SEARCH · Search PubMed

Results for “TRIPLETS”

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 379 records · Page 21Linked to original sources

PubMed1988

Determination of the dioxygen quenching constant for protein and model indole triplets.

The decay of the indole triplet of single tryptophan-containing proteins and model compounds can be readily determined at room temperature in solution by monitoring the triplet absorption or emission following an exciting laser pulse. The dioxygen triplet quenching constants, can be measured for all these molecules and compared to the analogous singlet values determined by fluorescence methods. The dioxygen triplet quenching constant (tkq) ranged from a high of 5.1.10(9) M-1.s-1 for the exposed indole of corticotropin to a low of 0.1.10(9) M-1.s-1 for the buried indole of asparaginase. The ratio of these values with their respective dioxygen singlet quenching constants (skq), tkq/skq, ranged from 0.3 to 0.6 for aqueous exposed polypeptide indoles. For globular proteins the tkq/skq value is observed to be 0.2 +/- 0.1. This lower value for protein indoles is not attributable to 'bulk' environmental or hydrogen bonding effects, since the magnitude of tkq/skq (= 0.5 +/- 0.1) for model indoles was independent of solvent dielectric constant, polarity, and proticity. Temperature-dependence studies were done to test whether tkq could be used to characterize the nature of the protein matrix. The activation energy (Ea) for tkq was found to be 11 +/- 2 kcal/mol for most proteins. This Ea was independent of whether the indole side-chain was solvent exposed or buried in the non-aqueous protein interior. Large Ea values were also obtained for model indoles, naphthalene and nalidixic acid, dissolved in water, whereas the same compounds dissolved in 95% ethanol exhibited much smaller Ea values. These data, in combination with the observation that the tkq of model indoles is insensitive to changes in solvent viscosity, indicate that dioxygen quenching at the triplet level can not be easily used to characterize the dynamics of proteins.

Indoles↗

PubMed1980

Hemin-catalyzed generation of triplet acetone.

Hemin can substitute for horseradish peroxidase as a catalyst for the aerobic oxidation of isobutanal to acetone and formate. Previous studies have shown that the chemiphosphorescent emission observed in the enzyme-catalyzed reaction is due to the production of acetone in its triplet state. Although no chemiphosphorescence is observed with the model system (hemin), generation of triplet acetone in this system is indicated by an analysis of data for energy transfer to the 9,10-dibromoanthracene-2-sulfonate ion and for interception of the excited species by the sorbate ion, a known triplet quencher. These data are compared to those obtained with triplet acetone generated by thermal cleavage of tetramethyldioxetane in aqueous solution. The results are in agreement with the hypothesis that the quenching of triplet acetone by oxygen is less efficient in the enzyme catalyzed reaction, pointing to a protective role for the apoenzyme in that system.

Acetone↗

PubMedDate not supplied

Subpopulations of neurons in the guinea-pig inferior mesenteric ganglia distinguished by the differential distribution of neurofilament triplet epitopes.

The distribution of the neurofilament protein triplet was examined in neurochemically identified subpopulations of neurons in the guinea-pig inferior mesenteric ganglion. A majority of the catecholamine-containing nerve cell bodies also contained the neurofilament protein triplet. However, a major proportion of the noradrenergic, neuropeptide Y-immunoreactive neurons did not contain neurofilament protein triplet immunoreactivity. Furthermore, a specific subpopulation of neurons that lacked catecholamines and were associated with the hypogastric nerve could be distinguished by the unusual feature of cell body content of post-translationally modified neurofilament protein triplet epitopes. These studies indicate that neurons in the inferior mesenteric ganglia can be distinguished by the presence of specific neurofilament protein triplet epitopes, and thus this class of intermediate filament proteins may confer specific properties to the neurons in which it is contained.

Animals↗

PubMed2005

HLAMatchmaker-based analysis of human monoclonal antibody reactivity demonstrates the importance of an additional contact site for specific recognition of triplet-defined epitopes.

