Hydrogen bond contribution to the isotropic hyperfine splitting constant of a nitroxide free radical

Gagua, A. V.; Маленков, Г. Г.; Timofeev, Vladimir P.

doi:10.1016/0009-2614(78)89018-6

Cited by 18 publications

(21 citation statements)

References 4 publications

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“…The polarity profile registered by the spin-label isotropic hyperfine splitting constants can, at least in part, be related to water penetration into the lipid membranes. Experiments in aprotic solvents with increasing concentrations of H-bond donor (15) …”

Section: Discussionmentioning

confidence: 99%

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Polarity and permeation profiles in lipid membranes

Marsh

2001

Proc. Natl. Acad. Sci. U.S.A.

175

208

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The isotropic 14 N-hyperfine coupling constant, a o N , of nitroxide spin labels is dependent on the local environmental polarity. The dependence of a o N in fluid phospholipid bilayer membranes on the C-atom position, n, of the nitroxide in the sn-2 chain of a spinlabeled diacyl glycerophospholipid therefore determines the transmembrane polarity profile. The polarity variation in phospholipid membranes, with and without equimolar cholesterol, is characterized by a sigmoidal, trough-like profile of the form {1 ؉ exp [(n ؊ no)͞]} ؊1 , where n ‫؍‬ no is the point of maximum gradient, or polarity midpoint, beyond which the free energy of permeation decreases linearly with n, on a characteristic length-scale, . Integration over this profile yields a corresponding expression for the permeability barrier to polar solutes. For fluid membranes without cholesterol, no Ϸ 8 and Ϸ 0.5-1 CH2 units, and the permeability barrier introduces an additional diffusive resistance that is equivalent to increasing the effective membrane thickness by 35-80%, depending on the lipid. For membranes containing equimolar cholesterol, n o Ϸ 9 -10, and the total change in polarity is greater than for membranes without cholesterol, increasing the permeability barrier by a factor of 2, whereas the decay length remains similar. The permeation of oxygen into fluid lipid membranes (determined by spin-label relaxation enhancements) displays a profile similar to that of the transmembrane polarity but of opposite sense. For fluid membranes without cholesterol no Ϸ 8 and Ϸ 1 CH2 units, also for oxygen. The permeation profile for polar paramagnetic ion complexes is closer to a single exponential decay, i.e., no lies outside the acyl-chain region of the membrane. These results are relevant not only to the permeation of water and polar solutes into membranes and their permeabilities, but also to depth determinations of site-specifically spin-labeled protein residues by using paramagnetic relaxation agents.T he permeation profiles of water and polar solutes into lipid membranes are fundamental not only to transport studies but also to the energetics of insertion of proteins into membranes. Additionally, the permeation profiles of paramagnetic relaxation agents are of practical importance for depth determinations in membranes by site-directed spin labeling (1). Trough-like polarity profiles across lipid membranes have been established by measuring isotropic 14 N-hyperfine splitting constants, a o N , or the principal hyperfine tensor element, A zz , in lipid membranes (2-4). These are probably determined to a large extent by the penetration of water into the hydrophobic interior. This will modulate the energetics of burying amino acid residues in membranes, but has largely been neglected in favor of a uniform hydrophobic effect when analyzing the stability of integral membrane proteins (e.g., ref 5). White and Wimley (6) are among the few authors who have addressed this problem specifically. Interestingly, experiments on paramagnetic enhancements of spin l...

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Section: Discussionmentioning

confidence: 99%

“…3 will depend on the local diffusion-controlled on-rate, specified by D(x) (see ref. 15). For the case of oxygen and paramagnetic complexes, considered later, the accessibilities measured from spin-label relaxation enhancements are explicitly proportional to the K(x)D(x), concentration-diffusion product (19).…”

Section: The Value Of K(x)mentioning

confidence: 99%

Polarity and permeation profiles in lipid membranes

Marsh

2001

Proc. Natl. Acad. Sci. U.S.A.

175

208

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“…[5] in the limit ε − n 2 i n 2 i (6). In Kirkwood's treatment for associated fluids, the second term on the right in Eq.…”

Section: Theoretical Backgroundmentioning

confidence: 98%

“…In Kirkwood's treatment for associated fluids, the second term on the right in Eq. [5], which describes the dipole contribution, is multiplied by a correlation parameter, g i ≥ 1. This extension allows for the mutual interactions of the dipoles and is a measure of their local ordering (10).…”

Section: Theoretical Backgroundmentioning

confidence: 99%

“…Determination of local proton donor (e.g., water) concentrations therefore requires allowance for the variation in effective local dielectric constant. In homogeneous mixtures of protic and aprotic solvents this may be done by direct measurement of the dielectric constant of the mixture (5). In heterogeneous systems, e.g., membranes and the interior of proteins, however, some estimate is needed of the effective local dielectric constant.…”

Section: Introductionmentioning

confidence: 99%

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Polarity Contributions to Hyperfine Splittings of Hydrogen-Bonded Nitroxides—The Microenvironment of Spin Labels

Marsh¹

2002

Journal of Magnetic Resonance

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A self-consistent treatment of reaction field effects on isotropic (14)N hyperfine coupling constants of nitroxide spin labels in mixtures of polar and apolar solvents is given based on the Onsager approach. It is shown that this works reasonably well for mixtures of water or methanol with dioxane, far better than do conventional approaches using the Clausius-Mossotti relation. Association constants, K(A,h), for hydrogen bonding of protic solvents to nitroxides are derived in this way from published EPR data. A value of K(A,h) approximately 1.0 M x (-1) is argued to be reasonable for water in a hydrophobic environment. Data from spin-labelled lipids can then be used to estimate effective water concentrations in biological membranes.

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