Proteins detection ranges, motions

Caspar et al. (1988) described an elegant analysis of the diffuse X-ray scattering from insulin crystals. They found two types of coupled motion one with a characteristic length of about 6 A and amplitude of about 0.4 A, the other with a characteristic length of about 20 A and smaller amplitude. The latter motion represents the jiggle of neighboring molecules of the lattice. The former represents the coupled fluidlike fluctuations within a protein molecule. The short-range motions appear to be similar to those detected by Mossbauer spectroscopy. 

The hydration of proteins at subzero temperatures is reviewed. The thermodynamics of the protein-water system and the water molecule dynamics are discussed. The hydration layer around a protein at low temperature is best thought of as being in a glass-like state with the water molecules selectively oriented near ionic and polar groups at the protein surface. Water motions in the nanosecond and microsecond range have been detected. 

Globular and membrane proteins in aqueous solution or Hpid bilayers/ biomembranes could undergo a wide range of motions at ambient temperatures. Such motions can be characterized by several types of NMR relaxation parameters, chemical exchange, dynamic interference including SRI [27—29]. Detection ranges for motions by the respective parameters are schematically illustrated in Fig. 1.1 for solution and soUd state NMR approaches. Naturally, fast motions are solely examined by solution NMR techniques for any portions of proteins. The experimental approaches to be able to detect intermediate or slow motions are obviously different between the solution and solid state NMR methods. 

Thus, although the nmr methods are important for comparison of the structure of proteins in the solid state and in solution, they are also of importance in areas where X-ray crystallography can provide little information. One of these areas concerns the time dependence of protein structure, for molecular motion over a wide range of time scales can be detected. Table IV indicates the methods and references to these studies. The range of the nmr technique is from about 10 10 s to slower than 10 s. Thus, although more restricted than X-ray crystallography in direct structure determination, the nmr studies can check the solution structure and complement the diffraction studies once the overall similarity between the solid and solution structure is proved. In this way nmr relates the static picture of a protein structure to the kinetic data of solution chemistry. 

Experimental observations, such as the coincidence of onset of function with onset of protein motion and long-range connectivity, lead one to infer that correlated fluctuations are among the fundamental principles of enzyme activity. The proof of this inference, through the detection of cross-correlations, remains an open and difficult problem. There is a substantial body of evidence, however, that bears on the correlation of solvent and protein motions, and understanding of the details of this coupling is likely to come soon. 

It has recently become more widely appreciated that the presence of rotational diffusional anisotropy in proteins and other macromolecules can have a significant affect on the interpretation of NMR relaxation data in terms of molecular motion. Andrec et al. used a Bayesian statistical method for the detection and quantification of rotational diffusion anisotropy from NMR relaxation data. Sturz and Dolle examined the reorientational motion of toluene in neat liquid by using relaxation measurements. The relaxation rates were analyzed by rotational diffusion models. Chen et al measured self-diffusion coefficients for fluid hydrogen and fluid deuterium at pressures up to 200 MPa and in the temperature range 171-372 K by the spin echo method. The diffusion coefficients D were described by the rough sphere (RHS) model invoking the rotation translational coupling parameter A = 1. 

Evidence for mobility within proteins comes from a variety of physical methods single crystal X-ray or neutron diffraction, electron microscopy, and spectroscopic techniques such as NMR, fluorescence depolarization, Mossbauer spectroscopy and H-exchange studies. Theoretical approaches such as potential-energy minimization and molecular-dynamics calculations may also be used to study flexibility. An illustration of the frequency range of the various thermal motions detected in proteins is given in Table 1. 

---