Laser chemically induced dynamic nuclear polarization study of the reaction between photoexcited flavins and tryptophan derivatives at 360 MHz.

Chemically induced dynamic nuclear polarization (CIDNP) is generated when tryptophan (Trp), its derivatives, or Trp-containing peptides react with photoexcited flavins in a 360-MHz NMR spectrometer. In contrast to tyrosine (Tyr), we find that the nuclear polarization of Trp originates in an electron-transfer reaction. By use of a series of Trp derivatives, the unpaired spin-density distribution of the Trp radical cation and the ground-state NMR spectrum of Trp are analyzed in detail. The signs and the relative magnitudes of the proton isotropic hyperfine coupling constants for each position around the indole ring in the radical cation deduced from these measurements are the following: position 3 greater than 2 approximately 4 approximately 6 greater than 1 greater than 5 greater than 7, with positions 1, 2, 3, 4, and 6 positive, 5 negative, and 7 essentially zero. This result is inconsistent with most available calculations of the unpaired spin-density distribution but is compatible with the pattern of electrophilic aromatic substitution. The origin of this discrepancy is discussed in detail. Possible mechanistic complications in the reaction leading to CIDNP are discussed. The laser CIDNP spectra of the Trp-rich peptides gramicidins A and B are presented as examples of the resolution enhancement obtained with this technique.