Proton Relaxation Enhancement Research Articles

Ion-binding by muscular parvalbumins of the alpha phylogenetic series. A proton relaxation enhancement study with pike 5.0 parvalbumin

Ion-binding by muscular parvalbumins of the alpha phylogenetic series. A proton relaxation enhancement study with pike 5.0 parvalbumin

Information obtainable from N.M.R. longitudinal relaxation as measured by a non-selective pulse sequence in an ABC system

From proton and carbon-13 longitudinal relaxation and 13C Overhauser enhancement measurements, the diffusion constants for the molecule acrylonitrile dissolved in carbon disulphide are found to be: Dxx =(0·11±0·06) × 1012 s-1; Dyy = (0·38 ± 0·12) × 1012 s-1; Dzz = (0·25 ± 0·02) × 1012 s-1 (the z axis is perpendicular to the molecular plane; the y axis is almost parallel to the C≡N bond). The diffusion and inertial principal axis systems are shown to be nearly coincident. From these diffusion constants, the nitrogen-14 quadrupolar coupling constant was calculated to be (4·9±0·8) MHz. Notwithstanding the fact that we can study more relaxation rates due to the presence of more lines in the proton ABC spectrum, as compared to an AB 2 or AX 2 system, we obtain less information. This is a result of the fact that the unknown spectral densities occur in linear combinations in the expression for the recovery rates. A procedure to obtain the maximum available information is described.

Characterization of Mg 2+-ATPase from sheep kidney medulla: Magnetic resonance and kinetic studies

Magnesium-dependent adenosine triphosphatase, purified from sheep kidney medulla using digitonin, has been characterized in a series of kinetic and magnetic resonance studies. Kinetic studies of divalent metal activation using either Mg 2+ or Mn 2+ indicate a biphasic response to divalent cations. Apparent K m values of 23 μ m for free Mg 2+ and 3.3 μ m for free Mn 2+ are obtained at low levels of added metal, while K m values of 0.50 m m for free Mg 2+ and 0.43 m m for free Mn 2+ are obtained at much higher levels of divalent cations. In all cases the kinetic data indicate that the binding of divalent metals is independent of the substrate, ATP. Kinetic studies of the substrate requirements of the Mg 2+-ATPase also yield biphasic Lineweaver-Burk plots. At low ATP concentrations, kinetic studies yield apparent K m values for free ATP of 6.0 and 1.4 μ m with Mg 2+ and Mn 2+, respectively, as the activating divalent metals. At much higher levels of ATP the response of the enzyme to ATP changes so that K m values for free ATP of 8.0 and 2.0 m m are obtained for Mg 2+ and Mn 2+, respectively. In both cases, however, the binding of ATP is independent of added metal. ADP inhibits the Mg 2+-ATPase and the kinetic data indicate that ADP competes with ATP at both the high and low affinity sites. Dixon plots of the data are consistent with competitive inhibition at both ATP sites, with K i values of 10.5 μ m and 4.5 m m. Electron paramagnetic resonance and water proton relaxation rate studies show that the enzyme binds 1 g ion of Mn 2+ per 469,000 g of protein. The Mn 2+ binding studies yield a K D for Mn 2+ at the single high affinity site of 2 μ m, in good agreement with the kinetically determined activator constant for Mn 2+ at low Mn 2+ levels. Moreover, the EPR binding studies also indicate the existence of 34 weak sites for Mn 2+ per single high affinity Mn 2+ site. The K D for Mn 2+ at these sites is 0.55 m m, in good agreement with the kinetic activator constant for Mn 2+ of 0.43 m m, consistent with additional activation of the enzyme by the large number of weaker metal binding sites. The enhancement of water proton relaxation by Mn 2+ in the presence of the enzyme is also consistent with the tight binding of a single Mn 2+ ion per 469,000 M r protein and the weaker binding of a large number of divalent metal ions. Analysis of the data yields a value for the enhancement for bound Mn 2+ at the single tight site, ϵ b, of 5 and an enhancement at the 34 weak sites of 11. The frequency dependence of water proton relaxation by Mn 2+ at the single tight site yields a dipolar correlation time (constant from 8–60 MHz) of 3.18 × 10 −9 s. The kinetics and metal binding studies, together with the effect of temperature on ATPase activity at high and low levels of ATP, are consistent with the existence in this preparation of a single Mg 2+-ATPase, with high and low affinity sites for divalent metals and for ATP. Observations of both high and low affinities for ATP have been made with two other purified ATPases. The similarities of these systems to the Mg 2+-ATPase described here are discussed.

