SEARCH · Search PubMed

Results for “Spin Labels”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 901 records · Page 50Linked to original sources

PubMed1988

A comparative study on the structural differences of primate hemoglobins by spin labeling technique.

The hemoglobins of human and five non-human primates were spin-labeled with N-(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)iodoacetamide, and the ESR spectra of their deoxy, oxy, and carbonmonoxy forms were measured. The analyses of the spectra indicated that the local protein conformation in the vicinity of the spin-labeled cysteine residue at position 93(F9) in the beta-chain is slightly but significantly different among species, and that each hemoglobin shows a similar change in conformation upon conversion from the oxy form to the carbonmonoxy one except for human hemoglobin. Human hemoglobin was suggested to undergo a significantly different conformational change upon this conversion, indicating that it has unique characteristics among the primate hemoglobins.

Animals↗

PubMed1999

Synthesis of a spin-labeled photoaffinity ATP analogue, and its use to specifically photolabel myosin cross-bridges in skeletal muscle fibers.

A spin-labeled photoaffinity ATP analogue 3'(2')-O-[4-[4-oxo-(4-amido-2,2,6,6-tetramethyl-piperidino-1-oxyl)]-benz oyl]benzoyl adenosine 5'-triphosphate (SL-Bz2ATP) was synthesized and used to photolabel myosin in muscle fibers. Previous work has shown that 3'(2')-O-(4-benzoyl)benzoyl adenosine 5'-triphosphate (Bz2ATP) photolabeled Ser-324 of the 50 kDa tryptic fragment of skeletal S1 heavy chain. In this work, [alpha-32P]SL-Bz2ATP was hydrolyzed and trapped as the diphosphate analogue with Co2+ and orthovanadate at the active site of myosin in rabbit psoas muscle fibers. After UV irradiation, the myosin heavy chain was the only protein band found to be significantly photolabeled as assayed by gel electrophoresis and radioactivity counting. The labeling was localized after brief trypsin digestion by SDS-PAGE to be on the 50 kDa tryptic fragment of the S1 heavy chain. Ca. 35% of the myosin in fibers was covalently photolabeled. The fibers photolabeled with SL-Bz2ATP had the same active tension and maximum shortening velocity as the control fibers. The resulting spin label on myosin was too mobile to report the orientation of the heads in fibers. Nonetheless, this is the first work to show the feasibility of utilizing active site binding and photoaffinity labeling to place covalent spectroscopic probes at the myosin active site in fibers with high specificity and yield without affecting mechanical function.

Adenosine Triphosphate↗

PubMed1998

Introduction of a nitroxide group on position 2 of 9-phenoxyacridine: easy access to spin labelled DNA-binding conjugates.

In the search for spin labelled intercalators of general use to construct DNA-binding conjugates, 6-chloro-2-[(1-oxyl-2,2,5,5-tetramethyl- pyrrolin-3-yl)methyloxy]-9-phenoxy-acridine 5, has been prepared. This key-intermediate reacts with amines to give the corresponding labelled 9-amino substituted acridines. Comparative EPR and fluorescence measurements show that the label causes only little modification of the binding properties of acridine.

Acridines↗

PubMed1988

Effects of AMPPNP on the orientation and rotational dynamics of spin-labeled muscle cross-bridges.

We have used electron paramagnetic resonance (EPR) to investigate the orientation, rotational motion, and actin-binding properties of rabbit psoas muscle cross-bridges in the presence of the nonhydrolyzable nucleotide analogue, 5'-adenylylimido-diphosphate (AMPPNP). This analogue is known to decrease muscle tension without affecting its stiffness, suggesting an attached cross-bridge state different from rigor. We spin-labeled the SH1 groups on myosin heads and performed conventional EPR to obtain high-resolution information about the orientational distribution, and saturation transfer EPR to measure microsecond rotational motion. At 4 degrees C and 100 mM ionic strength, we find that AMPPNP increases both the orientational disorder and the microsecond rotational motion of myosin heads. However, computer analysis of digitized spectra shows that no new population of probes is observed that does not match either rigor or relaxation in both orientation and motion. At 4 degrees C, under nearly saturating conditions of 16 mM AMPPNP (Kd = 3.0 mM, determined from competition between AMPPNP and an ADP spin label), 47.5 +/- 2.5% of myosin heads are dynamically disoriented (as in relaxation) without a significant decrease in rigor stiffness, whereas the remainder are rigidly oriented as in rigor. The oriented heads correspond to actin-attached heads in a ternary complex, and the disoriented heads correspond to detached heads, as indicated by EPR experiments with spin-labeled subfragment 1 (S1) that provide independent measurements of orientation and binding. We take these findings as evidence for a single-headed cross-bridge that is as stiff as the double-headed rigor cross-bridge. The data are consistent with a model in which, in the presence of saturating AMPPNP, one head of each cross-bridge binds actin about 10 times more weakly, whereas the remaining head binds at least 10 times more strongly, than extrinsic S1. Thus, although there is no evidence for heads being attached at nonrigor angles, the attached cross-bridge differs from that of rigor. The heterogeneous behavior of heads is probably due to steric effects of the filament lattice.

