Complexation of curium(III) by adenosine 5′-triphosphate (ATP): A time-resolved laser-induced fluorescence spectroscopy (TRLFS) study

Inorganica Chimica Acta 358 (2005) 2275–2282 www.elsevier.com/locate/ica

Complexation of curium(III) by adenosine 5 0 -triphosphate (ATP): A time-resolved laser-induced fluorescence spectroscopy (TRLFS) study Henry Moll *, Gerhard Geipel, Gert Bernhard Institute of Radiochemistry, Forschungszentrum Rossendorf e.V., P.O. Box 510119, D-01314 Dresden, Germany Received 4 October 2004; accepted 1 December 2004 Available online 7 February 2005

Abstract The complex formation of curium(III) with adenosine 5 0 -triphosphate (ATP) was determined by time-resolved laser-induced fluorescence spectroscopy (TRLFS). The interaction between soluble species of curium(III) with ATP was studied at trace Cm(III) concentrations (3 · 10
7 M). The concentrations of ATP were varied between 6.0 · 10
7 and 1.5 · 10
4 M in the pH range of 1.5–7.0 using 0.154 M NaCl as background electrolyte. Three Cm–ATP species, MpHqLr, could be identified from the fluorescence emission spectra: (i) CmH2ATP+ with a peak maximum at 598.6 nm, (ii) CmHATP with a peak maximum at 600.3 nm, and (iii) CmATP
with a peak maximum at 601.0 nm. The formation constants of these complexes were calculated from TRLFS measurements to be log b121 = 16.86 ± 0.09, log b111 = 13.23 ± 0.10, and log b101 = 8.19 ± 0.16. The hydrated Cm–ATP species showed fluorescence lifetimes between 88 and 96 ls; whereas the CmATP
complex has a significantly longer fluorescence lifetime of 187 ± 7 ls.
2005 Elsevier B.V. All rights reserved. Keywords: Adenosine triphosphate; Aqueous solution; TRLFS; Curium; Complexation

1. Introduction For a better understanding of the biosorption process of Cm(III) onto the cell envelope of the sulfate-reducing bacterial strain Desulfovibrio a¨spo¨ensis on a molecular level [1], we investigate the complexation of curium with selected bioligands of relevant functionalities as model compounds. The spectroscopic studies of uranium and other actinide complexes in several biological systems like microorganisms [2–4] and plants [5] indicated that one functionality, most likely the phosphate group, might be responsible for complexation.

*

Corresponding author. Tel.: +49 351 260 2433; fax: +49 351 260 3553. E-mail address: [email protected] (H. Moll). 0020-1693/$ - see front matter
2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2004.12.055

ATP (see Fig. 1) is an important enzymatic compound in the life sciences. This molecule is the major source of energy for cellular reactions. The amount of adenosine phosphates in living systems lies in the 2–10 millimolar concentration range [6]. The fact that a special ATP-synthesizing enzyme can be located in membrane systems of cells [7] points to the importance of ATP for a better understanding of biosorption processes of actinides on cell membranes of microorganisms. Imaginable would be also that heavy metals bound to adenosine phosphates can be transported into living cells and then deposited. Moreover, complexes of ATP with heavy metals can influence the behavior of some enzymes which catalyze important biochemical reactions. The ATP molecule can interact with metal ions via the oxygen atoms from the triphosphate chain and the

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Fig. 1. Structures of ATP considered in this paper.

nitrogen atoms of the purine nucleus (N-1, N-3 and N7). Du et al. [8] showed in the case of Fe(III) that besides the triphosphate chain the N-1 nitrogen is the predominant coordination site at pH 3 using 1H and 31P NMR. The results of a Raman study of the Fe(III)–ATP complex at pH 7.5 showed direct interactions of Fe(III) with the phosphate chain and the N-7 nitrogen and indirect interactions (via one water molecule) with the amide group [9]. The interactions with the adenine moiety are metal specific and strongly influenced by pH changes [10]. Stability constants are not reported in these papers. A few studies describe the complexation of lanthanide ions with ATP. A short overview will be given in relation to the presented work. Eads et al. [11] found spectroscopic evidence for the formation of Ln(III)–ATP2 (Ln:Gd(III) and Eu(III)) complexes at metal to nucleotide ratios of less than 0.5. The 1:2 complex dissociates to the 1:1 complex. The Eu(III) TRLFS measurements resulted in a value of 2.4 ± 0.5 for the number of water molecules coordinated to the metal in the 1:2 complex, whereas the hydration number increases to 4.1 in the 1:1 complex. The Eu(III) most likely coordinates via the b-phosphate oxygen to the ATP molecule. In contrast to these results, Gutman and Levy [12] report an average hydration number of 2.6 for Eu(III) in the 1:1 Eu3+–ATP complex in solution at pH 8. Furthermore, the formation of [Ln(III)ATP] and [Ln(III)(ATP)2] complexes for Eu(III) and Gd(III) determined by potentiometry and relaxation experiments is described in [13,14]. The structure of the proposed species is less discussed. Shanbhag and Choppin [6] describe the formation of