NMR studies of primary and secondary sites of parvalbumins using the two paramagnetic probes Gd (III) and Mn (II)

The binding of cations by parvalbumins was studied by the proton relaxation enhancement (PRE) method using the paramagnetic probes Gd(III) and Mn(II). Gd(III) appears as a specific probe of the primary sites CD and EF with the following binding parameters: n = 2, KdGd = 0.5 x 10(-11) M and epsilon b = 2.3. The low value of epsilon b is the result of a nearly complete dehydration of the protein bound ions. Competition experiments between Gd(III) and various diamagnetic cations show the following order of affinity for the EF and CD sites: Mg2+ less than Zn2+ less than Sr2+ less than Ca2+ less than Cd2+ less than La3+ less than or equal to Gd3+. Mn 2+ is a specific probe of a secondary site with the following binding parameters: n = 1, KdMn = 0.6 x 10(-3) M and epsilon b = 17. The high value of epsilon b suggests that the protein bound Mn(II) has retained most of its hydration shell. Competition experiments between (Mn(II) and different cations show similar affinities for this site: Ca2+ less than or equal to Mg2+ less than or equal to Cd2+ less than or equal to Mn2+. This secondary site is located near the EF primary site.

Proton relaxation enhancement to probe the effects of antigen binding on pig antibody structure

FEBS LettersVolume 102, Issue 2 p. 249-252 Full-length articleFree Access Proton relaxation enhancement to probe the effects of antigen binding on pig antibody structure Dennis R. Burton, Dennis R. Burton Physical Chemistry 2, Chemical Centre, University of Lund, Lund, SwedenSearch for more papers by this authorSture Forsén, Sture Forsén Physical Chemistry 2, Chemical Centre, University of Lund, Lund, SwedenSearch for more papers by this authorFrantišek Franěk, František Franěk Physical Chemistry 2, Chemical Centre, University of Lund, Lund, Sweden Institute of Molecular Genetics, Czechoslovak Academy of Sciences, Prague, CzechoslovakiaSearch for more papers by this authorJiří Novotný, Jiří Novotný Physical Chemistry 2, Chemical Centre, University of Lund, Lund, Sweden Institute of Molecular Genetics, Czechoslovak Academy of Sciences, Prague, CzechoslovakiaSearch for more papers by this author Dennis R. Burton, Dennis R. Burton Physical Chemistry 2, Chemical Centre, University of Lund, Lund, SwedenSearch for more papers by this authorSture Forsén, Sture Forsén Physical Chemistry 2, Chemical Centre, University of Lund, Lund, SwedenSearch for more papers by this authorFrantišek Franěk, František Franěk Physical Chemistry 2, Chemical Centre, University of Lund, Lund, Sweden Institute of Molecular Genetics, Czechoslovak Academy of Sciences, Prague, CzechoslovakiaSearch for more papers by this authorJiří Novotný, Jiří Novotný Physical Chemistry 2, Chemical Centre, University of Lund, Lund, Sweden Institute of Molecular Genetics, Czechoslovak Academy of Sciences, Prague, CzechoslovakiaSearch for more papers by this author First published: June 15, 1979 https://doi.org/10.1016/0014-5793(79)80011-3Citations: 3AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat References 1 Metzger, H. (1979) Contemp. Top. Mol. Immunol. in press. 2 R.A. Dwek, Nuclear Magnetic Resonance in Biochemistry (1973), Clarendon Press Oxford 3 S.K. Dower, R.A. Dwek, A.C. McLaughlin, L.E. Mole, E.M. Press, C.A. Sunderland, Biochem. J., 149, (1975), 73– 82. 4 D.R. Burton, S. Forsén, G. Karlström, R.A. Dwek, A.C. McLaughlin, S. Wain-Hobson, Eur. J. Biochem., 71, (1976), 519– 528. 5 D.R. Burton, R.A. Dwek, S. Forsén, G. Karlström, Biochemistry, 16, (1977), 250– 254. 6 D.R. Burton, R.A. Dwek, S. Forsén, G. Karlström, Progress in Nuclear Magnetic Resonance Spectroscopy (1979), Pergamon Press Oxford in press 7 F. Franěk, J. Doskočil, L. Sǐmek, Immunochemistry, 11, (1974), 803– 809. 8 J. Novotný, F. Franék, Abst. 4th Eur. Immunol. Meet. Budapest, (1978), 96– 9 L. Cser, F. Franěk, R.A. Gladkikh, R.S. Nezlin, J. Novotný, Yu.M. Ostanevich, FEBS Lett., 93, (1978), 312– 316. 10 C. Desaymard, M. Feldman, Eur. J. Immunol., 5, (1975), 537– 541. Citing Literature Volume102, Issue2June 15, 1979Pages 249-252 ReferencesRelatedInformation