Actins↗

PubMed1997

Oxidatively induced structural alteration of glutamine synthetase assessed by analysis of spin label incorporation kinetics: relevance to Alzheimer's disease.

The activity of the astrocytic enzyme glutamine synthetase (GS) is decreased in the Alzheimer's disease brain, which may have relevance to mechanisms of chronic excitotoxicity. The molecular perturbation(s) that results in GS inactivation is not known, although oxidative lesioning of the enzyme is one likely cause. To assess structural perturbation induced in GS by metal-catalyzed oxidation, a series of spin-labeling studies were undertaken. Ovine GS was oxidized by exposure to iron/hydrogen peroxide and subsequently labeled with the thiol-specific nitroxide probe MTS [(1-oxyl-2,2,5,5-tetramethyl-pyrroline-3-methyl)methanethiosulfonate]. The reaction of MTS with cysteine residues within GS was monitored in real time by electron paramagnetic resonance spectrometry. Structural perturbation of GS, manifested as decreased thiol accessibility, was inferred from an apparent decrease in the rate constant for the second-order reaction of MTS with protein thiols. A subsequent spin-labeling study was undertaken to compare the structural integrity of GS purified and isolated from Alzheimer's disease-afflicted brain (AD-GS) with that of GS isolated from nondemented, age-matched control brain (C-GS). The rate constant for reaction of MTS with AD-GS was markedly decreased relative to that for the reaction of spin label with C-GS. The kinetic data were partially corroborated by spectroscopic data obtained from circular dichroism analysis of control and peroxide-treated ovine GS. In an adjunct experiment, the interaction of GS with a synthetic analogue of the Alzheimer's-associated beta-amyloid peptide, known to induce free radical oxidative stress, indicated strong interaction of the enzyme with the peptide as reflected by a decrease in the rate constant for MTS binding to reactive protein thiols.

Alzheimer Disease↗

PubMed1975

Chemically induced lipid phase separation in model membranes containing charged lipids: a spin label study.

The lipid distribution in binary mixed membranes containing charged and uncharged lipids and the effect of Ca2+ and polylysine on the lipid organization was studied by the spin label technique. Dipalmitoyl phosphatidic acid was the charged, and spin labelled dipalmitoyl lecithin was the uncharged (zwitterionic) component. The ESR spectra were analyzed in terms of the spin exchange frequency, Wex. By measuring Wex as a function of the molar percentage of labelled lecithin a distinction between a random and a heterogeneous lipid distribution could be made. It is established that mixed lecithin-phosphatidic acid membranes exhibit lipid segregation (or a miscibility gap) in the fluid state. Comparative experiments with bilayer and monolayer membranes strongly suggest a lateral lipid segregation. At low lecithin concentration, aggregates containing between 25% and 40% lecithin are formed in the fluid phosphatidic acid membrane. This phase separation in membranes containing charged lipids is understandable on the basis of the Gouy-Chapman theory of electric double layers. In dipalmitoyl lecithin and in dimyristoyl phosphatidylethanolamine membranes the labelled lecithin is randomly distributed above the phase transition and has a coefficient of lateral diffusion of D = 2.8-10(-8) cm2/s at 59 degrees C. Addition of Ca2+ dramatically increases the extent of phase separation in lecithin-phosphatidic acid membranes. This chemically (and isothermally) induced phase separation is caused by the formation of crystalline patches of the Ca2+-bound phosphatidic acid. Lecithin is squeezed out from these patches of rigid lipid. The observed dependence of Wex on the Ca2+ concentration could be interpreted quantitatively on the basis of a two-cluster model. At low lecithin and Ca2+ concentration clusters containing about 30 mol % lecithin are formed. At high lecithin or Ca2+ concentrations a second type of precipitation containing 100% lecithin starts to form in addition. A one-to-one binding of divalent ions and phosphatidic acid at pH 9 was assumed. Such a one-to-one binding at pH 9 was established for the case of Mn2+ using ESR spectroscopy. Polylysine leads to the same strong increase in the lecithin segregation as Ca2+. The transition of the phosphatidic acid bound by the polypeptide is shifted from Tt = 47.5 degrees to Tt = 62 degrees C. This finding suggests the possibility of cooperative conformational changes in the lipid matrix and in the surface proteins in biological membranes.

