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1.
A fifteen minute incubation of spinach chloroplasts with the divalent Ca 2+ chelator, EGTA, in concentrations 50–250 μM, inhibits electron transport through both photosystems. All photosystem II partial reactions, including indophenol, ferricyanide and the DCMU-insensitive silicomolybdate reduction are inhibited from 70–100%. The photosystem II donor reaction, diphenyl carbazide → indophenol, is also inhibited, indicating that the inhibition site comes after the Mn 2+ site, and that the first Ca 2+ effect noted (site II) is not on the water oxidation enzyme, as is commonly assumed, but between the Mn 2+ site and plastoquinone A pool. The other photosystem II effect of EGTA (Ca 2+ site I), occurs in the region between plastoquinone A and P700 in the electron transport chain of chloroplasts. About 50% inhibition of the reaction ascorbate + TMPD → methyl viologen is given by incubation with 200 μM EGTA for 15 min. Ca 2+ site II activity can be restored with 20 mM CaCl 2. Ca 2+ site I responds to Ca 2+ and plastocyanin added jointly. More than 90% activity in the ascorbate + TMPD → methylviologen reaction can be restored. Various ways in which Ca 2+ ions could affect chloroplast structure and function are discussed. Since EGTA is more likely to penetrate chloroplast membranes than EDTA, which is known to remove CF 1, the coupling factor, from chloroplast membranes, and since Mg 2+ ions are ineffective in restoring activity, it is concluded that Ca 2+ may function in the electron transport chain of chloroplasts in a hitherto unsuspected manner. 相似文献
2.
The functional connection between redox component Y
z
identified as Tyr-161 of polypeptide D-1 (Debus et al. 1988) and P680 + was analyzed by measurements of laser flash induced absorption changes at 830 nm in PS II membrane fragments from spinach. It was found that neither DCMU nor the ADRY agent 2-(3-chloro-4-trifluoromethyl) anilino-3,5-dinitrothiophene (ANT 2p) affects the rate of P680 + reduction by Y
z
under conditions where the catalytic site of water oxidation stays in the redox state S 1. In contrast to that, a drastic retardation is observed after mild trypsin treatment at pH=6.0. This effect which is stimualted by flash illumination can be largely reversed by Ca 2+. The above mentioned data lead to the following conclusions: (a) the segment of polypeptide D-1 containing Tyr-161 and coordination sites of P680 is not allosterically affected by structural changes due to DCMU binding at the Q B-site which is also located in D-1. (b) ANT 2p as a strong protonophoric uncoupler and ADRY agent does not modify the reaction coordinate of P680 + reduction by Y
z
, and (c) Ca 2+ could play a functional role for the electronic and vibrational coupling between the redox groups Y
z
and P680. The electron transport from Y
z
to P680 + is discussed within the framework of a nonadiabatic process. Based on thermodynamic considerations the reorganization energy is estimated to be in the order of 0.5 V.Abbreviations ADRY
acceleration of the deactivation reactions of the water splitting enzyme system Y
- ANT 2p
2-(3-chloro-4-trifluoromethyl)anilino-3,5 dinitrothiophene
- DCMU
3-(3,4-dichlorophenyl)-1,1-dimethylurea
- MES
2[N-Morpholino]ethanesulfonic acid
- PS II
photosystem II
- Q A, Q B
primary and secondary plastoquinone acceptor of photosystem II
- S
i
redox states of the catalytic site of water oxidation
- Y
z
redox active Tyr-161 of polypeptide D-1 相似文献
3.
Depletion of Ca 2+ from Anacystis nidulans produces an inhibition of O 2 evolution that is accompanied both at 39°C and 77 K by a loss of chlorophyll fluorescence of variable yield. This indicates that Ca 2+-depletion causes disruption of normal photosystem II function, manifested by the disappearance of photoreduction of Q. Delayed light emission in the ms time range is also eliminated in Ca 2+-depleted cells, which confirms that Ca 2+ removal prevents charge separation and recombination in reaction centers of photosystem II. Readdition of Ca 2+ to depleted cells restores fully the fluorescence of variable yield and delayed light emission, as well as O 2 evolution. Thus, Ca 2+ may be a required component for photosystem II in A. nidulans. 相似文献
4.