Magnetic resonance studies of the binding of ATP and cations to human hemoglobin.

Reversible interactions of ATP, MgATP, and three cations (Mg”‘, Mn”+, and Zn’+) with human oxyand deoxyhemoglobins have been studied at physiological pH and salt concentration by 31P NMR, EPR, and water proton relaxation techniques. In the absence of hemoglobin a value of 45 f 8 pM was obtained for the dissociation constant of MgATP from the 31P chemical shifts of solutions containing sufficiently low levels of Mg2+ and ATP (0.5 to 5 mM) in the presence of 0.15 M KC1 (pH 7.2, T = 25°C). Although hemoglobin does not change the positions of the 31P NMR peaks of ATP or its Mg”+ complex at pH 7.2, the spectra do provide evidence of interactions under aerobic and anaerobic conditions. The presence of deoxyhemoglobin was found to result in appreciable displacement of the apparent equilibrium, Mg”+ + ATP4+ MgATP2-, as measured by phosphorus chemical shifts in favor of ATP thus showing competition between deoxyhemoglobin and Mg2+ for binding ATP. The presence of oxyhemoglobin produces only small changes in the apparent equilibrium as measured by ‘I’P NMR. Both oxyand deoxyhemoglobin significantly broaden the 31P resonances of ATP. A larger broadening of the 31P resonances is observed in the deoxyhemoglobin solutions compared to that in the oxyhemoglobin solutions under similar conditions and reflects the higher affinity of ATP for deoxyhemoglobin. While the affinities of ATP and MgATP for deoxyhemoglobin differ by more than an order of magnitude, our data indicate that their affinities for oxyhemoglobin are comparable. The NMR spectral data are generally consistent with the dissociation constants of Hamasaki and Rose ((1974) J. Biol. Chem. 249,7896). The binding of Mg’+ to human hemoglobin was examined by its effects on the EPR spectra and water proton relaxation enhancement (eg = 9.5) of a solution containing Mn2+ and hemoglobin. It is found that Mg’+ shows little affinity (KDM” = 0.4 -+ 0.1 M) for the four divalent cation sites per tetramer of oxyor deoxyhemoglobin detected here whereas Mn2+ and Zn2+ bind to an appreciable extent (Kn”” = 5 +1 mM; KD~” = 1.3 + 0.4 mM). These results argue against any significant binding of Mg2+ to hemoglobin in red blood cells.

The determination of molecular-motion parameters from proton-relaxation-enhancement measurements in a number of Gd(III) - antibody-fragment complexes. A comparative study.

Longitudinal and transverse proton relaxation rates for water in the hydration spheres of Gd(III) bound to the non-immune rabbit IgG fragments Fc (C-terminal half of heavy-chain dimer), pFc' (C-terminal quarter of heavy-chain dimer) and Fab (N-terminal half of heavy and light chain) have been measured at a number of frequencies and temperatures using pulsed nuclear magnetic resonance spectrometry. For the fragments Fc and pFc', a full computer analysis showed that the results could be fitted by parameters of similar magnitude to those found previously for IgG. In contrast to the results for the other complexes the Fab -Gd(III) complex showed no slow exchange contribution to the relaxation rates. Under these circumstances it was found possible to obtain an accurate value for the hydration number (q) from measurements of the longitudinal and transverse relaxation rates at a chosen frequency such that the product of the nuclear Larmor frequency (omega1) and the correlation time for the dipolar relaxation processes (tauc) was approximately unity. Water-proton relaxation rates were also determined for the complex of Gd(III) with the Fv fragment of the mouse myeloma protein MOPC 315. A computer analysis of the results revealed a slow exchange contribution to the rates and this gave errors in the variable parameters similar to those observed previously for IgG, Fc and pFc'. The conclusions drawn from the different systems are discussed in terms of the present state of application of the proton relaxation enhancement technique in biology.

