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 use d to investigate the dynamics of slow motional processes induced in bi layer cardiolipin upon binding with cytochrome c. P-31 NMR line shapes suggest that protein binding induces less restricted, isotropic-like motions in the lipid phosphates within the ms time scale of this measu rement. However, these motions impart rapid transverse relaxation to m ethylene deuterons adjacent to the phosphate in the lipid headgroup an d so did not feature strongly in the NMR line shapes recorded from the se nuclei by using the quadrupolar echo. Nonetheless, motional charact eristics of the headgroup deuterons were accessible to a dynamic NMR a pproach using the Carr-Purcell-Meiboom-Gill multiple-pulse experiment. Compared to the well-studied case of deuterons in fatty acyl chains o f bilayer phosphatidylcholine, the motions determining the H-2 spin tr ansverse relaxation in the headgroup of bilayer cardiolipin were much faster, having a lower limit in the 5-10 kHz range. On binding with cy tochrome c, the T2 effecting motions in the cardiolipin headgroup beca me faster still, with rates comparable to the residual quadrupolar cou pling frequency of the headgroup deuterons (approximately 25 kHz) and so coincided with the time scale for recording the quadrupolar echo (a pproximately 40 ps). It is concluded that the headgroup of cardiolipin does not exclusively report localized dynamic information but is part icularly sensitive to collective motions occurring throughout the bila yer molecules. Although the rates of collective modes of motion may be dependent on the lipid type in pure lipid bilayers, these low-frequen cy fluctuations appear to occupy a similar dynamic range in a variety of lipid-protein systems, including the natural membranes.