microwaves correspond to changes like transitions in rotational energies about bonds, while higher-energy photons can trigger rearrangements in electron levels or even shifting in bonds (like the UV-induced dimerization of thymidine that can lead to skin cancers). At x-ray energies you can remove electrons from inner shells of atoms. It is the structure of the molecule (and its solvent environment) that determine which energy-absorbing transitions are possible and therefore what sort of spectrum you can observe.