Ka. Earle et al., 250-GHZ ELECTRON-SPIN-RESONANCE STUDIES OF POLARITY GRADIENTS ALONG THE ALIPHATIC CHAINS IN PHOSPHOLIPID-MEMBRANES, Biophysical journal, 66(4), 1994, pp. 1213-1221
Rigid-limit 250-GHz electron spin resonance (FIR-ESR) spectra have bee
n studied for a series of phosphatidylcholine spin labels (n-PC, where
n = 5, 7, 10, 12, 16) in pure lipid dispersions of dipalmitoylphospha
tidylcholine (DPPC) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC)
, as well as dispersions of DPPC containing the peptide gramicidin A (
GA) in a 1:1 molar ratio. The enhanced g-tensor resolution of 250-GHz
ESR for these spin labels permitted a careful study of the nitroxide g
-tensor as a function of spin probe location and membrane composition.
In particular, as the spin label is displaced from the polar head gro
up, A(zz) decreases and g(xx) increases as they assume values typical
of a nonpolar environment, appropriate for the hydrophobic alkyl chain
s in the case of pure lipid dispersions. The field shifts of spectral
features due to changes in g(xx) are an order of magnitude larger than
those from changes in A(zz). The magnetic tenser parameters measured
in the presence of GA were characteristic of a polar environment and s
howed only a very weak dependence of A(zz) and g(xx) on label position
. These results demonstrate the significant influence of GA on the loc
al polarity along the lipid molecule, and may reflect increased penetr
ation of water into the alkyl chain region of the lipid in the presenc
e of GA. The spectra from the pure lipid dispersions also exhibit a br
oad background signal that is most significant for 7-, 10-, and 12-PC,
and is more pronounced in DPPC than in POPC. It is attributed to spin
probe aggregation yielding spin exchange narrowing. The addition of G
A to DPPC essentially suppressed the broad background signal observed
in pure DPPC dispersions.