Five HLA-A3 reactive human monoclonal antibodies (mAbs) originating from a parous woman were screened against HLA-typed panels by means of complement-dependent lymphocytotoxicity, high-definition ELISA, and flow cytometry with single antigen beads. Antibody reactivity profiles were compared with triplet amino acid sequence polymorphisms identified by HLAMatchmaker, and a three-dimensional structural modeling program (Cn3D of the National Center for Biotechnology Information) was used to determine the topography of epitopes recognized by each mAb. These mAbs originated from a woman who during pregnancy developed antibodies to the paternal HLA-A3 antigen of her child. Each mAb was specific for one mismatched triplet on HLA-A3, and the reactivity patterns of these IgM-type mAbs were practically the same in lymphocytotoxicity and antigen-binding assays. One mAb was specific for 163dT, a unique triplet present only on A3. The other mAbs reacted with 62Qe, 142mI, or 144tKr; these triplets are present on different groups of HLA-A alleles, some of which, however, did not react. Topographic modeling of triplet-defined epitopes identified clusters of polymorphic surface residues that were shared between reactive alleles. These clusters may serve as primary contact sites for the specificity-determining complementarity-determining region (CDR) loops of antibody. The reactivity with these mAbs required also the presence of self-sequence elsewhere on the HLA molecular surface as a critical secondary contact site for antibody, likely through another CDR loop. For instance, the reactivity of the 62Qe-specific mAb required the presence of a glycine residue in position 56 and the reactivity of the 142mI-specific mAb required the presence of the GTLRG sequence in positions 79-83. Conversely, there were many other amino acid differences between the mAb-reactive alleles and HLA-A3 that did not prevent antibody binding. For instance, the 62Qe-specific mAb-reactive alleles had 35 and the 142mI-reactive alleles had 50 of such "permissive" residue differences. An HLAMatchmaker-based analysis of the reactivity of human mAbs will increase our understanding of the structural definition of HLA epitopes and their reactivity with alloantibodies.

Alleles↗

PubMed2004

The interaction of RNA with TRAP: the role of triplet repeats and separating spacer nucleotides.

The trp RNA-binding attenuation protein (TRAP) regulates expression of the tryptophan biosynthetic genes in several Bacilli by binding to the leader region of the nascent trp mRNA, inhibiting continued transcription. The 11 subunit TRAP molecule is active in complex with tryptophan, and binds consequently an RNA target segment consisting of 11 (G/U)AG triplets, each separated by two or three non-conserved "spacer" nucleotides. Here, we report the first crystal structures of TRAP in a complex with RNA containing UAG triplets separated by two nucleotides and in a complex with RNA containing GAG triplets separated by three nucleotides. Comparison with known structures of TRAP-RNA complexes shows that both substitution of G-1 with U-1 in the triplet and addition of an extra spacer nucleotide lead to a more flexible complex. This suggests an explanation why, in the trp leader RNA, all three-nucleotide spacer regions are followed by a G-1 nucleotide. Taken together, the structures demonstrate that RNA binding to TRAP is mediated by specific interactions involving the A-2 and G-3 nucleotides of the triplet. This is accompanied by the disruption of stacking interactions between the bases of the other nucleotides, contributing to the increase in entropy that drives binding.

Bacillus subtilis↗

PubMed2005

Ab initio study of low-lying triplet states of the lithium dimer.

Observation of Bose-Einstein condensation in Li27 initiated the interest in the scattering length of two ground state lithium atoms when they approach each other as a radical pair triplet aSigmau+3 state. But some properties of this state are still unknown. In present work, a number of low-lying triplet states of lithium molecule are calculated by multi-configuration self-consistent field (MCSCF) and response techniques with account of spin-orbit coupling, spin-spin coupling and some other magnetic perturbations. The singlet-triplet transition probabilities to the ground state are also presented. Most results are connected with the weakly bound lowest triplet a3Sigmau+ state, whose radiative lifetime and spin-splitting are unknown so far in spite of its great importance in Bose-Einstein condensation. Calculations indicate that this state has a very small spin-splitting, lambdass=-0.01 cm-1, which is negligible in comparison with the line-width in experimental Fourier transform spectra published so far. Similar splitting is obtained for the upper state of the 1(3)Sigmag+--a3Sigmau+ transition. This is in agreement with experimental rovibronic analysis of the 1(3)Sigmag+--a3Sigmau+ band system in which the triplet structure was not resolved. The radiative lifetime of the a3Sigmau+ state is predicted to exceed 10 h.

