Photo-CIDNP and protein folding
The work described in this thesis is concerned with the development of new applications of the photo-CIDNP (photochemically induced dynamic nuclear polarization) technique to aspects of protein structure and folding. Chapters 1 and 2 are introductory chapters; Chapter 1 describes the theoretical basis of the CIDNP phenomenon in terms of the underlying spin chemistry of the radical pair mechanism, while Chapter 2 presents the apparatus, photosensitizer and pulse sequences used, along with some important experimental considerations. Chapter 3 describes how 15N CIDNP can be used to probe the accessibility of tryptophan side-chains in both native and denatured states of proteins. The polarization of indole nitrogens in uniformly 15N labeled protein is detected in a two-dimensional 15N-1 H NMR heteronuclear correlation experiment. Chapter 4 describes two new techniques offering considerable improvements in the quality of photo-CIDNP spectra of proteins. Both focus on the problem of progressive photo-degradation of the flavin dye and in both cases a larger number of scans can be accumulated before the flavin is exhausted than would otherwise be possible. In Chapter 5, the potential of stopped-flow photo-CIDNP spectroscopy for the study of protein folding is explored. Rapid dilution of denatured protein into a buffer solution is used to initiate a refolding process which is followed using short laser pulses to generate 1H CIDNP in the side-chains of exposed aromatic residues. In Chapter 6, the field dependence of amino acid photo-CIDNP intensities is investigated using a stopped-flow CIDNP device that allows sample irradiation over a range of magnetic fields (0.1-7 T) within the bore of a 9.4 T NMR magnet and rapid transfer into the NMR tube for detection. Finally, in Chapter 7 two photo-CIDNP techniques that probe the exposure of aromatic residues in partially folded states are described. Both involve transfer of polarization to the native state for detection. One approach achieves this kinetically by rapid refolding, and the other involves monitoring exchange cross peaks in a two-dimensional CIDNP spectrum under conditions where the two states are interconverting.