After acid-treatment of spinach ( Spinacia oleracea) chloroplasts, various partial electron transport reactions are inactivated from 25 to 75%. Divalent cations in concentrations from 10 to 50 millimolar can partially restore electron transport rates. Two cation-specific sites have been found in photosystem II: one on the 3-(3,4-dichlorophenyl)-1, 1-dimethylurea-insensitive silicomolybdate pathway, which responds better to restoration by Mg 2+ than by Ca 2+ ions, the other on the forward pathway to photosystem I, located on the 2,5-dimethylbenzoquinone pathway. This site is selectively restored by Ca 2+ ions. When protonated chloroplasts are treated with N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)aziridine, a carboxyl group modifying reagent, presumed to react with glutamic and aspartic acid residues of proteins, restoration of electron transport at the Ca 2+-selective site on the 2,5-dimethylbenzoquinone pathway is impaired, while no difference in restoration is seen at the Mg 2+ site on the 3-(3,4-dichlorophenyl)-1,1-dimethylurea-insensitive silicomolybdate pathway. Trypsin treatment of chloroplasts modifies the light-harvesting pigment-protein complex, destroys the dibromothymoquinone-insensitive 2,5-dimethyl-benzoquinone reduction, but does not interfere with the partial restoration of activity of this pathway by Ca2+ ions, implying that the selective Ca2+ effect on photosystem II (selective Ca2+ site) is different from its effects as a divalent cation on the light-harvesting pigment-protein complex involved in the excitation energy distribution between the two photosystems. 相似文献
5.
The photosystem II (PSII) manganese-stabilizing protein (PsbO) is known to be the essential PSII extrinsic subunit for stabilization
and retention of the Mn and Cl − cofactors in the oxygen evolving complex (OEC) of PSII, but its function relative to Ca 2+ is less clear. To obtain a better insight into the relationship, if any, between PsbO and Ca 2+ binding in the OEC, samples with altered PsbO-PSII binding properties were probed for their potential to promote the ability
of Ca 2+ to protect the Mn cluster against dark-inhibition by an exogenous artificial reductant, N, N-dimethylhydroxylamine. In the absence of the PsbP and PsbQ extrinsic subunits, Ca 2+ and its surrogates (Sr 2+, Cd 2+) shield Mn atoms from inhibitory reduction (Kuntzleman et al., Phys Chem Chem Phys 6:4897, 2004). The results presented here show that PsbO exhibits a positive effect on Ca 2+ binding in the OEC by facilitating the ability of the metal to prevent inhibition of activity by the reductant. The data
presented here suggest that PsbO may have a role in the formation of the OEC-associated Ca 2+ binding site by promoting the equilibrium between bound and free Ca 2+ that favors the bound metal. 相似文献
6.
Phospholamban (PLB) inhibits the activity of SERCA2a, the Ca 2+-ATPase in cardiac sarcoplasmic reticulum, by decreasing the apparent affinity of the enzyme for Ca 2+. Recent cross-linking studies have suggested that PLB binding and Ca 2+ binding to SERCA2a are mutually exclusive. PLB binds to the E2 conformation of the Ca 2+-ATPase, preventing formation of E1, the conformation that binds two Ca 2+ (at sites I and II) with high affinity and is required for ATP hydrolysis. Here we determined whether Ca 2+ binding to site I, site II, or both sites is sufficient to dissociate PLB from the Ca 2+ pump. Seven SERCA2a mutants with amino acid substitutions at Ca 2+-binding site I (E770Q, T798A, and E907Q), site II (E309Q and N795A), or both sites (D799N and E309Q/E770Q) were made, and the effects of Ca 2+ on N30C-PLB cross-linking to Lys 328 of SERCA2a were measured. In agreement with earlier reports with the skeletal muscle Ca 2+-ATPase, none of the SERCA2a mutants (except E907Q) hydrolyzed ATP in the presence of Ca 2+; however, all were phosphorylatable by P i to form E2P. Ca 2+ inhibition of E2P formation was observed only in SERCA2a mutants retaining site I. In cross-linking assays, strong cross-linking between N30C-PLB and each Ca 2+-ATPase mutant was observed in the absence of Ca 2+. Importantly, however, micromolar Ca 2+ inhibited PLB cross-linking only to mutants retaining a functional Ca 2+-binding site I. The dynamic equilibrium between Ca 2+ pumps and N30C-PLB was retained by all mutants, demonstrating normal regulation of cross-linking by ATP, thapsigargin, and anti-PLB antibody. From these results we conclude that site I is the key Ca 2+-binding site regulating the physical association between PLB and SERCA2a. 相似文献
7.
