| Abstract
| - We have used ab initio calculations to compute all of the tensor elements of the electric fieldgradient for each carbon−deuterium bond in the ring of deuterated 3-methyl-indole. Previous analyseshave ignored the smaller tensor elements perpendicular to principal component Vzz which is aligned withthe C−2H bond (local bond z-axis). At each ring position, the smallest element Vxx is in the molecularplane and Vyy is normal to the plane of the ring. The asymmetry parameter η = (|Vyy| − |Vxx|)/|Vzz| rangesfrom 0.07 at C4 to 0.11 at C2. We used the perpendicular (off-bond) tensor elements, in concert with animproved understanding of the indole ring geometry, to analyze prototype 2H NMR spectra from well-oriented, hydrated peptide/lipid samples. For each of the four tryptophans of membrane-spanning gramicidinA (gA) channels, the inclusion of the perpendicular elements changes the deduced ring tilt by nearly 10°and increases the ring principal order parameter Szz for overall “wobble” with respect to the membranenormal (molecular z-axis). With the improved analysis, the magnitude of Szz for the outermost indole ringsof Trp13 and Trp15 is indistinguishable from that observed previously for backbone atoms (0.93 ± 0.03).For the Trp9 and Trp11 rings, which are slightly more buried within the membrane, Szz is slightly lower (0.86± 0.03). The results show that the perpendicular elements are important for the detailed analysis of 2HNMR spectra from aromatic ring systems.
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