Protein dynamics and the NMR relaxation time T1 of water in biological systems

The relaxation rate R, = T;' for water in many biological tissues and protein solutions is characterized by a weak frequency dependence, R ₁ ∞ ω ^{- {1}/{2}} for ω/2π > 5 MHz. We propose a mechanism for the water-polymer interaction that leads to this frequency dependence in a natural way without the introduction of ad hoc assumptions of water fractions with arbitrary distributions of correlation times. The mechanism involves the wavelength-dependent correlation time for the fluctuations of the highly damped excitation modes of a long protein segment.