Fe(II) cations bind with high efficiency and specificity at the high-affinity (HA), Mn-binding site (termed the “blocking effect” since Fe blocks further electron donation to the site) of the oxygen-evolving complex (OEC) in Mn-depleted, photosystem II (PSII) membrane fragments (Semin et al. in Biochemistry 41:5854, 2002). Furthermore, Fe(II) cations can substitute for 1 or 2Mn cations (pH dependent) in Ca-depleted PSII membranes (Semin et al. in Journal of Bioenergetics and Biomembranes 48:227, 2016; Semin et al. in Journal of Photochemistry and Photobiology B 178:192, 2018). In the current study, we examined the effect of Ca2+ cations on the interaction of Fe(II) ions with Mn-depleted [PSII(-Mn)] and Ca-depleted [PSII(-Ca)] photosystem II membranes. We found that Ca2+ cations (about 50 mM) inhibit the light-dependent oxidation of Fe(II) (5 µM) by about 25% in PSII(-Mn) membranes, whereas inhibition of the blocking process is greater at about 40%. Blocking of the HA site by Fe cations also decreases the rate of charge recombination between QA? and YZ?+ from t1/2?=?30 ms to 46 ms. However, Ca2+ does not affect the rate during the blocking process. An Fe(II) cation (20 µM) replaces 1Mn cation in the Mn4CaO5 catalytic cluster of PSII(-Ca) membranes at pH 5.7 but 2 Mn cations at pH 6.5. In the presence of Ca2+ (10 mM) during the substitution process, Fe(II) is not able to extract Mn at pH 5.7 and extracts only 1Mn at pH 6.5 (instead of two without Ca2+). Measurements of fluorescence induction kinetics support these observations. Inhibition of Mn substitution with Fe(II) cations in the OEC only occurs with Ca2+ and Sr2+ cations, which are also able to restore oxygen evolution in PSII(-Ca) samples. Nonactive cations like La3+, Ni2+, Cd2+, and Mg2+ have no influence on the replacement of Mn with Fe. These results show that the location and/or ligand composition of one Mn cation in the Mn4CaO5 cluster is strongly affected by calcium depletion or rebinding and that bound calcium affects the redox potential of the extractable Mn4 cation in the OEC, making it resistant to reduction. 相似文献
8.
A new hypothesis describing the role of the redox inactive Ca 2+ ion in the expression of physiological oxidative damage is described. The hypothesis is based on the optimization of the chelation characteristics of iron complexes for pro-oxidant activity. In a previous investigation it was found that an excess of ligand kinetically hindered the Fenton reaction activity of the Fe II/IIIEDTA complex (Bobier et al. 2003). EDTA, citrate, NTA, and glutamate were selected as models for the coordination sites likely encountered by mobile iron, i.e. proteins. The optimal [EDTA]:[Fe III] ratio for Fenton reaction activity as measured by electrocatalytic voltammetry in a solution was found to be 1:1. An excess of EDTA in the amount of 10:1 [ligand]:[metal] suppresses the Fenton reaction activity to nearly the control. It is expected that the physiological coordination characteristics of mobile Fe would have a very large excess of [ligand]:[metal] and thus not be optimized for the Fenton reaction. Introduction of Ca 2+ in to a ratio of 10:10:1 [Ca 2+]:[EDTA]:[Fe III] to the system reinvigorated the Fenton reaction activity to nearly the value of the optimal 1:1 [EDTA]:[Fe III] complex. The pH distribution diagrams of Ca 2+ in the presence of EDTA and Fe II/III indicate that Ca 2+ has the ability to uptake excess EDTA without displacing either Fe II of Fe III from their respective complexed forms. The similarity in the presence for hard ligand sites albeit with a lower binding constant for Ca 2+ accounts for this action. 相似文献
9.
