NAD+ / NADH Question: Absorption peaks and spectra

[Jake578]

Hi guys,

Im new here so I am going to get right to it. Excuse my grammar I am on my mobile.

So I am trying to understand the difference in absorbance spectra of NAD+ and NADH. I have seen this touched on on this site but not directly answered.

NAD+, with an apparent two aromatic structures, produces one spectral peak.

NADH, with an apparent singular aromatic structures, produces two spectral peaks.

Now I believe for abs. a molecule must have aromatic structures... I have a feeling perhaps resonance structures in both molecules are causing the number of spectra not to match the number of aromatic structures?

What are your thoughts?

Jake

 

[Jake578]

Bump

 

[DrDu]

Each of the broad peaks you see in the spectrum (UVVis for sure) correspond to dozens of different transitions in the molecules this becomes apparent once you look at carefully taken spectra at the temperature of liquid nitrogen and there is not only one aromatic molecule present but several, so that a detailed assignment isn't possible.
My best guess is that the second bump in NAD+ is due to a charge transfer transition involving the N+.

 

[TeethWhitener]

The peak at 260 nm in both spectra is the adenine $\pi\to\pi^*$ transition. The NAD+ does have a second peak for the aromatic pyridine ring $\pi\to\pi^*$ transition, but it's deeper in the UV (I think around 220 nm or so). The peak at 340 nm that appears in the NADH is (if I recall correctly), usually assigned to the $n\to\pi^*$ transition of the conjugated ketone/alkene structure (cf. the broad resonance at around 340 nm in acrolein: http://webbook.nist.gov/cgi/cbook.cgi?ID=C107028&Mask=400#UV-Vis-Spec).