Photo-CIDNP nuclear magnetic resonance as a probe for conformational changes in epidermal growth factor

Photo-chemically induced dynamic nuclear polarization (CIDNP)-NMR spectroscopy at 360 MHz has been used to investigate pH-induced conformational transitions in mouse epidermal growth factor. At about pH 9, all five tyrosine residues and both tryptophan residues are, to various extents, solvent-exposed, while the His-22 residue is buried in the protein matrix. Tyr-13 is the least exposed of the tyrosine residues and also the most immobilized. As the pH is decreased to 5.9, the tryptophan residues gradually become less exposed, while the Tyr-13 residue becomes internalized in the protein. These data suggest that the C-terminus and part of the N-terminal structural domain are affected by a conformational transition in mouse epidermal growth factor occurring between pH 6 and 8 via breakage of the His-22 inter-residue linkage. Above pH 9, a decreased photo-CIDNP effect is evident for both tryptophans and for Tyr-10 and Try-13; this information suggests that a second conformational change takes place at basic pH, which may simply be incipient denaturation.