Modulation of cytochrome c spin states by lipid acyl chains: a continuous-wave electron paramagnetic resonance (CW-EPR) study of haem iron.

This work is a systematic study, showing a clear correlation between the nature of the lipid acyl chain and the spin states of cytochrome c interacting with different types of lipid membranes. According to the lipid acyl chain type, and the head group charge present in the bilayer, three spin states of cytochrome c were observed in different proportions: the native cytochrome c low spin state with rhombic symmetry (spin 1/2, g axially=3.07 and g radially=2.23), a low spin state with less rhombic symmetry (spin 1/2, g(1)=2.902, g(2)=2.225, and g(3)=1.510) and the high spin state (spin 5/2, g axially=6.0 and g radially=2.0). The proportion of the spin states of cytochrome c bound to bilayers was also dependent on the lipid/protein ratio, suggesting the existence of two or more protein sites interacting with the lipids. The lipid-induced alterations in the symmetry and spin states of cytochrome c exhibited partial reversibility when the ionic strength was increased, which reinforces the crucial role played by the electrostatic interaction with the lipid bilayer. Different cytochrome c spin states exhibited corresponding modifications in the haemprotein UV/visible spectra, particularly in the Q-band associated with loss of the 695 nm band and appearance of a band in the region of 600-650 nm. The observed reactivity of cytochrome c with oxidized forms of unsaturated lipids reinforces the possibility of the acyl chain insertion in the haemprotein structure.