Calcium↗

PubMed1990

Investigations of the horse conceptus via magnetic resonance imaging (MRI) and nitroxide spin labels as contrast agents.

Results are presented which illustrate the usefulness of Magnetic Resonance Imaging as applied to the study of living embryos. Nitroxide spin labels were employed as contrast agents to study the structure and properties of the embryos. These spin labels offer the additional advantage that they may potentially be bound to biologically important molecules thereby imparting the ability to produce contrast in the MR images to these new molecules. The horse conceptus was chosen over other embryos due to its large size. Whereas the embryos of cattle and swine are sub-millimetre in size, the horse conceptus is on the order of 10 millimetres in diameter. The availability of microscopic imaging gradient coils will allow the techniques developed in this study to be applied to the smaller embryos of other species.

Animals↗

PubMed2000

Spin labeling analysis of structure and dynamics of the Na(+)/proline transporter of Escherichia coli.

With respect to the functional importance attributed to the N-terminal part of the Na(+)/proline transporter of Escherichia coli (PutP), we report here on the structural arrangement and functional dynamics of transmembrane domains (TMs) II and III and the adjoining loop regions. Information on membrane topography was obtained by analyzing the residual mobility of site-specifically-attached nitroxide spin label and by determination of collision frequencies of the nitroxide with oxygen and a polar metal ion complex using electron paramagnetic resonance (EPR) spectroscopy. The studies suggest that amino acids Phe45, Ser50, Ser54, Trp59, and Met62 are part of TM II while Gly39 and Arg40 are located at a membrane-water interface probably forming the cytoplasmic cap of the TM. Also Ala67 and Glu75 are at a membrane-water interface, suggesting a location close to the periplasmic ends of TMs II and III, respectively. Ser71 between these residues is clearly in a water-exposed loop (periplasmic loop 3). Spin labels attached to positions 80, 86, and 91 show EPR properties typical for a TM location (TM III). Leu97 may be part of a structured loop region while Ala107 is clearly located in a water-exposed loop (cytoplasmic loop 4). Finally, spin labels attached to the positions of Asp33 and Leu37 are clearly on the surface of the transporter and are directed into an apolar environment. These findings strongly support the recently proposed 13-helix model of PutP [Jung, H., Rübenhagen, R., Tebbe, S., Leifker, K., Tholema, N., Quick, M., and Schmid, R. (1998) J. Biol. Chem. 273, 26400-26407] and suggest that TMs II and III of the transporter are formed by amino acids Ser41 to Gly66 and Ser76 to Gly95, respectively. In addition to the topology analysis, it is shown that binding of Na(+) and/or proline to the transporter alters the mobility of the nitroxide group at the positions of Leu37 and Phe45. From these findings, it is concluded that binding of the ligands induces conformational alterations of PutP that involve at least parts of TM II and the preceding cytoplasmic loop.

Amino Acid Sequence↗

PubMed1991

Effect of polar carotenoids on the oxygen diffusion-concentration product in lipid bilayers. An EPR spin label study.

The oxygen diffusion-concentration product was determined in phosphatidylcholine (PC) bilayers from oxygen broadening of the spin label EPR spectra. The use of fatty acid spin labels makes it possible to do structural and oximetric measurements with the same sample. We find that polar carotenoids, zeaxanthin and violaxanthin, increase ordering of hydrocarbon chains in saturated (dimyristoyl-PC) and unsaturated (egg yolk PC) membranes and also significantly decrease the oxygen diffusion-concentration product in the hydrocarbon region of these membranes. At 25 degrees C in the presence of 10 mol% of carotenoids, the product is about 30% smaller than in pure PC membranes. Intercalation of carotenoids decreases the oxygen diffusion-concentration product in the central part of the bilayer and has little effect on the product in the polar head group region. In contrast, cholesterol molecules significantly reduce the product on and near the membrane surface, and do not change it (saturated PC) or increase it (unsaturated PC) in the middle of the bilayer (Subczynski, W.K., Hyde, J.S. and Kusumi, A. (1989) Proc. Natl. Acad. Sci. USA 86, 4474-4478). The decrease of oxygen diffusion-concentration product may be a mechanism of carotenoid protective activity, which should be effective in plant and animal cells in the light as well as in the dark.

