1H photo-CIDNP enhancements in heteronuclear correlation NMR spectroscopy.

Photochemically induced dynamic nuclear polarization (photo-CIDNP) is usually employed as a probe of solvent exposure in biomolecular NMR. The potential of the photo-CIDNP effect for sensitivity enhancement, however, remains poorly explored. Here, we introduce (1)H-photo-CIDNP in heteronuclear correlation spectroscopy at low laser irradiation power (1 W), and compare the sensitivity of various (1)H-photo-CIDNP-enhanced- (HPE) (1)H-(15)N heteronuclear correlation pulse sequences, including HSQC, HMQC, and SOFAST-HMQC, in terms of their ability to detect the Trp indole H(epsilon1) resonance. Both Trp and the Trp-containing protein apoHmpH were analyzed using flavin mononucleotide as photosensitizer in aqueous solutions either containing or lacking urea. We find that (1)H-(15)N photo-CIDNP-SOFAST-HMQC, denoted here as HPE-SOFAST-HMQC, yields a 2-fold higher signal-to-noise per unit time than the parent SOFAST-HMQC, for the solvent-exposed Trp of urea-unfolded apoHmpH. Thus, HPE-SOFAST-HMQC is the most sensitive heteronuclear correlation pulse sequence for the detection of solvent-exposed Trp.