Geminal methyl groups
The (CC) measurements do not show large significant differences for geminal methyl groups in the two valines and four leucines for which both order parameters were available, except for the very flexible L73. The question whether geminal methyl groups exhibit similar mobility in proteins has been addressed by several different methods, but has led to conflicting interpretations. A study on 13C–13C cross-relaxation on serine-protease PB92 (Houben and Boelens 2004) reports consistently higher order parameters for the methyl group that is trans in the main chain as compared to the methyl group that is gauche. In support of this, relaxation studies on thioredoxin (LeMaster and Kushlan 1996; LeMaster 1999) have led to the observation of important differences in S2, which were interpreted as concerted motions. This effect has however been observed to a much smaller extent from 2H-relaxation measurements for various other proteins (Millet et al. 2003; Mittermaier et al. 1999; Flynn et al. 2001). For ubiquitin, there is no indication of this behaviour from 2H-relaxation measurements either. Interestingly, the anisotropy of motion η (Fig. 2b) show some differences for geminal methyl groups and could also underline the presence of different dynamic properties. This point is addressed again in the rotameric analysis (vide infra).