Dimerization↗

PubMed2003

Photosystem II single crystals studied by transient EPR: the light-induced triplet state.

Transient electron paramagnetic resonance (TR EPR) at 9.8 GHz has been used to study the light-induced triplet state in single crystals of Photosystem II (PS II). The crystals were grown from a solution of PS II core complexes from the thermophilic cyanobacterium Synechococcus elongatus. The core complexes contain at least 17 subunits, including the water-oxidizing complex, and 32 chlorophyll a molecules per PS II complex. The PS II complexes are active in light-induced electron transfer and water oxidation. The crystals belong to the orthorhombic space group P2(1)2(1)2(1), with four dimers of PS II complexes per unit cell. Laser excitation was used to generate the recombination triplet state in PS II which was then studied by EPR at low temperatures (10 K). The crystal spectra show the same magnitude of the zero-field splitting (ZFS) values D, E as spectra obtained earlier for the triplet state of PS II in frozen solution. The orientation of the ZFS tensor D of the triplet state with respect to the crystallographic axes has been deduced from the analysis of angular-dependent EPR spectra. Knowledge of the orientation of the D tensor component perpendicular to the plane of the chlorophyll (D(Z)) allows an assignment on which chlorophyll of the reaction centre the triplet state is localized at low temperatures. Furthermore, the orientation of the D(X) and D(Y) components of the D tensor yielded the in-plane orientation of the respective chlorophyll in the reaction centre providing first experimental evidence for the orientation of this molecule in the PS II.

Cyanobacteria↗

PubMed1994

Temperature-dependent triplet and fluorescence quantum yields of the photosystem II reaction center described in a thermodynamic model.

A key step in the photosynthetic reactions in photosystem II of green plants is the transfer of an electron from the singlet-excited chlorophyll molecule called P680 to a nearby pheophytin molecule. The free energy difference of this primary charge separation reaction is determined in isolated photosystem II reaction center complexes as a function of temperature by measuring the absolute quantum yield of P680 triplet formation and the time-integrated fluorescence emission yield. The total triplet yield is found to be 0.83 +/- 0.05 at 4 K, and it decreases upon raising the temperature to 0.30 at 200 K. It is suggested that the observed triplet states predominantly arise from P680 but to a minor extent also from antenna chlorophyll present in the photosystem II reaction center. No carotenoid triplet states could be detected, demonstrating that the contamination of the preparation with CP47 complexes is less than 1/100 reaction centers. The fluorescence yield is 0.07 +/- 0.02 at 10 K, and it decreases upon raising the temperature to reach a value of 0.05-0.06 at 60-70 K, increases upon raising the temperature to 0.07 at approximately 165 K and decreases again upon further raising the temperature. The complex dependence of fluorescence quantum yield on temperature is explained by assuming the presence of one or more pigments in the photosystem II reaction center that are energetically degenerate with the primary electron donor P680 and below 60-70 K trap part of the excitation energy, and by temperature-dependent excited state decay above 165 K. A four-compartment model is presented that describes the observed triplet and fluorescence quantum yields at all temperatures and includes pigments that are degenerate with P680, temperature-dependent excited state decay and activated upward energy transfer rates. The eigenvalues of the model are in accordance with the lifetimes observed in fluorescence and absorption difference measurements by several workers. The model suggests that the free energy difference between singlet-excited P680 and the radical pair state P680+l- is temperature independent, and that a distribution of free energy differences represented by at least three values of about 20, 40, and 80 meV, is needed to get an appropriate fit of the data.