Ln(III)ATP
(Ln:Y, La, Nd, Eu, Dy and Tm) and Ln(III)HATP. In the LnATP
complex, the lanthanide is bound to the phosphate chain. However, the low DS values and negative DH give evidence for Ln-ring interactions probably through a water molecule. The LnHATP complexes showed larger positive DS values with little change in DH supporting a model of Ln interaction with both, the phosphate chain and the adenosine ring system. A 1H and 31P NMR study showed that the Eu(III) in the Eu(III)–ATP complex is coordinated via the oxygens to at least two of the phosphorus atoms of the phosphate chain and not directly bound to the adenosine [15]. Nevertheless, there is an interaction of Eu(III) with the H-8 proton from the C-8 atom of the adenine ring via one water molecule. In contrast to the lanthanide elements, the formation of a 1:2 complex was not observed for Al3+ and Fe3+. Jackson and Voyi [16] identified the species AlHATP using potentiometry and discussed the coordination of Al(III) to the b- and c-phosphates. A NMR and infrared study investigating the interaction of Al(III) with ATP showed that Al(III) forms a long-lived complex with ATP at pH 6.2 [17]. 31P NMR gives direct evidence for metal ion coordination to the c-phosphate of ATP. However, due to the large effects in the proton NMR spectra, Al-ring interactions cannot be excluded. Presently, little is known about the complexation of ATP with actinides. Stability constants for the formation of NpO2ATP3
, NpO2HATP2
and UO2ATP2
are reported in [18,19]. However, the complex formation with trivalent actinides is unknown up to now.

H. Moll et al. / Inorganica Chimica Acta 358 (2005) 2275–2282

Therefore, we present the results of curium(III) complexation with ATP, obtained by TRLFS. In contrast to the complexation studies of ATP with lanthanides described above, where the metal ion concentrations are in the millimolar range, we performed our TRLFS experiments with trace amounts of Cm(III) (3 · 10
7 M). The results are compared to the relevant literature data.

2. Experimental 2.1. Solutions and reagents Adenosine 5 0 -triphosphate disodium salt was purchased from ACROS ORGANICS (analytical grade). The stock solutions were prepared freshly for each experiment. A stock solution of the long-lived curium isotope Cm-248 (t1/2 = 3.4 · 105 years) was used. This solution had the following composition: 97.3% Cm248, 2.6% Cm-246, 0.04% Cm-245, 0.02% Cm-247, and 0.009% Cm-244 in 1.0 M HClO4. The experiments were performed in a glove box under N2 atmosphere at 25 C. As a background electrolyte, 0.154 M NaCl was used. To avoid carbonate complexation of Cm(III), carbonate free water and NaOH solution was used. The Cm(III) concentration was fixed to 3 · 10
7 M in all TRLFS measurements. The ATP concentration was varied between 3 · 10
7 and 1.5 · 10
4 M. The pH was changed between 1.5 and 7.0 by adding analytical grade NaOH (carbonate free) or HClO4. The pH was measured using a glass electrode (type: Mettler Toledo InLab 427) calibrated in H+ concentration units.

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3 · 10
5 M, by varying the pH between 1.5 and 7.0. In the other series, we varied the ATP concentration between 6.0 · 10
7 and 1.5 · 10
4 M at pH 4.5 and 6.5. The TRLFS spectra were measured after an equilibration time of 0.5 h.