A novel approach to water proton relaxation in paramagnetic ion-macromolecule complexes.

The use of the proton relaxation enhancement (PRE) technique in solutions of macromolecules and paramagnetic ions to obtain information on molecular motion and ion-hydration numbers can be hazardous with the possibility of large errors in the derived parameters. Here we describe a method based on solvent proton-deuteron comparative relaxation measurements at a limited number of different magnetic fields for obtaining, reasonably accurately, rapidly, and unambiguously, certain of the parameters normally obtained from a PRE study. In the non-immune rabbit immunoglobulin G(IgG)-Gd(iii) system, we are able to obtain values for the ion hydration number (q), the residence lifetime of water molecules in the hydration sphere of the ion (tauM) and a correlation time of 5 ns, which can be associated to a large part with a rotational correlation time (tauR) for the Fc portion (C-terminal half of heavy-chain dimer) of the IgG molecule. This shows that there is a rapid internal motion in the Fc region. The limitations and possible applications of the method are discussed.

Difficulties in Determining Accurate Molecular Motion Parameters from Proton Relaxation Enhancement Measurements as Illustrated by the Immunoglobulin G · Gd(III) System

Longitudinal and traverse proton magnetic relaxation rates for water in the hydration sphere of Gd(III) bound to non-immune rabbit IgG (immunoglobulin G) have been determined over a wide range of frequencies (4-84 MHz) at constant temperature (19 degrees C) using pulsed nuclear magnetic resonance spectrometry. The rates have also been determined at temperatures between 0 and 40 degrees C for two frequencies (61 and 84 MHz). The rates were fitted to existing theory using a computer least-squares procedure. Further computer analysis was then carried out to determine the sensitivity of the best-fit error to variation in the variable parameters in the theoretical expressions used. These include the water co-ordination number (q) for which it was found large variations could occur (between approximately 2 and 8) for only small changes in the error of best-fit. It was concluded that a slow exchange contribution to the relaxation rates was important in deciding which parameters are poorly determined. A rotational correlation time (tau r) was obtained which suggested there might be considerable internal motion of the Fc region (C-terminal half of heavy-chain dimer) of the IgG molecule. However the possibility of large errors in this value prevented unequivocal conclusions being drawn.

Proton and phosphorus‐31 magnetic relaxation studies on the interaction of polyriboadenylic acid with Mn2+

AbstractProton and phosphorus‐31 nuclear spin–lattice relaxation times T1 and spin–spin relaxation times T2 have been measured on the single‐stranded polyriboadenylic acid [poly(A)]–Mn2+ system in a neutral D2O solution in the temperature range 10°–90°C at 100 and 40.5 MHz, respectively, with the Fourier transform nmr method. Minimum values of T1 have been found for all these nuclei, which have enabled the exact estimation of apparent distances from Mn2+ to H2, H8, H1′, and the phosphorus nucleus to be 4.7, 4.1, 5.2, and 3.0 Å, respectively. The electron spin of Mn2+ penetrates into the phosphorus nucleus, giving 31P hyperfine coupling of more than 106 Hz. Evidence of penetration of the electron spin into H8 and H2 is also obtained, suggesting direct coordination of nitrogen atoms of the adenine ring to the Mn2+ Ion. Combined with the result from proton relaxation enhancement of water, it is concluded that every Mn2+ ion added is bound directly to two phosphate groups with a Mn2+–phosphorus distance of 3.3 Å, while a part of the Mn2+ ions are simultaneouly bound to the adenine ring. It is estimated that 39 ± 13% and 13 ± 5% of Mn2+ are coordinated by N7 and N3 (or N1), respectively. The motional freedom of poly(A) in the environment of the Mn2+ binding site has been found to be quenched to the extent that the rotational motion becomes several times slower than that of the corresponding Mn2+–free poly(A). The activation energies for the molecular motion are, however, practically unchanged from those for Mn2+–free poly(A), and are found to be 8.3, 8.5, 6.1, and 8.7 kcal/mol for H8, H2, H1′, and phosphorus, respectively. T2 of phosphorus is determined by the dissociation rate (k−1) of Mn2+ from the phosphate group for the whole temperature range studied with activation enthalpy of 6.5 kcal/mol. The dissociation rates of Mn2+ from the adenine ring are also estimated from proton T2 values below 50°C.

