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W L Hubbell

Publications and source records attributed to W L Hubbell.

134 records · Page 8Linked to original sources
PubMed1969

Motion of steroid spin labels in membranes.

Steroid spin labels have been found to undergo rapid and sometimes anisotropic rotational motion in membranes and phospholipid vesicle preparations. N-oxyl-4',4'-dimethyloxazolidine derivatives of 5alpha-androstan-3-one-17beta-ol, 5beta-androstan-3-one-17beta-ol, and 5alpha-androstan-3-one were prepared according to the procedure described by Keana. The paramagnetic five-membered ring is rigidly attached to the steroid nucleus at the 3-position, and thus the resonance spectrum of the nitroxide group reflects the motion of the entire steroid nucleus. This rapid motion, with rotational diffusion frequencies of the order of 10(7) to 10(8) second(-1) is considered to lend plausibility to models of membrane transport involving rotations and/or translations of carriers within the hydrophobic region of the membrane. The steroid spin labels also form a convenient class of probes for studying conformational changes in membranes, since (a) their resonance spectra often fall in the category of "intermediate immobilization," which is most sensitive to small changes, and (b) these labels are quite soluble in a wide variety of membranes.

Journal Article↗
PubMed1969

Orientation and motion of amphiphilic spin labels in membranes.

The paramagnetic resonance spectra of N-oxyl-4','-dimethyloxazolidine derivatives of 12-ketostearic acid, 5-ketostearic acid, 5-ketotricosanoic acid, and 5alpha-androstan-3-one-17beta-ol have been observed in sonicated phospholipid dispersions, the walking-leg nerve fibers of Homarus americanus, and in erythrocytes oriented by hydrodynamic shear. The preferred orientation of these spin labels in the nerve and erythrocyte membranes is one in which the long amphiphilic axis is perpendicular to the membrane surface. The resonance spectra indicate that these labels in the nerve fiber are bound to a lipid bilayer very similar to that found in the sonicated phospholipid dispersion, whereas any lipid bilayer in the erythrocyte must be much more rigid.

Animals↗
PubMedDate not supplied

Site directed spin labeling studies of structure and dynamics in bacteriorhodopsin.

Site-directed spin labeling of membrane proteins has been used to determine: (1) the topography of the polypeptide chain with respect to the membrane/solution interface, and (2) the identity and orientation of secondary structure in selected regions. These features are deduced from the collision rates of nitroxide side chains with paramagnetic reagents in solution, and the principles of the method are reviewed with reference to bacteriorhodopsin. The dynamics of the nitroxide side chains relative to the backbone reveal tertiary interactions of the labeled site, and provide a promising means of time-resolving conformational changes. This aspect is illustrated by recent studies of structural changes in bacteriorhodopsin during the photocycle. In these experiments, nitroxide side chains were introduced at residues 72, 101 and 105 after replacement of the original residues by cysteine. Upon flash photolysis, the electron paramagnetic resonance spectrum of a nitroxide at 101, but not those at 72 or 105, is time-dependent. The spectral change develops during the decay of the M-intermediate, and reverses upon return to the ground state. The results suggest a movement of the C-D or E-F interhelical loops during the protonation changes of aspartate 96.

Bacteriorhodopsins↗