Tri- and Diglycine Backbone Rotational Dynamics Investigated by ¹³C NMR Multiplet Relaxation and Molecular Dynamics Simulations

Backbone motional dynamics in tri- and diglycine have been investigated by using ¹³C NMR multiplet relaxation spectroscopy. Dipolar auto- and cross-correlation times were determined as a function of pH, ionic strength, and temperature. Molecular dynamics simulations and φ,ψ bond rotation energy profiles were calculated for insight into the physical nature of backbone rotations that could contribute to ¹³C relaxation. Various motional models were used to fit the experimental data. For internal glycine G2 in triglycine, restricted and unrestricted rotational diffusion models both underestimate internal correlation times, although they do agree that the axis of fastest internal rotation is directed closely along the C_α-C bond. For di- and triglycine, significant pH dependencies in cross-correlation times for C-terminal glycines, and more so for those of N-terminal glycines, indicate the importance of the ionization state in internal mobility of terminal backbone positions. For terminal glycines, rotational jump models which allow for diffusive-like fluctuations within minima best explain the experimental data. φ,ψ rotational fluctuation amplitudes and internal rotational energy barriers derived from the temperature dependence of ¹³C relaxation parameters, which range from 3 to 5 kcal/mol, agree well with those values calculated in rotational energy profiles.