A proton-relaxation enhancement study of the interaction of manganous ions with phospholipids in aqueous dispersions

An interaction of dipalmitoylphosphatidylcholine (PC) and phosphatidylserine (PS) with manganous ions has been investigated by measuring the effect of bound manganese upon the longitudinal relaxation rate. 1/ T 1, of the solvent water protons and evaluating the enhancement factor ϵ b. The observed enhancement values were used to determine the number of interacting sites per polar head group, n. and the values of association constants, K A, of manganese to PC and PS. Changes in ϵ b correlate with structural changes at the interacting site. By increasing the temperature one can see an abrupt decrease in ϵ b within the temperature interval from 40 to 50° C indicating the thermal phase transition of PC as established by calorimetry, fluorescence and high-resolution NMR measurements. That an enhancement of 1/ T 1 of the solvent-watet protons occurs at all is explained by assuming a restricted rotation of the Mn 2+—aquo complex in the bound state. In addition we suppose that the rotation of the Mn 2+-aquo complex is the mechanism which dominates the relaxation of the waier protons in the bulk solvent when phospholipids are present.

NMR study of -17-O from H2-17-O in human erythrocytes.

Human erythrocytes were incubated in a Ringer's solution enriched with 10--18 per cent H2-17-O. The longitudinal relaxation time (T1) of the -17-O was determined separately in samples of red cell suspensions, packed cells, and supernatant. The longitudinal relaxation of -17-O in erythrocyte suspensions was non-exponential, reflecting water exchange across the cell membranes as well as relaxation processes inside and outside the cell. The T1 of intracellular -17-O is 4--5 times shorter than in the supernatant, similar to the enhancement of proton relaxation by hemoglobin in erythrocytes and free solution at the frequency applied (8.13 MHz). This datum is consistent with tht thesis that hemoglovin modifies the NMR relaxation behavior of water inside cells and in free solution in the same way. The rate constant (kx) for water exchange was calculated to be 60 and 107 s- minus 1 at 25 and at 37 degrees C, respectively. The apparent activation energy for kx over the temperature range 23--37 degrees C was 8.7 plus or minus 1.0 kcal/mole.

Magnetic resonance studies of concanavalin A:CONFORMATIONAL CHANGES INDUCED BY Ca2+ and alpha-methyl-D-mannopyranoside.

Three independent solution spectroscopic techniques (solvent proton relaxation enhancement, circular dichroism, and high resolution 220 MHz proton magnetic resonance spectroscopy) have been utilized to demonstrate mental ion- and monosaccharide inhibitor-induced structural perturbations for the dimeric form of the plant lectin concanavalin A (Con A). The results indicate that (i) the occupation of the transition metal ion site S1 by Mn-2+ or Zn-2+ does not detectably perturn the demetallized protein conformation, (ii) the binding of Ca=2+ to the Con A-Mn-2+ or Con A-Zn-2+ complexes perturbs the protein structure in the vicinity of the S1 site as well as at points remote from the S1-S2 double ion site, and (iii) the binding of the monosaccharide inhibitor alpha-methyl-D-mannopyranoside to the fully metallized Con A complex also significantly perturbs the structural features of the protein. A detailed radio frequency dependence analysis of the Ca-2+ effect on the solvent proton relaxation enhancement properties of the Con A-Mn-2+ complex indicates that the considerable reduction in the observed enhancement upon Ca-2+ binding principally results from an approximate 120-fold decrease in the single Mn-2+ water of hydratio- exchange rate; The 220 MHz proton magnetic resonance spectra for Con A indicate that this form of spectroscopy is the most useful of those utilized in detailing the solution structural features of this lectin, and a tentative assignment for the C-2-H proton of histidine residue 24 (the S1 site ligand) has been proposed.