Heart sarcolemma has been shown to possess three catalytic sites (I, II and III) for methyl transferase activity (Panagia V, Ganguly PK and Dhalla NS. Biochim Biophys Acta 792: 245–253, 1984). In this study we examined the effect of phosphatidylethanolamine N-methylation on ATP-independent Ca 2+ binding and ATPase activities in isolated rat heart sarcolemma. Both low affinity (1.25 mM Ca 2+) and high affinity (50 µM Ca 2+) Ca 2+ binding activities were decreased following incubation of sarcolemmal membranes with AdoMet under optimal conditions for site II and III. Similarly, Ca 2+ ATPase activities measured at 1.25 mM and 4 mM Ca 2+ were depressed by phospholipid N-methylation. S-adenosyl homocysteine, a specific inhibitor of phospholipid N-methylation, prevented the depression of low affinity Ca 2+ binding and Ca 2+ ATPase activities, whereas the methylation-induced effect on the high affinity Ca 2+ binding was not influenced by this agent. Pretreatment of sarcolemma with methyl acetimidate hydrochloride, an amino group blocking agent, also prevented the methylation-induced inhibition of both Ca 2+ binding and Ca 2+ ATPase. A further decrease in Ca 2+ binding and Ca 2+ ATPase activities together with a marked increase in the intramembranal level of PC was seen when membranes were methylated under the site III conditions in the presence of phosphatidyldimethylethanolamine as exogenous substrate. There was no effect of phospholipid methylation on sarcolemmal Na +-K + ATPase and Mg 2+ ATPase activities. These results indicate a role of phospholipid N-methylation in the regulation of sarcolemmal Ca 2+ ATPase and low affinity ATP-independent Ca 2+ binding. 相似文献
10.
The O 2-evolution deficient mutant (LF-1) of Scenedesmus obliquus inserts an unprocessed D1 protein into the thylakoid membrane and binds less than half the wild type (WT) level of Mn. LF-1 photosystem II (PS II) membrane fragments lack that part of the high-affinity Mn 2+-binding site found in WT membranes which may be associated with histidine residues on the D1 protein (Seibert et al. 1989 Biochim Biophys Acta 974: 185–191). Hsu et al. (1987 Biochim Biophys Acta 890: 89–96) purport that the high-affinity site (characterized by competitive inhibition of DPC-supported DCIP photoreduction by M concentrations of Mn 2+) in Mn-extracted PS II membranes is also the binding site for Mn functional in O 2 evolution. Proteases (papain, subtilisin, and carboxypeptidase A) can be used to regenerate the high-affinity Mn 2+-binding site in LF-1 PS II membranes but not in thylakoids. Experiments with the histidine modifier, DEPC, suggest that the regenerated high-affinity Mn 2+-binding sites produced by either subtilisin or carboxypeptidase A treatments were the same sites observed in WT membranes. However, none of the protease treatments produced LF-1 PS II membranes that could be photoactivated. Reassessment of the processing studies of Taylor et al. (1988 FEBS Lett 237: 229–233) lead us to believe that their procedure also does not result in substantial photoactivation of LF-1 PS II membranes. We conclude that (1) the unprocessed carboxyl end of the D1 protein in LF-1 is located on the lumenal side of the PS II membrane, (2) the unprocessed fragment physically obstructs or perturbs that part of the high-affinity Mn 2+-binding site undetectable in LF-1, and (3) the D1 protein must be processed at the time of insertion into the membrane for normal O 2-evolution function to result.Abbreviations Chl
chlorophyll
- DCBQ
2,6-dichloro-1,4-benzoquinone
- DCIP
2,6-dichlorophenol indophenol
- DEPC
diethylpryocarbonate
- DPC
1,5-diphenylcarbazide
- HEPES
4-(2-hydroxyethyl)-1-piperazine-ethanesulfonic acid
- LDS-PAGE
lithium dodecylsulfate polyacrylamide gel electrophoresis
- LF-1
a low-fluorescent mutant of Scenedesmus obliquus
- MES
4-morpholineethanesulfonic acid
- PS II
photosystem II
- PMSF
phenylmethylsulfonyl fluoride
- RC
photosystem II reaction center
- Tris
tris(hydroxymethyl)aminomethane
- WT
wild type
Operated by the Midwest Research Institute for the U.S. Department of Energy under contract DE-AC-02-83CH10093. 相似文献
11.