Kinetics↗

PubMed1996

Influence of the triplet excited state on the photobleaching kinetics of fluorescein in microscopy.

The investigation in this report aimed at providing photophysical evidence that the long-lived triplet excited state plays an important role in the non-single-exponential photobleaching kinetics of fluorescein in microscopy. Experiments demonstrated that a thiol-containing reducing agent, mercaptoethylamine (MEA or cysteamine), was the most effective, among other commonly known radical quenchers or singlet oxygen scavengers, in suppressing photobleaching of fluorescein while not reducing the fluorescence quantum yield. The protective effect against photobleaching of fluorescein in the bound state was also found in microscopy. The antibleaching effect of MEA let to a series of experiments using time-delayed fluorescence spectroscopy and nanosecond laser flash photolysis. The combined results showed that MEA directly quenched the triplet excited state and the semioxidized radical form of fluorescein without affecting the singlet excited state. The triplet lifetime of fluorescein was reduced upon adding MEA. It demonstrated that photobleaching of fluorescein in microscopy is related to the accumulation of the long-lived triplet excited state of fluorescein and that by quenching the triplet excited state and the semioxidized form of fluorescein to restore the dye molecules to the singlet ground state, photobleaching can be reduced.

Biophysical Phenomena↗

PubMed1997

Sequence fossils, triplet expansion, and reconstruction of earliest codons.

mRNA sequences are known to carry a hidden periodical pattern (GCU)n, which may be considered a remnant of sequence organization of mRNA early in its evolution, dominated by codons for alanine and their point mutation derivatives. A similar pattern is characteristic of the master (consensus) tRNA sequence derived in 1981 by Eigen and Winkler-Oswatitsch. The master tRNA sequence is thought to represent one of the earliest mRNA. From analysis of literature and from our own calculations presented in this work, the (GCU)n pattern appears to be the most expandable in the norm and in disease. The speculation is put forward that (GCU)n and polyalanine have been key players at the beginning of the triplet code, and the first codons, apart from the GCU triplet, were point change derivatives of the generic triplet GCU, coding for amino acids present in the early prebiotic-biotic environment. The set of the earliest amino acids is derived on the basis of structural simplicity, presence in imitated prebiotic conditions and involvement with class II aminoacyl-tRNA synthetases. The set consists of six amino acids: Ala, Asp, Gly, Pro, Ser and Thr. All these amino acids are, indeed, encoded by the GCU triplet and its derivatives, as predicted. Thus, the pairs GCN (Ala), GAU (Asp), GGU (Gly), CCU (Pro), UCU (Ser) and ACU (Thr) can be viewed as an early triplet code.

Codon↗

PubMed1998

Perinatal outcome of triplet gestation: does prophylactic cerclage make a difference?

Objective: To compare the perinatal outcome of triplet gestations with and without prophylactic cerclage.Methods: A retrospective chart review of all triplet gestations delivered between January 1988 and June 1997 was performed. Only women initiating prenatal care before 15 weeks gestation were included. Management guidelines for triplets were uniform with the exception of use of prophylactic cerclage placed at 14 +/- 1 weeks gestation. Hospitalization was reserved for maternal medical and obstetric conditions, or fetal indications. Tocolytics were initiated only for persistent uterine contractions with intact membranes. All deliveries were effected by cesarean section at 37 weeks or less for maternal or fetal indications. All neonates were cared for at our institution. The cerclage group was compared to the no cerclage group for gestational age at delivery and maternal and neonatal complications. Descriptive statistics, Student t test, and Mann-Whitney U test were used for analysis.Results: Fourteen of the 51 (27.4%) sets of triplets had cerclage. There were no differences between groups when compared for maternal age, length of hospital stay, preterm labor rate, steroid use, histologic or clinical chorioamnionitis, and postoperative complications. In addition, there were no differences in mean birth weight, Apgar scores, respiratory distress syndrome, intraventricular hemorrhage/periventricular leucomalacia, NICU stay, and neonatal mortality. Although the mean gestational age at delivery for the cerclage group (33.1 +/- 2.7 weeks) was not different from the no cerclage group (31.7 +/- 3.7 weeks), the proportion of pregnancies delivered at 31 weeks or more was significantly higher in the cerclage group (13/14 vs 23/37, P =.03).Conclusions: Prophylactic cerclage may influence the proportion of triplet gestations delivered at 31 weeks or more. Lack of differences in the other parameters may be reflective of the small sample size.