3. Results and discussion Fluorescence emission spectra of 3 · 10
7 M Cm(III) with 3 · 10
6 M ATP in 0.154 M NaCl in the pH range 1.5–7.0 are shown in Fig. 2. The Cm3+ aquo ion is characterized by a fluorescence emission band maximum at 593.8 nm. When adding ATP and changing the pH to 7.0, there is a pronounced red-shift of the emission as a result of the complex formation. This observation is accompanied by an increase in the fluorescence emission lifetime from 65 ls for the Cm3+ aquo ion to 95 ls for the sample at pH 7.0 containing 3 · 10
6 M ATP. The TRLFS spectra are indicating an interaction between Cm(III) and ATP species already at a pH of 1.5. The fluorescence emission spectra of 3 · 10
7 M Cm(III) at various concentrations of ATP at pH 4.5 and 6.5 are shown in Fig. 3. Again, the TRLFS spectra (see Figs. 2 and 3) give evidence for interactions of Cm(III) with ATP already at a ligand concentration of 6 · 10
7 M. At pH 4.5, one Cm(III) peak with a maximum at 600.4 nm is observed at ATP concentrations above 3 · 10
6 M (see Fig. 3(a)). At pH 6.5, the peak maximum is shifted to 601.8 nm at ATP concentrations above 1.2 · 10
5 M (see Fig. 3(b)). This might indicate the formation of different Cm–ATP species. If no ATP

2.2. TRLFS measurements The time-resolved laser-induced fluorescence spectra were recorded at 25 C using a flash lamp pumped Ti:sapphire laser (Elight, Titania). Details on the experimental set-up are summarized in [20]. The excitation wavelength was 395 nm using a laser energy of less than 2 mJ controlled by a photodiode. The fluorescence emission spectra were detected by an optical multi-channel analyzer. The system consists of a monochromator and spectrograph (Oriel; MS 257) with a 300 or 1200 lines mm
1 grating and an ICCD camera (Andor). The Cm(III) emission spectra were recorded in the 500– 700 nm (300 lines mm
1 grating) and 570–650 nm (1200 lines mm
1 grating) ranges, respectively. A constant time window of 1 ms was used. For time dependent emission decay measurements, the delay time between laser pulse and camera grating was scanned with time intervals between 10 and 15 ls. Four series of experiments were made; in the first series, we investigated the complex formation of Cm3+ and ATP at two concentrations of ATP, 3 · 10
6 and

Fig. 2. Fluorescence emission spectra of 3 · 10
7 M Cm(III) in 0.154 M NaCl solution containing 3 · 10
6 M ATP at various pH; the spectra are scaled to the same peak area.

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Fig. 3. Fluorescence emission spectra of 3 · 10
7 M Cm(III) in 0.154 M NaCl solution at various concentrations of ATP; (a) at pH 4.5 and (b) at pH 6.5; the spectra are scaled to the same peak area.

is present in the system, speciation calculations using the formation constants for the Cm(III) hydrolysis species published in [21] show that 5% of the Cm(III) exist as Cm(OH)2+ at pH 7.0. Due to the strong interaction of Cm3+ with ATP species (Figs. 2 and 3), we conclude that hydrolysis plays no role in the reaction mechanism. Factor analysis of spectroscopic data is a powerful tool for the determination of the number of independent absorbing/emitting species in a series of mixtures [22]. To describe the complex formation reactions in the system Cm(III)–ATP, we applied two different factor analysis program codes: (a) iterative transformation factor analysis [23] and (b) SPECFIT [24]. The approach of both programs to analyze, e.g., TRLFS spectra, is a quantitative decomposition of the spectra of mixtures into different spectral components/constituents. The speciation of Cm(III) changes with physicochemical parameters like pH and concentration of ATP (see Figs. 2 and 3). Due to the spectroscopic properties of each individual chemical species, the spectra of the four series (spectra measured at 3 · 10
7 M Cm(III) with 3 · 10
5 M ATP at varying pH from 1.5 to 6.85 not shown) of samples showed variations depending on the physicochemical

