Mapping of collision frequencies for stearic acid spin labels by saturation-recovery electron paramagnetic resonance. Biophys J 1990 Sep;58(3):713-20
Date
09/01/1990Pubmed ID
2169919Pubmed Central ID
PMC1281012DOI
10.1016/S0006-3495(90)82414-0Scopus ID
2-s2.0-0025006656 (requires institutional sign-in at Scopus site) 29 CitationsAbstract
Short pulse saturation-recovery electron paramagnetic resonance methods have been used to measure interactions of 14N:15N stearic acid spin label pairs in multilamellar liposomal dispersions composed of dimyristoyl-phosphatidylcholine (DMPC) and dielaidoylphosphatidylcholine (DEPC). Pairs consisting of various combinations of [14N]-16-, [14N]-12- or [14N]-5-doxylstearate, and [15N]-16-, [15N]-12-, or [15N]-5-doxylstearate were studied. SR experiments were performed at 27 degrees and 37 degrees C, and recovery signals were analyzed for initial conditions and multiexponential time constants by computer fitting using a damped least-squares approach. The time constants contain combinations of the electron spin lattice relaxation time, Tle, for each member of the spin-label pair, and the Heisenberg exchange rate constant, Kx. Spin-lattice relaxation times for each of the 14N and 15N stearic acid spin labels were determined, and it is noted that Tle for a given 15N-SASL was always slightly greater than that of the corresponding 14N-SASL. From Kx the bimolecular collision frequency was calculated, providing a detailed picture of molecular interactions. For both lipid systems the bimolecular collision rates were ordered as 12:5 less than 16:5 less than 5:5 less than 16:12 less than 12:12 less than 16:16. For all spin-label pairs studied, interaction frequencies were greater in DMPC than in DEPC. For the 16:16, 12:12, and 16:12 pairs, Kx was approximately 30% greater in DMPC than in DEPC, a significantly greater difference than is observed by conventional EPR methods. Further confirmation of the existence of vertical fluctuation of nitroxide moieties that are at the 16- (or 12) position toward the polar surfaces was obtained, with the frequency of 16:5 (or 12:5) interactions ~40% of the 16:16 (or 12:12) interaction frequency. In both DMPC and DEPC, Kx for homogeneous pairs increases as the doxyl moiety is moved further down the alkyl chain (i.e.,5:5 < 12:12 < 16:16), suggesting that steric effects or the local rotational mobility of the nitroxide group influence the frequency at which spin exchange occurs.
Author List
Yin JJ, Feix JB, Hyde JSAuthor
Jimmy B. Feix PhD Professor in the Biophysics department at Medical College of WisconsinMESH terms used to index this publication - Major topics in bold
Cyclic N-OxidesElectron Spin Resonance Spectroscopy
Membranes, Artificial
Spin Labels