Journal Article↗

PubMed2000

Reinvestigation of the triplet-minus-singlet spectrum of chloroplasts.

A comparison of the triplet-minus-singlet (TmS) absorption spectrum of spinach chloroplasts, recorded some thirty years ago, with the more recently published TmS spectrum of isolated Chla/b LHCII (light-harvesting complexes associated with photosystem II of higher plants) shows that the two spectra are very similar, which is to be expected, since only the carotenoid pigments contribute to each spectrum. Be that as it may, the comparison also reveals a dissimilarity: photoexcitation of the sample does, or does not, affect the absorbance in the Qy region (650-700 nm), depending on whether the sample is a suspension of chloroplasts or of isolated LHCII. The Qy-signal in the TmS spectrum of LHCII decays, it should be noted, at the same rate as the rest of the difference spectrum, and its most prominent feature is a negative peak. As the carotenoids do not absorb in the Qy region, the presence of a signal in this region calls for an explanation: van der Vos, Carbonera and Hoff, the first to find as well as fathom the phenomenon, attributed the Qy-signal to a change, in the absorption spectrum of a chlorophyll a (Chla) molecule, brought about by the presence of triplet excitation on a neighbouring carotenoid (Car). The difference in the behaviours of chloroplasts and LHCII, if reproducible, would imply that the Car triplets which give rise to the TmS spectrum of chloroplasts do not influence the absorption spectra of their Chla neighbours. With a view to reaching a firm conclusion about this vexed issue, spinach chloroplasts and thylakoids have been examined with the aid of the same kinetic spectrometer as that used for investigating LHCII; the TmS spectra of both chloroplasts and thylakoids contain prominent bleaching signals centred at 680 nm, and the triplet decay time in each case is comparable to that of the Chla/b LHCII triplets. Results pertaining to other closely related systems are recalled, and it is concluded that, so far as the overall appearance of the TmS spectrum is concerned, spinach chloroplasts are by no means abnormal.

Carotenoids↗

PubMed2004

Neurologic course, endocrine dysfunction and triplet repeat size in spinal bulbar muscular atrophy.

OBJECTIVE: To study the role of diabetes, gynecomastia and CAG triplet repeat size as disease modifying factors of neurologic expression in spinal bulbar muscular atrophy (SBMA, Kennedy's disease). METHODS: Twenty unrelated SBMA patients with confirmatory genetic testing were reviewed. Patterns of neurologic involvement were assessed (e.g. bulbar, asymmetric, proximal, distal, motor and sensory). Slopes of disease progression were calculated from serial quantified neurologic examinations. Patterns of neurologic involvement and course were correlated to the presence of diabetes, gynecomastia and triplet repeat size. RESULTS: Diabetes or glucose impairment occurred in nine and 11 had gynecomastia. Patterns of neurologic involvement and rates of progression did not correlate with these endocrine diseases or triplet repeat sizes. Correlation was seen between number of CAG repeats and age of onset weakness (r = -0.53, r2 = 29%, p = 0.01). CONCLUSIONS: The specific neurotoxic effect of expanded CAGs appears limited to age of onset weakness in SBMA. Although significant, only 29% of the variability in onset age could be accounted for by polyglutamine size suggesting the importance of other unidentified factors. In this series diabetes or glucose impairment was more common than previously reported and, like gynecomastia, did not correlate with size of triplet repeats, severity or patterns of neurologic involvement. Modifying factors other than diabetes, gynecomastia or triplet repeat size are suggested in disease expression.

