13C NMR relaxation study of segmental motion of poly(L-histidine) in aqueous solution

To study the segmental motion Of poly (L-histidine) (PLH) in aqueous solution, the C-13 spin-lattice relaxation time (T-1) was measured at six resonance frequencies (omega(C)/2pi) ranging from 15 to 100 MHz at temperatures from 10 to 80degreesC. For backbone C-alpha, plots of log(T-1/omega(C)) against log(omega(C)) gave the well-superposed master curve, showing that the time-temperature reduction rule is realized. The shift factor obeyed the Arrhenius-type temperature dependence with the activation energy of 25.0 kJmol(-1). Using this activation energy for the temperature dependence of the correlation time, the master curve was well reproduced by the Dejean-Laupretre-Monnerie (DLM) model. One of the parameters relating to the segmental motion was tau(0)/tau(1) = 15, where tau(0) and tau(1) are the correlation times for the isolated single and correlated pair conformational transitions, respectively. It was found that the spectral density function J(omega(C)) has the exponent to the correlation time tau(1) as omega(C)j(omega(C)) similar to (omega(C)tau(1))0.71 in the region of omega(C)tau(1) much less than 1.