parameter varied. These spectral variations are used in both programs to determine the spectra of the single components and their concentration distribution depending on the physicochemical parameter varied. All data sets, 32 individual spectra, were taken for the calculations using SPECFIT ; whereas for the iterative transformation factor analysis each data set was evaluated separately. A satisfactory agreement of the single component spectra calculated with both factor analysis program codes could be observed. The spectra of these single components derived by deconvolution of the mixed spectra using SPECFIT are shown in Fig. 4. SPECFIT provides a new application of factor analysis called evolving factor analysis and is described in detail in [22]. SPECFIT calculates besides the determination of the independent components (see Fig. 4) and their relative species distribution curves, with the aid of some chemical reasoning, reasonable equilibrium constants. As a consequence, the single component spectra derived from SPECFIT can be attributed to the formed species (see Fig. 4). Gampp et al. demonstrated in [25] the successful application of the program SPECFIT to describe the complexation of Cu2+ with 3,7-diazanonanediamide (DANA) using spectrophotometric and ESR (electron spin resonance) data. Recently, the speciation of Mg2+ and Ca2+ with tetracycline ligands was investigated by UV–Vis absorption and fluorescence spectroscopy. The species distribution could be clarified using the SPECFIT software [26]. Input parameters for the data fitting in the present study were the known total concentrations of Cm3+, ATP, the pH of each sample, and the protonation constants for ATP, log KHATP3
= 6.87, log KH2ATP2- = 11.34 and log KH3ATP
= 13.51 (see Table 2) determined at an ionic strength of 0.1 M NaCl and published by Oscarson et al. [27]. Furthermore, the known fluorescence emission spectrum of the Cm3+ aquo ion was used in the SPECFIT calculations. In

Fig. 4. Fluorescence emission spectra of the single components in the Cm–ATP-system: Cm3+, Cm/complex 1 (CmH2ATP+), Cm/complex 2 (CmHATP) and Cm/complex 3 (CmATP
) as derived by peak deconvolution using SPECFIT ; the spectra are scaled to the same peak area.

H. Moll et al. / Inorganica Chimica Acta 358 (2005) 2275–2282

evaluation of relevant complexation studies of ATP with actinides [18,19] and lanthanides [6,14] and taking into consideration the deprotonation of the ATP molecule, possible Cm–ATP species of the type MpHqLr were introduced in the data analysis procedure. As a result, we could develop a chemical model describing the ongoing processes in the Cm(III)–ATP system. It could be demonstrated that predominantly 1:1 complexes of the type MpHqLr were formed under the given experimental conditions. The dependencies found in the TRLFS data could be expressed by the following equilibria:

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Formation constants for reactions (1)–(3) were calculated to be log b121 = 16.86 ± 0.09, log b111 = 13.23 ± 0.10 and log b101 = 8.19 ± 0.16, respectively. At metal to ligand ratios up to 1:500, no indications were found for the formation of 1:2 species like observed for, e.g., Eu(III) in [11,13,14]. However, in test solutions with an excess of ATP greater than 1:500 the TRLFS spectra give evidence for the existence of a further Cm–ATP species, presumably a 1:2 complex. In Fig. 5, we are comparing the species distributions of the Cm(III)– ATP system obtained with both factor analysis programs. The iterative factor analysis applied for series one (constant concentration of ATP of 3 · 10
6 M and varying pH) calculates for every composite spectrum at pH 1.5, 2.0, 2.5, 3.1, 4.0, 5.1, and 7.0 the relative amount of each single component. Whereas SPECFIT also determined the formation constants of the three Cm(III)–ATP species. The species distribution curves were calculated by means of the program SOLGASWATER [28] using these constants. In general, a good

agreement between both methods could be achieved. This gives further support for the chemical model developed for the Cm(III)–ATP system. Fluorescence lifetime measurements provide information on the composition of the first coordination sphere of Cm(III) [29]. A linear correlation between the decay rate and the number of H2O molecules in the first coordination sphere of Cm(III) was found by Kimura and Choppin [30]. The Cm(III) aquo ion is characterized by a measured lifetime of 68 ± 3 ls [31–33]. Increasing lifetimes of Cm(III) species are reflecting the exclusion of water molecules out of the first coordination sphere of the Cm(III) due to complex formation reactions. 68 ls [31–33] measured for the Cm(III) aquo ion corresponds to nine water molecules and a value of 1300 ls corresponds to zero water molecules in the first coordination sphere of Cm(III) [34]. Besides the bonding in the first coordination sphere of the metal ion, the time dependence of the fluorescence decay contains information about the kinetics of the complex formation reactions [29]. We expect a mono-exponential fluorescence decay with an average lifetime of the species in equilibrium, if the rate of ligand exchange is high compared to the lifetime of the excited Cm(III). If the ligand exchange rate is low in comparison to the fluorescence decay rate of the excited Cm(III), we expect at least a bi-exponential decay. The emission decay of all test solutions was mono-exponential (see Fig. 6 and Table 1). This indicates a fast exchange between the different Cm(III)-ATP species. NMR investigations of the Fe(III)–ATP complex showed a fast exchange on the NMR time scale between bound ATP and free ATP [8]. Transwell et al. [15] observed a fast exchange between the Eu(III)–ATP complex and the free ligand at pH 6 using 1H NMR. At pH 8.5, the exchange is still fast but close to the intermediate/slow exchange borderline. At both pH values, a single 1:1 complex between