Carotenoids↗

PubMed1985

Magnetic resonance imaging of acute myocardial infarction using a nitroxyl spin label (PCA).

The effects of an intravenously administered nitroxyl spin label (PCA) on the magnetic resonance imaging (MRI) appearance and relaxation times of acute canine myocardial infarctions were studied. Twenty-four hours after ligation of the left anterior descending coronary artery (LAD), animals were either sacrificed immediately (three dogs) or injected with 3.0 mmol/kg of PCA prior to sacrifice (six dogs). The PCA group dogs were sacrificed at either 5 minutes postinjection (three dogs) or 15 minutes postinjection (three dogs). Magnetic resonance imaging (0.35 T) using spin-echo techniques demonstrated high signal intensity in the infarct relative to normal myocardium in all three groups. In the control group, the T1 and T2 relaxation times were longer in infarcted compared with normal myocardium, but only the measure in T2 reached statistical significance (P less than .05). PCA produced infarct-avid T1 shortening in the six dogs that received it. Contrast in the group sacrificed at 15 minutes postcontrast administration was greater than that in the control group due to T1 shortening in the infarct. Thus, PCA produces differential effects on normal and infarcted myocardium. Between 5 and 15 minutes after IV administration, it causes greater changes in the infarct due to prolonged retention in this region.

Animals↗

PubMed1994

A transmembrane potential does not affect the vertical location of charged lipid spin labels with respect to the surface of a phosphatidylcholine bilayer.

The effect of a transmembrane potential on the vertical location of a charged lipid in a neutral phosphatidylcholine (PC) lipid bilayer has been investigated using negatively and positively charged spin-labeled lipids. A transmembrane potential was generated across extruded large unilamellar vesicles either by using a K+/Na+ ion gradient and a K+ ionophore or by using a pH gradient. Since a transmembrane potential could have opposing effects on lipids in the inner and outer monolayer, some of the acidic spin labels were asymmetrically located in the inner monolayer as a result of a pH gradient. No significant effect on their order parameters was observed upon applying a transmembrane potential. The internal dipole potential of the bilayer was modified by using dialkyl-PC or by incorporating 10 mol% phloretin, or 6-ketocholestanol in the PC, but a transmembrane potential still had no detectable effect on the spin labeled lipids. Therefore, it is concluded that the electrochemical potential across membranes probably does not cause a significant change in the vertical location of charged lipids with respect to the surface of a PC bilayer. This suggests that polar interactions and/or van der Waals interactions between the spin probe and the surrounding lipids stabilize the overall structure of the membranes and these interactions are not disrupted by a selective effect of the transmembrane potential on the charged lipids.

Electron Spin Resonance Spectroscopy↗

PubMed1995

Determination of local protein structure by spin label difference 2D NMR: the region neighboring Asp61 of subunit c of the F1F0 ATP synthase.

Purified subunit c from the H(+)-transporting F1F0 ATP synthase of Escherichia coli folds as an antiparallel pair of extended helices in a solution of chloroform-methanol-water. A similar hairpin-like folding is predicted for the native protein in the multisubunit transmembrane Fo sector of the ATP synthase. A single Cys variant (A67C) of subunit c was created and modified with a maleimido-PROXYL [[3-(maleimidomethyl)-2,2,5,5-tetramethyl-1-pyrrolidinyl]oxy] spin label. Pairs of 1H 2D correlation and NOE spectra were collected with the nitroxide oxidized (paramagnetic) and reduced (diamagnetic). The pairs of spectra were subtracted, yielding difference spectra containing only cross-peaks from 1H within 15 A of the spin label. These greatly simplified spectra were easily analyzed to provide complete assignments for residues 10-25 and 52-79 of the protein, 150 NOE distance restraints, and 27 hydrogen-bonding restraints. The chemical shifts and NOE patterns observed in the derivatized mutant were virtually identical to those which were resolved in the unmodified wild-type protein, strongly suggesting that the spin label was not perturbing the protein structure. The restaints enabled us to calculate a detailed structure for this region of subunit c. The structure consisted of two gently curved helices, crossing at a slight (30 degrees) angle. The C-terminal helix was disrupted from Val60 to Ala62 near the essential Pro64. Asp61, the residue thought to undergo protonation--deprotonation with each H+ transported across the membrane, was in ver der Waals contact with Ala24. The proximity of these residues had been predicted from mutant analyses, where H+ translocation was retained on moving the Asp from position 61 to 24.