Anacystis nidulans exhibits a total loss of photosystem II (PSII) activity upon incubation in a nutrient medium deficient in Ca 2+ and Na + and containing a divalent cation chelator. This loss of activity is light-dependent, which corresponds to an energy requirement. Likewise, Ca 2+ efflux takes place only in cells incubated in light. The loss of PSII activity is reversible by addition of submillimolar amounts of either Ca 2+ or Na + to the external medium but not by the addition of any other cation. Restoration of lost PSII activity also requires light. Light saturation curves for partially depleted cells demonstrate both lower maximum O 2 evolution rates and decreased relative quantum yields when compared to control cells. Partial electron transport reactions isolate the site of the Ca 2+/Na + effect to the reaction center itself or immediately on its oxidizing side and exclude the water-splitting complex. O 2 flash yields decline during cation depletion, indicating a decrease in the number of functional PSII reaction centers, but the maximum turnover rate for still functional reaction centers does not decline. Thus, PSII of A. nidulans exhibits an all-or-none cation requirement, satisfied only by Ca 2+ or Na +. 相似文献
13.
The minimal requirements and characteristics of epididymal sperm binding to the zona pellucida of the mouse egg were investigated using a new stop-fix centrifugation technique. This assay provided a precise physical definition of the association between the spermatozoon and the zona and permitted quantitation of the binding reaction at short time intervals. The results demonstrated that Ca 2+ is an essential physiological component required for binding to occur. Sperm preincubated for 60 min in a simplified medium lacking Ca 2+ did not acquire the ability to bind to eggs. In contrast, if sperm preincubation occurred in this medium supplemented with 1.7 m M Ca 2+, binding was identical to that observed following sperm preincubation in the complete culture medium which supports both capacitation and fertilization in vitro. The Ca 2+-dependent binding reaction was rapid, reversed by EGTA, specific for Ca 2+, and did not require the transport of Ca 2+ into the cell. Sperm bound to the zona surface following preincubation with Ca 2+ were capable of fertilization in vitro when the eggs were subsequently transferred to the culture medium. It is proposed that this binding reaction represents a part of capacitation and not the acrosome reaction. 相似文献
14.
