Dynamics in a protein-lipid complex: nuclear magnetic resonance measurements on the headgroup of cardiolipin when bound to cytochrome c.

Deuterium and phosphorus nuclear magnetic resonance (NMR) has been used to investigate the dynamics of slow motional processes induced in bilayer cardiolipin upon binding with cytochrome c. 31P NMR line shapes suggest that protein binding induces less restricted, isotropic-like motions in the lipid phosphates within the ms time scale of this measurement. However, these motions impart rapid transverse relaxation to methylene deuterons adjacent to the phosphate in the lipid headgroup and so did not feature strongly in the NMR line shapes recorded from these nuclei by using the quadrupolar echo. Nonetheless, motional characteristics of the headgroup deuterons were accessible to a dynamic NMR approach using the Carr-Purcell-Meiboom-Gill multiple-pulse experiment. Compared to the well-studied case of deuterons in fatty acyl chains of bilayer phosphatidylcholine, the motions determining the 2H spin transverse relaxation in the headgroup of bilayer cardiolipin were much faster, having a lower limit in the 5-10 kHz range. On binding with cytochrome c, the T2 effecting motions in the cardiolipin headgroup became faster still, with rates comparable to the residual quadrupolar coupling frequency of the headgroup deuterons (approximately 25 kHz) and so coincided with the time scale for recording the quadrupolar echo (approximately 40 microseconds). It is concluded that the headgroup of cardiolipin does not exclusively report localized dynamic information but is particularly sensitive to collective motions occurring throughout the bilayer molecules. Although the rates of collective modes of motion may be dependent on the lipid type in pure lipid bilayers, these low-frequency fluctuations appear to occupy a similar dynamic range in a variety of lipid-protein systems, including the natural membranes.