Fig. 5. Experimental species distribution of the Cm(III)–ATP system as a function of pH. Open symbols: species distribution obtained from SPECFIT . Solid symbols: result from the iterative transformation factor analysis of series 1 (3 · 10
7 M Cm(III) + 3 · 10
6 M ATP).

Fig. 6. Fluorescence emission lifetimes of Cm3+ and the Cm(III)–ATP species showing mono-exponential decay behavior.

Cm3þ þ ATP4
þ 2Hþ $ CmH2 ATPþ

ð1Þ

Cm3þ þ ATP4
þ Hþ $ CmHATP

ð2Þ

Cm3þ þ ATP4
$ CmATP


ð3Þ

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Table 1 Fluorescence properties of Cm(III) in selected systems Species

Medium

Cm3+ 82% CmH2 ATP+ 66% CmHATP 96% CmATP
100% Cm–D. a¨spo¨ensis surface complex Cm(H2PO4)2+

0.154 M 0.154 M 0.154 M 0.154 M 0.154 M

a

NaCl NaCl NaCl NaCl NaCl

pH 6.5 3 · 10
5 M ATP pH 2.5 3 · 10
5 M ATP pH 4.1 1.5 · 10
4 M ATP pH 4.1 pH 7.55

0.1 M H3PO4 pH 2.0

Emission (nm)

Number of coordinated watersa

Lifetime (ls)

Reference

593.8 598.6 600.3 601.0 600.1

9.0 6.5 6.3 2.6 3.1

66 89 92 187 162

This work This work This work This work Moll et al. [1]

601.0

6.0

95

Cavellec et al. [35]

According to the Kimura and Choppin equation [30].

Eu(III) and ATP is formed. If the complexes CmH2ATP+ and CmHATP are dominating the speciation in the test solutions, the resulting fluorescence lifetime is 92 ± 4 ls. The complex formation leads to an exclusion of water molecules out of the first coordination sphere of the Cm3+ aquo ion. According to the Kimura and Choppin equation, a fluorescence lifetime of 92 ± 4 ls corresponds to 6.2 ± 0.3 coordinated water molecules. Now, estimates concerning the structure of these two complexes can be made. The number of remaining water molecules can be explained by a favored bounding of Cm to the partly deprotonated phosphate groups of the ATP molecule. An interaction with the ring structure via water molecules concluded for LnHATP [6] cannot be excluded. The lifetime of the hydrated Cm(III)–ATP species is in the range of those measured for inorganic Cm(III)–phosphate complexes, 95 ls [35]. This might support our conclusions. In contrast to the hydrated Cm–ATP species, we obtained an increased fluorescence lifetime of 187 ± 7 ls for CmATP
. According to the Kimura and Choppin equation, this lifetime corresponds to 2.6 coordinated water molecules. This value is in excellent agreement with 2.6 water molecules measured for the Eu(III)–ATP complex published in [12]. This low value of bound water mole-

cules to Cm(III) in the 1:1 complex, CmATP
, is difficult to explain by a pure binding of the Cm(III) to the oxygen atoms of the phosphate chain. We favor a structural model where Cm(III) binds to ATP through interactions with the phosphate chain and the N-7 and –NH2 of the adenine like suggested for the Fe(III)–ATP complex [9]. Table 2 gives an overview of relevant stability constants of actinide and lanthanide complexes with ATP in comparison with the complexation constants determined in the present work. As shown in Table 2, it exhibits a very limited knowledge concerning the complex formation reactions of actinide elements with ATP. The strength of the formation of the Ac–ATP complexes follows the pattern: þ Cm3þ > UO2þ 2  NpO2

Our results agree with the general assumption that Np(V) complexes ligands poorly. Due to the effective charge of the uranyl ion of +3.2 [36], one would expect that UO22+ forms the strongest species with ATP. However, the results of this study show slightly stronger complexes formed with Cm3+. One reason for this observation could be that steric effects of the O@U@O

