Determination of the solvent accessibility of specific aromatic residues in gamma-crystallin by photo-CIDNP NMR measurements.

The surface or solvent accessibility of certain individual aromatic residues of calf-gamma II crystallin in solution (1 mM) were measured by the dramatic intensity enhancements of NMR lines generated by the interactions of cyclic radical pair formation of the 3-N-carboxymethyl lumiflavin (flavin I) dye excited (488nm) by an argon laser (5 watts) with the protein. This effect is called photo-chemically induced dynamic nuclear polarization: photo-CIDNP. The "light" and "dark" NMR spectra were taken in alternating scans in the pulsed Fourier transform mode on a Bruker 360 MHz instrument. Subtraction results in the photo-CIDNP difference spectrum containing lines of the polarized residues. With flavin dyes only tyrosine, histidine, and tryptophan can be polarized. The respective theoretical static accessibility of these residues based upon van der Waal's contact radii have been calculated from the atomic coordinates and provide a basis for evaluating the dynamic NMR photo-CIDNP results and for assigning the resonances. These results suggest that while the four tryptophan residues are completely buried, His-113 and His-14 of the five histidines; and Tyr-165 and Tyr-62 of the fifteen tyrosines are sufficiently exposed to elicit a photo-CIDNP effect. These results confirm and extend the observations previously obtained with theoretical electrostatic programs and FT-NMR measurements.