The H +-translocating inorganic pyrophosphatase (H +-PPase) associated with vesicles of the vacuolar membrane (tonoplast) isolated from beet ( Beta vulgaris L.) is subject to direct inhibition by Ca 2+ and a number of other divalent cations (Co 2+, Mn 2+, Zn 2+). By contrast, the H +-translocating ATPase (H +-ATPase) located on the same membrane is insensitive to Ca 2+. Here we examine the mechanism and feasibility of regulation of the vacuolar H +-PPase by cytosolic free Ca 2+ under the conditions thought to prevail in vivo with respect to Mg 2+, inorganic pyrophosphate (PPi), and pH. The minimal reaction scheme that satisfactorily describes the effects of elevated Ca 2+ or CaPPi on the enzyme is one that invokes equilibrium binding of substrate (Mg 2PPi) at one site, inhibitory binding of Mg 2PPi to a lower-affinity second site, binding of activator (Mg 2+) at a third site, and direct binding of Ca 2+ or CaPPi to a fourth site. Changes in enzyme activity in response to selective manipulation of either Ca 2+ or CaPPi are explicable only if Ca 2+, rather than CaPPi, is the inhibitory ligand. This conclusion is supported by the finding that CaPPi fails to mimic substrate in protection of the enzyme from inhibition by N-ethylmaleimide. Furthermore, the reaction scheme quantitatively and independently predicts the observed noncompetitive effects of free Ca 2+ on the substrate concentration dependence of H +-PPase activity. The results are discussed in relation to the previous proposal that CaPPi is the principal inhibitory ligand of the vacuolar H +-PPase (M. Maeshima [1991] Eur J Biochem 196: 11-17) and the possibility that in vivo modulation of cytosolic free Ca 2+ might constitute a specific mechanism for selective regulation of this enzyme, and consequently for stabilization of PPi levels in the cytoplasm of plant cells. 相似文献
15.
The toxic effect of Ni 2+ on photosynthetic electron transport was studied in a photosystem II submembrane fraction. It was shown that Ni 2+ strongly inhibits oxygen evolution in the millimolar range of concentration. The inhibition was insensitive to NaCl but significantly
decreased in the presence of CaCl 2. Maximal chlorophyll fluorescence, together with variable fluorescence, maximal quantum yield of photosystem II, and flash-induced
fluorescence decays were all significantly declined by Ni 2+. Further, the extrinsic polypeptides of 16 and 24 kDa associated with the oxygen-evolving complex of photosystem II were
depleted following Ni 2+ treatment. It was deduced that interaction of Ni 2+ with these polypeptides caused a conformational change that induced their release together with Ca 2+ from the oxygen-evolving complex of photosystem II with consequent inhibition of the electron transport activity. 相似文献
16.
The sarco(endo)plasmic reticulum Ca 2+‐ATPase (SERCA) couples ATP hydrolysis to transport of Ca 2+. This directed energy transfer requires cross‐talk between the two Ca 2+ sites and the phosphorylation site over 50 Å distance. We have addressed the mechano‐structural basis for this intramolecular signal by analysing the structure and the functional properties of SERCA mutant E309Q. Glu 309 contributes to Ca 2+ coordination at site II, and a consensus has been that E309Q only binds Ca 2+ at site I. The crystal structure of E309Q in the presence of Ca 2+ and an ATP analogue, however, reveals two occupied Ca 2+ sites of a non‐catalytic Ca 2E1 state. Ca 2+ is bound with micromolar affinity by both Ca 2+ sites in E309Q, but without cooperativity. The Ca 2+‐bound mutant does phosphorylate from ATP, but at a very low maximal rate. Phosphorylation depends on the correct positioning of the A‐domain, requiring a shift of transmembrane segment M1 into an ‘up and kinked position’. This transition is impaired in the E309Q mutant, most likely due to a lack of charge neutralization and altered hydrogen binding capacities at Ca 2+ site II. 相似文献
17.
The nature of Cu 2+ inhibition of photosystem II (PSII) photochemistry in pea ( Pisum sativum L.) thylakoids was investigated monitoring Hill activity and light emission properties of photosystem II. In Cu 2+-inhibited thylakoids, diphenyl carbazide addition does not relieve the loss of Hill activity. The maximum yield of fluorescence induction restored by hydroxylamine in Tris-inactivated thylakoids is markedly reduced by Cu 2+. This suggests that Cu 2+ does not act on the donor side of PSII but on the reaction center of PSII or on components beyond. Thermoluminescence and delayed luminescence studies show that charge recombination between the positively charged intermediate in water oxidation cycle (S 2) and negatively charged primary quinone acceptor of pSII (Q A−) is largely unaffected by Cu 2+. The S 2Q B− charge recombination, however, is drastically inhibited which parallels the loss of Hill activity. This indicates that Cu 2+ inhibits photosystem II photochemistry primarily affecting the function of the secondary quinone electron acceptor, Q B. We suggest that Cu 2+ does not block electron flow between the primary and secondary quinone acceptor but modifies the Q B site in such a way that it becomes unsuitable for further photosystem II photochemistry. 相似文献
18.
