Segmental Motion at the N-Terminal Structural Domain in Epidermal Growth Factor
The downfield aromatic region of a 500 MHz proton NMR spectrum of mouse epidermal growth factor is shown below. Several of the resonances assigned to the Tyr-10, Try-13 and His-22 residues in mouse epidermal growth factor (mEGF) seem to have one or more associated “minor” resonances (labelled ‘B’ and ‘C’) (Mayo, 1984). The term “minor” defines those resonances in the NMR spectrum whose integrated intensities are less than one proton each. Based primarily on the observation of saturation transfer between these minor resonances and the Tyr-10, tyr-13 and His-22 resonances, respectively, a process of slow chemical exchange on a 500 MHz NMR time scale must be occurring between these minor resonances and their respective major resonance counterparts (labelled ‘A’). Selective saturation transfer and spin-lattice relaxation (T 1 experiments allow estimation of the exchange rate constants which range from 10 s-1 to 700 s-1 at 303K. The effect of temperature on the exchange kinetics gives the transition enthalpies (~5 kcal/mole) and entropies (~2 x10-1 kcal/mole °K) of the exchange process and allows insight into the most probable exchange mechanism. The unequal resonance intensities of the exchanging resonances rule-out 180° “flip” motions about the C α — C β bond of the tyrosines as the cause for exchange. Rotation about the C α — C β tyrosine bond also seems unlikely.