Adult↗

PubMed1995

Dynamics of parvalbumin studied by fluorescence emission and triplet absorption spectroscopy of tryptophan.

Fluorescence emission and triplet-triplet absorbance spectroscopy of the single tryptophan in cod parvalbumin were used to study the stability and dynamics of the protein as influenced by Ca2+ binding and interaction with a chaotropic agent. The concentrations for half-saturation for Ca binding were 3.6 x 10(-9), 3.3 x 10(-4), 7.1 x 10(-3), and 0.14 M in the presence of 0, 2, 3, and 4 M guanidine hydrochloride, respectively. As predicted for thermodynamic reversibility, the guanidine hydrochloride unfolding reaction depends upon Ca2+, and the delta G are as follows: 22.9, 29.3, 35.2, and 44.2 kJ/mol for no added Ca2+, 1, 2, and 5 mM Ca2+, respectively. The stability toward denaturation imparted by the binding of two Ca2+ is about -60 kJ/mol. For Ca(2+)-bound parvalbumin in the presence of excess Ca2+, the decay of the triplet state tryptophan is approximately exponential, and the lifetime decreases from 6.5 to 3.8 ms as the temperature increases from 10 to 40 degrees C. In contrast, the triplet decay of the calcium-free protein is nonexponential over the time range of microseconds to milliseconds, a result that may indicate that the Ca-free protein is molten-globule-like. At Ca2+ concentrations where the protein is partially saturated with Ca2+, the lifetime of the longest decay component is less than that for the Ca-saturated protein; this finding suggests an exchange of Ca2+ and a conformational change during the triplet lifetime. From these data, a rate constant for the process that includes calcium-related protein conformational change can be surmised to range between 200 and 500 s-1.

Animals↗

PubMed1992

Intramolecular triple-helix formation at (PunPyn).(PunPyn) tracts: recognition of alternate strands via Pu.PuPy and Py.PuPy base triplets.

Triple-helical DNA shows increasing potential for applications in the control of gene expression (including therapeutics) and the development of sequence-specific DNA-cleaving agents. The major limitation in this technology has been the requirement of homopurine sequences for triplex formation. We describe a simple approach that relaxes this requirement, by utilizing both Pu.PuPy and Py.PuPy base triplets to form a continuous DNA triple helix at tandem oligopurine and oligopyrimidine tracts. [Triplex formation at such a sequence has been previously demonstrated only with the use of a special 3'-3' linkage in the third strand [Horne, D. A., & Dervan, P. B. (1990) J. Am. Chem. Soc. 112, 2435-2437].] Supporting evidence is from chemical probing experiments performed on several oligonucleotides designed to form 3-stranded fold-back structures. The third strand, consisting of both purine and pyrimidine blocks, pairs with purines in the Watson-Crick duplex, switching strands at the junction between the oligopurine and oligopyrimidine blocks but maintaining the required strand polarity without any special linkage. Although Mg2+ ions are not required for the formation of Pu.PuPy base triplets, they show enhanced stability in the presence of Mg2+. In the sequences observed. A.AT triplets appear to be more stable than G.GC triplets. As expected, triplex formation is largely independent of pH unless C+.GC base triplets are required.

Base Composition↗

PubMed1991

Population of the triplet states of bacteriorhodopsin and of related model compounds by intramolecular energy transfer.

In variance with chlorophyll-based photosynthetic pigments, the triplet states of rhodopsins, either visual or photosynthetic, have not been observed experimentally. This is due to the ultrafast crossing from S1 to S0, which effectively competes with intersystem crossing to the triplet (T1) state. In order to populate T1 indirectly, laser photolysis experiments are performed with model protonated Schiff bases of retinal in solution, in which both inter- and intramolecular energy transfer to the polyene chromophore are carried out from an appropriate triplet energy donor. The experiments are then extended to bacteriorhodopsin (bR) by detaching the native retinal chromophore from the protein-binding site and replacing it by an analogous (synthetic) protonated Schiff base polyene, attached in a nonconjugated way to a naphthone triplet donor. Pulsed laser excitation of the latter moiety led, for the first time, to the observation of the triplet state of a rhodopsin. Possible locations and roles of the T1 state in bR and in visual pigments are discussed briefly.

