EPR Spin Labeling in Carotenoid–Membrane Interactions - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 194

and from the quadratic term

⎛ ⎞

h h

− 10 0 0

τ= 6.51 × 10 ×Δ H + − 2 (10.3)

⎜ ⎟ 2C 0

⎜ ⎟

h h

⎝ ⎠

− +

Δ H 0 is the peak-to-peak width of the central line in gauss, and h + , h 0 , h − are heights of the low,

central, and high i eld peaks, respectively (see Figure 10.2). When τ 2C are similar, it is

argued that the motional model is fairly good and motion is isotropic. Figure 10.4 presents correla-
tion times for 16-SASL in the DMPC bilayer calculated from the linear term (

τ 2B and

τ 2B ) and the quadratic

zeaxanthin decreases motional freedom of the 16-SASL free-radical moiety which is monitored
by a large increase in correlation times. At lower temperatures (25°C and 35°C), zeaxanthin also
increases the anisotropy of spin-label movement, which is manifested as a difference between

term ( τ 2C ) of the line width as a function of mole fraction of zeaxanthin. The addition of 10 mol %

τ 2B

and τ 2C

are very similar, indicating that zeaxanthin decreases the rate of spin-label motion, but does not
inl uence its isotropy. Additionally, from the Arrhenius display of the temperature dependence of
the rotational correlation time (log

τ 2C . However, at 45°C—well above the phase-transition temperature—calculated τ 2B and

versus reciprocal temperature), the activation energy of the
rotational motion of the nitroxide moiety of 16-SASL or 16-PC can be calculated as shown in
Subczynski et al. (1993).
We would like to point out that an order parameter indicates the static property of the lipid
bilayer, whereas the rotational motion, the oxygen transport parameter (Section 4.1), and the
chain bending (Section 4.4) characterize membrane dynamics (membrane l uidity) that report
on rotational diffusion of alkyl chains, translational diffusion of oxygen molecules, and fre-
quency of alkyl chain bending, respectively. The EPR spin-labeling approach also makes it
possible to monitor another bulk property of lipid bilayer membranes, namely local membrane
hydrophobicity.

τ

2

25° C

1.6

35° C

1.2

45° C

0.8

0.4

01 3

Zeaxanthin (mol%)

10

FIGURE 10.4 Effective rotational correlation time of 16-SASL in DMPC membranes plotted as a function
of mole fraction of zeaxanthin at different temperatures (τ 2B (○) and τ 2C ( ● )). (From Subczynski, W.K. et al.,
Biochim. Biophys. Acta , 1105, 97, 1992. With permission.)