Proton relaxation rate and fluorometric studies of manganese and rare earth binding to concanavalin A

The binding of Mn 2+, Ca 2+, and rare earth ions to apoconcanavalin A has been studied by water proton relaxation enhancement, electron paramagnetic resonance spectroscopy, and fluorescence spectroscopy. An electron paramagnetic resonance and water proton relaxation rate study of the titration of apoconcanavalin A with Mn 2+ gives evidence of two equivalent binding sites per monomer with K D = 50 μm ± 4 μm. When a similar Mn 2+ titration of apoconcanavalin A is performed in the presence of Ca 2+ ion, very little free Mn 2+ is detected by electron paramagnetic resonance until the two Mn 2+ binding sites per monomer are filled. The substitution of a rare earth ion for Ca 2+ ion in the above experiment often resulted in a slight displacement of Mn 2+ from the transition metal site as detected by electron paramagnetic resonance. A water proton relaxation rate study of the titration of apoconcanavalin A with Gd 3+ reflects two binding sites with a K D = 40 μm ± 4 μm and two with a K D = 200 μm ± 50 μm. The fluorescence emission spectrum of concanavalin A (λ em = 340 nm) is slightly quenched by the addition of Tb 3+ while Tb 3+ fluorescence is greatly enhanced. A fluorometric titration of apoconcanavalin A with Tb 3+ also reflects two sites with a K D = 40 μm ± 15 μm and two with a K D = 270 μm ± 50 μm.

Probes for the conformational transitions of phosphorylase . Effect of ligands studied by proton-relaxation enhancement, and chemical reactivities.

1 The properties of rabbit muscle phosphorylase a have been studied by proton relaxation enhancement and chemical reactivity. 2 The response of this enzyme to temperature changes and to binding of ligands (AMP and glucose 1-phosphate) has been observed. 3 There is a single Mn2+-binding site per protomer of phosphorylase a having a dissociation constant, Kd of 135 μM at 20°C. All other sites are at least 10 times weaker. 4 The limiting proton relaxation enhancement factor for phosphorylase a is 18.4. This is not significantly altered by AMP, but changes to 12.3 on saturation with glucose 1-phosphate. 5 The variation in proton relaxation enhancement parameters with temperature shows an inflection at around 15°C, suggesting the occurrence of a conformational change in the enzyme at this temperature. 6 Two groups per protomer react with the specific SH reagent 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, the loss of activity corresponding to the reaction of the slower group. 7 AMP-binding decreases the rate of reaction of the slower SH group, while glucose 1-phosphate increases this rate. That of the faster group is unaffected. 8 These properties and their relationship to the corresponding properties of phosphorylase b reported previously are used to relate phosphorylase a conformations to those of phosphorylase b, and to refine the earlier conclusions about the conformational states observed in muscle phosphorylase.

Proton-relaxation-enhancement studies on the binding to yeast pyruvate kinase of a substrate and effectors.

The binding to yeast pyruvate base of Mn2+ has been determined from the enhancement of the proton relaxation rate which it causes. There were further changes in this enhancement in the presence of a substrate (phosphoenolpyruvate) and the effector (fructose diphosphate) and, from these changes, information was derived about their binding. Both strong and weak binding sites for Mn2+ on the enzyme were detected. The affinity of the enzyme for phosphoenolpyruvate was increased when the fructose diphosphate · Mn complex was bound to the enzyme. The proton relaxation enhancement results indicated that there were not more than two sites for fructose diphosphate · Mn, and two sites for phosphoenolpyruvate in the presence of the fructose diphosphate · Mn complex, but that there may have been more than two sites for phosphoenolpyruvate in the presence of Mn2+ only.

The lanthanide cations as probes in biological systems. Proton relaxation enhancement studies for model systems and lysozyme.

A study has been made over a range of temperatures at 20 MHz and 35 MHz of the spin relaxation rates of water protons in aqueous solutions of ethylene glycol bis-(aminoethyl)-tetraacetic acid containing Gd(III) and Eu(II) ions. The results suggest that the dominant relaxation process arises from rotation of the solvated complexes. Relaxation enhancement has been observed for Gd(III) in the presence of lysozyme and values of ɛ*, Kd and τc have been determined. An application of these results to obtain distances of substrate analogues from the active site of this and other enzymes is discussed.

Proton magnetic relaxation in solutions ofE. coliribosomal RNA containing Mn2+ions

A study has been made over a range of temperatures and magnetic field strengths of the spin relaxation of water protons in aqueous solutions of E. coli ribosomal RNA containing Mn2+ ions. The effects of the paramagnetic ions are enhanced in the presence of the RNA. As the temperature falls T 1 passes through a minimum value, the magnitude of which is field dependent, and this is attributed to a change in dipolar relaxation mechanism from rotation of the aquocomplex to electron spin relaxation. The relevance of this work is assessed in relation to other work on proton relaxation enhancement in Mn2+-containing solutions of biopolymers.