Marsh D
Max-Planck-Institut für biophysikalische Chemie, Abteilung Spektroskopie, D-37070 Göttingen, Germany.
Biophys J. 1998 Jul;75(1):354-8. doi: 10.1016/S0006-3495(98)77519-8.
In polytopic alpha-helical transmembrane proteins, the distribution of amide vibrational transition moments can be nonaxial, if the helix axes are tilted relative to the symmetry axis of the helix bundle. The infrared dichroic ratios from oriented samples then contain nonaxial terms and, in the most general case, require a second-order parameter for the axis of the helix bundle. The extent of nonaxiality depends on the summation over the individual amide transition moments along the helix. Because this is strongly oscillatory, with a 3.6-residue periodicity, complete axial symmetry is not achieved rapidly on progressive summation. Expressions for the contributions of residual nonaxiality to the dichroic ratios are derived. A similar situation arises for oligomers of transmembrane beta-barrel proteins, e.g., the porin trimer. In this case, the extent of nonaxiality depends not only on the number of residues in the beta-barrel, but also on the tilt of the beta-strands relative to the barrel axis and the characteristic dimensions of a beta-sheet, which together determine the axial periodicity. The nonaxial contributions to the dichroic ratios of beta-barrel oligomers are also derived. Estimates are given of the likely size of the nonaxial contributions for the different alpha-helical and beta-sheet systems.
在多聚体α-螺旋跨膜蛋白中，如果螺旋轴相对于螺旋束的对称轴倾斜，酰胺振动跃迁矩的分布可能是非轴向的。来自取向样品的红外二向色性比率随后包含非轴向项，并且在最一般的情况下，需要一个用于螺旋束轴的二阶参数。非轴向程度取决于沿着螺旋的各个酰胺跃迁矩的总和。由于这是强烈振荡的，具有3.6个残基的周期性，在逐步求和时不能迅速实现完全的轴对称。推导了残余非轴向性对二向色性比率贡献的表达式。跨膜β-桶蛋白的寡聚体，如孔蛋白三聚体，也会出现类似情况。在这种情况下，非轴向程度不仅取决于β-桶中的残基数，还取决于β-链相对于桶轴的倾斜度以及β-折叠的特征尺寸，它们共同决定了轴向周期性。还推导了β-桶寡聚体对二向色性比率的非轴向贡献。给出了不同α-螺旋和β-折叠系统中非轴向贡献可能大小的估计。