Thylakoids and Photosystem II particles prepared from the cyanobacterium Synechococcus PCC 7942 washed with a HEPES/glycerol buffer exhibited low rates of light-induced oxygen evolution. Addition of either Ca 2+ or Mg 2+ to both thylakoids and Photosystem II particles increased oxygen evolution independently, maximal rates being obtained by addition of both ions. If either preparation was washed with NaCl, light induced O 2 evolution was completely inhibited, but re-activated in the same manner by Ca 2+ and Mg 2+ but to a lower level. In the presence of Mg 2+, the reactivation of O 2 evolution by Ca 2+ allowed sigmoid kinetics, implying co-operative binding. The results are interpreted as indicating that not only Ca 2+, but also Mg 2+, is essential for light-induced oxygen evolution in thylakoids and Photosystem II particles from Synechococcus PC 7942. The significance of the reactivation kinetics is discussed. Reactivation by Ca 2+ was inhibited by antibodies to mammalian calmodulin, indicating that the binding site in Photosystem II may be analogous to that of this protein.Abbreviation HEPES
n-2-Hydroxyethylpiperazine--2-ethane sulphonic acid 相似文献
19.
Two new bismacrocyclic Gd 3+ chelates containing a specific Ca 2+ binding site were synthesized as potential MRI contrast agents for the detection of Ca 2+ concentration changes at the millimolar level in the extracellular space. In the ligands, the Ca 2+-sensitive BAPTA-bisamide central part is separated from the DO3A macrocycles either by an ethylene (L 1) or by a propylene (L 2) unit [H 4BAPTA is 1,2-bis( o-aminophenoxy)ethane- N, N, N′, N′-tetraacetic acid; H 3DO 3A is 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid]. The sensitivity of the Gd 3+ complexes towards Ca 2+ and Mg 2+ was studied by 1H relaxometric titrations. A maximum relaxivity increase of 15 and 10% was observed upon Ca 2+ binding to Gd 2L 1 and Gd 2L 2, respectively, with a distinct selectivity of Gd 2L 1 towards Ca 2+ compared with Mg 2+. For Ca 2+ binding, association constants of log K = 1.9 (Gd 2L 1) and log K = 2.7 (Gd 2L 2) were determined by relaxometry. Luminescence lifetime measurements and UV–vis spectrophotometry on the corresponding Eu 3+ analogues proved that the complexes exist in the form of monohydrated and nonhydrated species; Ca 2+ binding in the central part of the ligand induces the formation of the monohydrated state. The increasing hydration number
accounts for the relaxivity increase observed on Ca 2+ addition. A 1H nuclear magnetic relaxation dispersion and 17O NMR study on Gd 2L 1 in the absence and in the presence of Ca 2+ was performed to assess the microscopic parameters influencing relaxivity. On Ca 2+ binding, the water exchange is slightly accelerated, which is likely related to the increased steric demand of the central
part leading to a destabilization of the Ln–water binding interaction.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
20.
The degree of inhibition of respiration-dependent vs respiration-independent Ca 2+ binding by rat liver mitochondria by different members of the lanthanide family was used to establish the existence of two different classes of Ca 2+ binding sites. The distinction is based on the differences in cation:site interactions between the two classes of sites and the members of the lanthanide series. Lanthanide inhibition of respiration-dependent Ca 2+ uptake suggests that the binding site is specific for the calcium ion. Those members of the lanthanide family whose ionic radii are nearer that of Ca 2+ are the best inhibitors. The inhibition of respiration-independent Ca 2+ binding is much different, indicating non-specific cation absorption. 相似文献
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