DYNAMICS OF AN INTEGRAL MEMBRANE PEPTIDE - A DEUTERIUM NMR RELAXATIONSTUDY OF GRAMICIDIN

Solid state deuterium (H-2) NMR inversion-recovery and Jeener-Broekaer t relaxation experiments were performed on oriented multilamellar disp ersions consisting of 1,2-dilauroyl-sn-glycero-3-phosphatidylcholine a nd H-2 exchange-labeled gramicidin D, at a lipid to protein molar rati o (L/P) of 15:1, in order to study the dynamics of the channel conform ation of the peptide in a liquid crystalline phase. Our dynamic model for the whole body motions of the peptide includes diffusion of the pe ptide around its helix axis and a wobbling diffusion around a second a xis perpendicular to the local bilayer normal in a simple Maier-Saupe mean field potential. This anisotropic diffusion is characterized by t he correlation times, tau(R parallel to) and tau(R perpendicular to). Aligning the bilayer normal perpendicular to the magnetic field and gr aphing the relaxation rate, 1/T-1Z, as a function of (1 - S-N-2H(2)), where S-N-2H(2) represents the orientational order parameter, we were able to estimate the correlation time, tau(R parallel to)or rotational diffusion. Although in the quadrupolar splitting, which varies as (3 cos(2) theta(D) - 1), has in general two possible solutions to theta(D ) in the range O less than or equal to theta(D) less than or equal to 90 degrees, the 1/T-1Z vs. (1 - S-N-2H(2)) curve can be used to determ ine a single value of theta(D), in this range. Thus, the 1/T-1Z vs. (1 - S-N-2H(2)) profile can be used both to define the axial diffusion r ate and to remove potential structural ambiguities in the splittings. The T-1Z anisotropy permits us to solve for the two correlation times (tau(R parallel to) = 6 8 X 10(-9) s and tau(R perpendicular to) = 6 x 10(-6) s). The simulated parameters were corroborated by a Jeener-Bro ekaert experiment where the bilayer normal was parallel to the princip al magnetic field. At this orientation the ratio, J(2)(2 omega(o))/J(1 )(omega(o)) was obtained in order to estimate the strength of the rest oring potential in a model-independent fashion. This measurement yield s the rms angle, (theta(2))(1/2) (= 16 +/- 2 degrees at 34 degrees C), formed by the peptide helix axis and the average bilayer normal.