Table 2 Relevant stability constants of lanthanide- and actinide–ATP complexes Complex

Method

log b

T (C)

I (M)

Reference

HATP3
H2ATP2
H3ATP


Potentiometry

6.87 11.34 13.51

25

0.1 (NaCl)

Oscarson et al. [27]

TRLFS

7.44 ± 0.44

25

0.1 (NaClO4)

Geipel et al. [19]

NpO2HATP NpO2ATP3


Potentiometry

8.87 ± 0.06 3.53 ± 0.04

25

0.1 (NaClO4)

Rizkalla et al. [18]

GdH2ATP+ GdHATP GdATP


Potentiometry

14.81 ± 0.06 11.54 ± 0.04 6.34 ± 0.04

25 ± 0.1

0.15 M (NaCl)

Bianchi et al. [14]

EuHATP EuATP


Potentiometry

10.16 6.31

25 ± 0.1

0.1 (NaClO4)

Shanbhag et al. [6]

CmH2ATP+ CmHATP CmATP


TRLFS

16.86 ± 0.09 13.23 ± 0.10 8.19 ± 0.16

25 ± 2

0.154 M (NaCl)

This work

UO2ATP2
2


H. Moll et al. / Inorganica Chimica Acta 358 (2005) 2275–2282

structure hamper interactions of U(VI) with the ring structure of the ATP molecule. Humic acids represent another group of organic molecules of great interest for environmental research. If one compares the stability constants of these actinides with humic acids, log bCmHA = 6.20 ± 0.15 [37], log bU(VI)HA = 6.16 ± 0.22 [38] and log bNp(V)HA = 3.50 ± 0.15 [39], a similar trend is observed. Furthermore, Cm(III) forms stronger complexes compared to those formed with the lanthanide elements Eu(III) and Gd(III), which serve as homolog for curium. One explanation could be the larger charge to radius ratio of Cm(III) [40]. In Fig. 7, the fluorescence emission spectra of CmH2ATP+, CmHATP, and CmATP
are compared to the spectrum of the Cm(III)–D. a¨spo¨ensis-surface complex [1]. The shape of the spectrum and the peak maximum, 600.1 nm, of the sorbed Cm(III) on the cell envelope of D. a¨spo¨ensis show a good agreement with those of the Cm–ATP species. Moreover, the fluorescence emission lifetime of the Cm(III)–D. a¨spo¨ensis-surface complex of 162 ± 5 ls lies in the same range like the lifetime of CmATP
, 187 ± 7 ls (see Table 1). In both cases, only approximately three water molecules remain in the first coordination sphere. These results support the conclusions drawn in [1] for a favored interaction of Cm(III) with organic phosphato-groups of the cell envelope of D. a¨spo¨ensis. Anyhow, for a detailed explanation of the binding mechanism, more investigations of many different model compounds are still necessary. To summarize: TRLFS in combination with the factor analysis software SPECFIT is a sensitive tool to investigate the speciation of Cm(III) in the aqueous ATP system. Three different Cm–ATP species, CmH2ATP+, CmHATP, and CmATP
, could be identified by their individual fluorescence emission spectra. The long fluorescence emission lifetime of 187 ± 7 ls determined for CmATP
gives evidence for interactions of Cm(III) with the phosphate chain and the ring structure of adenosine 5 0 -triphosphate. We have noticed the close similarity of

Fig. 7. Fluorescence emission spectra of CmH2ATP+, CmHATP, CmATP
and Cm–D. a¨spo¨ensis-surface complex [1] as derived by peak deconvolution; the spectra are scaled to the same peak area.

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the Cm–ATP and Cm–D. a¨spo¨ensis systems. In both systems, emission spectra (peak maximum and shape) and fluorescence lifetimes are comparable. This might indicate that Cm(III) forms similar structures after the complex formation reactions.

Acknowledgments This work was funded by the BMWi under contract number 02E9491. We thank Andre´ Roßberg for help and fruitful discussions in using the factor analysis program code designed for the analysis of EXAFS, UV–vis and TRLFS spectra. The authors are indebted for the use of the Cm-248 to the US Department of Energy, Office of Basic Energy Sciences, through the transplutonium element production facilities at Oak Ridge National Laboratory, which was made available as part of a collaboration between the Forschungszentrum Rossendorf (FZR) and the Lawrence Berkeley National Laboratory (LBNL).

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