Escherichia coli↗

PubMedDate not supplied

Implementation of quantitative perfusion imaging techniques for functional brain mapping using pulsed arterial spin labeling.

We describe here experimental considerations in the implementation of quantitative perfusion imaging techniques for functional MRI using pulsed arterial spin labeling. Three tagging techniques: EPISTAR, PICORE, and FAIR are found to give very similar perfusion results despite large differences in static tissue contrast. Two major sources of systematic error in the perfusion measurement are identified: the transit delay from the tagging region to the imaging slice; and the inclusion of intravascular tagged signal. A modified technique called QUIPSS II is described that decreases sensitivity to these effects by explicitly controlling the time width of the tag bolus and imaging after the bolus is entirely deposited into the slice. With appropriate saturation pulses the pulse sequence can be arranged so as to allow for simultaneous collection of perfusion and BOLD data that can be cleanly separated. Such perfusion and BOLD signals reveal differences in spatial location and dynamics that may be useful both for functional brain mapping and for study of the BOLD contrast mechanism. The implementation of multislice perfusion imaging introduces additional complications, primarily in the elimination of signal from static tissue. In pulsed ASL, this appears to be related to the slice profile of the inversion tag pulse in the presence of relaxation, rather than magnetization transfer effects as in continuous arterial spin labeling, and can be alleviated with careful adjustment of inversion pulse parameters.

Arteries↗

PubMed1997

Spin-labeled extracellular loop from a seven-transmembrane helix receptor: studies in solution and interaction with model membranes.

A spin-labeled pentadecapeptide was synthesized containing 2,2,6,6-tetramethylpiperidine-N-oxyl-4-amino-4-carboxylic acid (TOAC) as the N-terminal amino acid and residues 253-266 (EYWSTFGNLHHISL) of the mass oncogene receptor, a membrane-bound protein from the G-protein coupled receptors family. According to predictions, this protein folds into seven transmembrane helices connected by three extra- and three intracellular loops, and the peptide encompasses part of the third extracellular loop and part of the seventh helix. Electron paramagnetic resonance (EPR) spectra of the spin-labeled peptide (TOAC-14) were obtained in aqueous solution as a function of pH and temperature, in a secondary structure-inducing solvent [trifluoroethanol (TFE)], and in the presence of detergent micelles and phospholipid bilayers. The charged and uncharged amino groups of TOAC and TOAC-14 yielded spectra with different isotropic hyperfine splittings (aN). The slow exchange between protonated and unprotonated forms in the EPR time scale gave rise to composite spectra weighted by the Henderson-Hasselbalch equation. Plots of aN vs pH allowed the determination of the amino group pK values (8.4 and 4.5, for TOAC and TOAC-14, respectively). A small change in aN centered at pH 6.5 was ascribed to the titration of the histidines. Values of calculated rotational correlation times were indicative of a pH-induced conformational change. A conformational change was also observed in TFE. TOAC-14 bound to micelles irrespective of peptide and detergent head group charge. In contrast, the peptide bound to phospholipid bilayers only when both carried opposite charges. The slow exchange (in the EPR time scale) between membrane-bound and free TOAC-14 allowed the calculation of the peptide's partition coefficient. The spectral line shapes were affected by aggregate size and degree of packing of the constituent molecules. It is proposed that pH, polarity, and lipid environment can affect the conformation of water-exposed regions of membrane-bound receptors, thereby playing a role in the mechanism of signal transduction.

Amino Acid Sequence↗

PubMed1987

Physical state abnormality of erythrocyte membrane in myotonic dystrophy: a spin label study.

Biophysical abnormalities of the erythrocyte membrane in muscular dystrophies have been described by numerous authors. This work presents the results we have obtained on 23 subjects suffering from myotonic muscular dystrophy (MyD, Steinert disease) by the spin label method. Two types of fatty acid spin labels were used: 5-nitroxide stearic acid (5NS) and 16-nitroxide stearic acid (16NS) which probe the membranes respectively near their polar heads and in their hydrophobic core. We measured the classical order parameter, the saturation behaviour of the electron paramagnetic resonance signal, and the label apparent rotation correlation time as a function of the temperature, on fresh and in vitro stored red blood cells. With the 5NS label, no differences were found between controls and patients. With the 16NS label, a highly significant variation in the thermic behaviour of the membrane is observed in its hydrophobic and fluid core. This last result may suggest some similarities between the red blood cell membranes of adult MyD's and healthy children.

