Figure 1: 15N-PASS spectrum of {N1,N3,N5}-15N-labeled tetracetyl-riboflavin, collected at 2.5kHz MAS.

Figure 2: Cartoon of the active site of NR, showing hydrogen bonds between the protein and the flavin. These interactions help to specify the reactivty and Emof the bound flavin.

      Electron redistribution in flavins as a means of protein control over flavin reactivity.

Flavins are the essence of the reactivity of a wide variety of crucial enzymes.  These mediate extremely diverse reactions including oxidation/reduction, oxygenation, isomerization, hydroxylation and electron transafer.  Thus, interactions with the protein modify the reactivity of the bound flavin.  In addition to allowing only select substrates access to the flavin, the protein also causes redistribution of the valence electron density within the highly delocalized flavin p system, and modulation of the flavin valence orbital energies, via the non-covalent interactions usually used to bind the flavin.  Thus, although redox activity is concentrated between N5 and N1, it is modulated by interactions with remote functionalities.  We propose to determine how the valence MOs that reside at N5 change in nature and energy in response to H bonds and distortion of the flavin ring system, by SS-NMR.  The 'paramagnetic' contribution to NMR shielding depends on the energy separating the HOMO from low-lying vacant MOs, as well as the MOs' orbital angular momenta, and thus their AO natures.  Moreover the three different principal values of the shielding tensor tend to reflect different orbitals.  Thus, principal NMR shielding values obtained from SS NMR are fundamentally related to valence orbital natures and energy separations, which in turn underlie reactivity.  Therefore, we propose to measure the three principal values of the NMR shielding of N5, N3 and N1 of flavins engaged in different H bonding interactions and possessing different reactivities, and relate them via DFT calculations, to variations in flavin valence electron distribution.  Thus we will develop SS-NMR as a new and highly-sensitive tool for probing flavin electronics in proteins and understanding variations in flavin reactivity at a fundamental level.

Figure 3: NBO-electron densities based on NMR-validated DFT calculations are also shown, note the significantly greater excess negative charge at N5 than at N1, which is consistent with observed differences in reactivity.

There is/was story about flavins at the website http://access.ncsa.uiuc.edu/CoverStories/flavins/

last updated: April, 2008 comments: send email to AFM 
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