PMC:2728246 / 25381-26858 JSONTXT

{"project":"2_test","denotations":[{"id":"19652920-731698-56190974","span":{"begin":458,"end":462},"obj":"731698"},{"id":"19652920-15014229-56190975","span":{"begin":820,"end":824},"obj":"15014229"}],"text":"As shown in Fig. 2a, methyl groups located at positions distant from the rigid backbone are more mobile and show smaller . This is expected since their orientational fluctuation will depend on the combined rotameric jumps of all intervening torsion angles between the methyl group and the backbone. It is also known that the first χ1 angle (N–Cα–Cβ–Cγ) has a preference for the gauche− position (−60°) due to steric hindrance with the backbone (Janin et al. 1978), whereas similar effects are less pronounced for all subsequent torsional angles (χ2,χ3,…). Similar properties have been seen already from relaxation studies of protein side chains, in which there is a general tendency for decreasing order parameters with increasing distance from the backbone (LeMaster 1999; LeMaster and Kushlan 1996; Houben and Boelens 2004; Mittermaier et al. 1999). The extent of anisotropy (Fig. 2b) can also be quite revealing; for instance, it is expected that geminal methyl groups of leucine and valine side chains not only have similar order parameters but also similar motional anisotropy in the simplistic model where their mobility depends on a single torsion angle. However, exceptions would be expected for cases where adjacent side chain and backbone dihedral angles undergo substantial concerted motion. In the following, these and other MF parameters for methyl groups are examined in the context of their distance to the backbone, of residue type and of solvent accessibility."}