Bacteriorhodopsins↗

PubMed2002

Aggregation of LHCII Leads to a Redistribution of the Triplets over the Central Xanthophylls in LHCII.

We present laser flash-induced triplet-minus-singlet (TmS(flash)) and absorbance-detected-magnetic-resonance (TmS(ADMR)) measurements on the light-harvesting chlorophyll a/b pigment-protein complex (LHCII) from pea. We investigated the influence of LHCII aggregation on xanthophyll triplet formation. The effect of aggregation was previously studied using TmS(ADMR) [van der Vos et al. (1994) Biochim. Biophys. Acta 1208, 243-250] for LHCII from spinach, and it was concluded that aggregation leads to a large increase of the amount of intertrimer triplet transfer. However, a similar study on LHCII from pea with the use of TmS(flash) measurements [Barzda et al. (1998) Biochemistry 37, 546-561] showed much smaller effects. To resolve this apparent discrepancy and to compare the results of TmS(ADMR) and TmS(flash) measurements, we used both techniques to study LHCII from pea, applying an identical aggregation procedure in both cases. It appears that aggregation does not lead to an increase of intertrimer triplet transfer as thought before but to a redistribution of the triplets over the two central xanthophylls (mainly lutein) that are present in each monomeric subunit of LHCII. Moreover, it is argued that the TmS band at 525 nm is due to lutein instead of violaxanthin as was reported in earlier studies. It is concluded that aggregation leads to a change in chlorophyll-xanthophyll interactions, which might explain the large change in excited-state lifetime of chlorophyll a in LHCII upon aggregation. This change in lifetime is possibly related to the phenomenon of nonphotochemical quenching in green plants, which is an important protective regulatory mechanism, that lowers the probability of photoinhibition.

Kinetics↗

PubMed2006

Analysis of the Cob(II)alamin-5'-deoxy-3',4'-anhydroadenosyl radical triplet spin system in the active site of diol dehydrase.

A triplet spin system (S=1) is detected by low-temperature electron paramagnetic resonance (EPR) spectroscopy in samples of diol dehydrase and the functional adenosylcobalamin (AdoCbl) analogue 5'-deoxy-3',4'-anhydroadenosylcobalamin (anAdoCbl). Different spectra are observed in the presence and absence of the substrate (R,S)-1,2-propanediol. In both cases, the spectra include a prominent half-field transition (DeltaM(S) = 2) that is a hallmark of strongly coupled triplet spin systems. The appearance of 59Co hyperfine splitting in the EPR signals and the positions (g values) of the signals in the spectra show that half of the triplet spin is contributed by the low-spin Co2+ of cob(II)alamin. Line width effects from isotopic labeling (13C and 2H) in the 5'-deoxy-3',4'-anhydroribosyl ring demonstrate that the other half of the spin triplet is from an allylic 5'-deoxy-3',4'-anhydroadenosyl (anhydroadenosyl) radical. The zero-field splitting (ZFS) tensors describing the magnetic dipole-dipole interactions of the component spins of the triplets have rhombic symmetry because of electron spin delocalization within the organic radical component and the proximity of the radical to the low-spin Co2+. The dipole-dipole interaction was modeled as a summation of point-dipole interactions involving the spin-bearing orbitals of the anhydroadenosyl radical and cob(II)alamin. Geometries which are consistent with the ZFS tensors in the presence and absence of the substrate position the 5'-carbon of the anhydroadenosyl radical 3.5 and 4.1 A from Co2+, respectively. Homolytic cleavage of the cobalt-carbon bond of the analogue in the absence of the substrate indicates that, in diol dehydrase, binding of the coenzyme to the protein weakens the bond prior to binding of the substrate.

Binding Sites↗