Adolescent↗

PubMed2002

Interaction of human serum albumin with membranes containing polymer-grafted lipids: spin-label ESR studies in the mushroom and brush regimes.

The adsorption of human serum albumin (HSA) to dipalmitoyl phosphatidylcholine (DPPC) bilayer membranes containing poly(ethylene glycol)-grafted dipalmitoyl phosphatidylethanolamine (PEG-DPPE) was studied as a function of content and headgroup size of the polymer lipid. In the absence of protein, conversion from the low-density mushroom regime to the high-density brush regime of polymer-lipid content is detected by the change in ESR outer hyperfine splitting, 2A(max), of chain spin-labelled phosphatidylcholine in gel-phase membranes. The values of 2A(max) remain constant in the mushroom regime, but decrease on entering the brush regime. Conversion between the two regimes occurs at mole fractions X(PEG)(m-->b) approximately 0.04, 0.01-0.02 and 0.005-0.01 for PEG-DPPE with mean PEG molecular masses of 350, 2000 and 5000 Da, respectively, as expected theoretically. Adsorption of HSA to DPPC membranes is detected as a decrease of the spin label 2A(max) hyperfine splitting in the gel phase. Saturation is obtained at a protein/lipid ratio of ca. 1:1 w/w. In the presence of polymer-grafted lipids, HSA adsorbs to DPPC membranes only in the mushroom regime, irrespective of polymer length. In the brush regime, the spin-label values of 2A(max) are unchanged in the presence of protein. Even in the mushroom regime, protein adsorption progressively becomes strongly attenuated as a result of the steric stabilization exerted by the polymer lipid. These results are in agreement with theoretical estimates of the lateral pressure exerted by the grafted polymer in the brush and mushroom regimes, respectively.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed1980

Characterization of octapeptin-membrane interactions using spin-labeled octapeptin.

Octapeptin is a membrane-active peptide antibiotic that contains a C10 fatty acid covalently attached to the peptide through an amide bond. Interactions of octapeptin with bacterial membranes and phospholipids were characterized by using spin-labeling techniques and octapeptin derivatives containing fatty acids of varying chain length. Acyl modification of octapeptin demonstrated that the fatty acid of the antibiotic contributed to the antimicrobial activity of octapeptin and its affinity for membranes. The influence of octapeptin and C2 acyloctapeptin on the rates of ascorbate reduction of several membrane-bound doxyl stearates was also examined. These studies demonstrated that octapeptin increaed the rate of diffusion of ascorbate into the lipid bilayer and suggested that the acyl chain contributed to this activity. In addition, an acyl spin-labeled analogue of octapeptin was prepared and shown to retain biological activity. Spectral analysis showed that octapeptin does not aggregate in solution over a wide concentration range. However, the isotropic splitting constant indicated that the acyl chain of octapeptin is not completely exposed to water. It is proposed that the acyl chain of octapeptin in solution interacts with hydrophobic amino acids in the peptide, which partially shields the acyl chain from water. Spectral features of the spin-labeled antibiotic bound to phospholipid dispersions were consistent with directional binding of octapeptin to lipid bilayers with insertion of the fatty acid into the hydrocarbon domain.

Anti-Bacterial Agents↗

PubMed2006

Dynamics of the nitroxide side chain in spin-labeled proteins.

The dynamics of the tether linking methanethiosulfonate (MTSSL) spin probes to alpha-helices has been investigated with the purpose of rationalizing its effects on ESR line shapes. Torsional profiles for the chain bonds have been calculated ab initio, and steric interactions with the alpha-helix and the neighboring residues have been introduced at the excluded-volume level. As a consequence of the restrictions deriving from chain geometry and local constraints, a limited number of allowed conformers has been identified that undergo torsional oscillations and conformational jumps. Torsional fluctuations are described as damped oscillations, while transition rates between conformers are calculated according to the Langer multidimensional extension of the Kramers theory. The time scale and amplitude of the different motions are compared; the major role played by rotations of the outermost bonds of the side chain emerges, along with the effects of substituents in the pyrroline ring on the conformer distribution and dynamics. The extent and symmetry of magnetic tensor averaging produced by the side chain motions are estimated, the implications for the ESR spectra of spin-labeled proteins are discussed, and suggestions for the introduction of realistic features of the spin probe dynamics into the line shape simulation are presented.

Models, Molecular↗