Quantum receiver enhanced by adaptive learning.

Quantum receivers aim to effectively navigate the vast quantum-state space to endow quantum information processing capabilities unmatched by classical receivers. To date, only a handful of quantum receivers have been constructed to tackle the problem of discriminating coherent states. Quantum receivers designed by analytical approaches, however, are incapable of effectively adapting to diverse environmental conditions, resulting in their quickly diminishing performance as the operational complexities increase. Here, we present a general architecture, dubbed the quantum receiver enhanced by adaptive learning, to adapt quantum receiver structures to diverse operational conditions. The adaptively learned quantum receiver is experimentally implemented in a hardware platform with record-high efficiency. Combining the architecture and the experimental advances, the error rate is reduced up to 40% over the standard quantum limit in two coherent-state encoding schemes.

Calcium-binding properties of wild-type and EF-hand mutants of S100B in the presence and absence of a peptide derived from the C-terminal negative regulatory domain of p53.

S100B is a dimeric Ca(2+)-binding protein that undergoes a 90 +/- 3 degrees rotation of helix 3 in the typical EF-hand domain (EF2) upon the addition of calcium. The large reorientation of this helix is a prerequisite for the interaction between each subunit of S100B and target proteins such as the tumor suppressor protein, p53. In this study, Tb(3+) was used as a probe to examine how binding of a 22-residue peptide derived from the C-terminal regulatory domain of p53 affects the rate of Ca(2+) ion dissociation. In competition studies with Tb(3+), the dissociation rates of Ca(2+) (k(off)) from the EF2 domains of S100B in the absence and presence of the p53 peptide was determined to be 60 and 7 s(-)(1), respectively. These data are consistent with a previously reported result, which showed that that target peptide binding to S100B enhances its calcium-binding affinity [Rustandi et al. (1998) Biochemistry 37, 1951-1960]. The corresponding Ca(2+) association rate constants for S100B, k(on), for the EF2 domains in the absence and presence of the p53 peptide are 1.1 x 10(6) and 3.5 x 10(5) M(-)(1) s(-)(1), respectively. These two association rate constants are significantly below the diffusion control ( approximately 10(9) M(-)(1) s(-)(1)) and likely involve both Ca(2+) ion association and a Ca(2+)-dependent structural rearrangement, which is slightly different when the target peptide is present. EF-hand calcium-binding mutants of S100B were engineered at the -Z position (EF-hand 1, E31A; EF-hand 2, E72A; both EF-hands, E31A + E72A) and examined to further understand how specific residues contribute to calcium binding in S100B in the absence and presence of the p53 peptide.

Europium luminescence of EF-hand helix-turn-helix chimeras: impact of pH and DNA-binding on europium coordination.

A series of Eu(III) metallopeptides, designed on the basis of the structural similarity of the helix-turn-helix and EF-hand motifs, have been studied by Eu(III) (7)F(0) --> (5)D(0) excitation spectroscopy. The impact of EF-hand ligand set differences on the hydration number and Eu(III) coordination environment are compared among the peptides. The conditional binding affinities were determined by Eu titration (P3, log K(a) = 6.0 +/- 0.4; P3W, log K(a) = 5.9 +/- 0.2; P5b, log K(a) = 5.3 +/- 0.1). Two similar coordination environments occur in each case, consistent with structural flexibility about the metal site. The coordination environments are consistent with 8- or 9-coordinate Eu(III), including six peptide-based ligands and two to three water molecules (P3, q = 1.9 +/- 0.2; P3W, q = 2.3 +/- 0.2; P4a, q = 1.9 +/- 0.3; P5b, q = 2.6 +/- 0.2). The Eu(III) (7)F(0) --> (5)D(0) excitation spectra are pH-dependent, as reported for several EF-hand proteins (oncomodulin, parvalbumin). A higher energy transition occurs at pH > 6, and has been assigned to deprotonation of coordinated water. The pK(a) leading to this new transition is dependent on Eu(III) Lewis acidity, which varies with the inner and outer sphere ligand set. The noncoordinating ninth position of the Eu-binding loop, which is poised to make second-sphere contacts to the coordinated water, stabilizes the deprotonated form of the coordinated solvent more effectively when it is Thr (P5b) than Asp (P3W). Upon DNA-binding by the metallopeptides, the pK(a) of the pH-dependent peak increases, but no new DNA-dependent transitions are observed. This indicates no DNA-based Eu(III) ligands are introduced, such as phosphate oxygen atoms of the DNA backbone. The hydration number decreases in the presence of DNA (P3W + DNA, q = 1.9 +/- 0.2; P5b + DNA, q = 1.7 +/- 0.2), indicating that DNA-binding by the metallopeptides organizes rather than compromises the Eu-binding site within the peptide.

Lanthanide complexes of [alpha-2-P2W17O61]10-: solid state and solution studies.

We have isolated the 1:1 Ln:[alpha-2-P2W17O61]10- complexes for a series of lanthanides. The single-crystal X-ray structure of the Eu3+ analogue reveals two identical [Eu(H2O)3(alpha-2-P2W17O61)]7- moieties connected through two Eu-O-W bonds, one from each polyoxometalate unit. An inversion center relates the two polyoxometalate units. The Eu(III) ion is substituted for a [WO]4+ unit in the "cap" region of the tungsten-oxygen framework of the parent Wells-Dawson ion. The point group of the dimeric molecule is Ci. The extended structure is composed of the [Eu(H2O)3(alpha-2-P2W17O61)]214- anions linked together by surface-bound potassium cations. The space group is P, a = 12.7214(5) A, b = 14.7402(7) A, c = 22.6724(9) A, alpha = 71.550(3), beta = 84.019(3)degrees, gamma = 74.383(3), V = 3883.2(3) A3, Z = 1. The solution studies, including 183W NMR spectroscopy and luminescence lifetime measurements, show that the molecules dissociate in solution to form monomeric [Ln(H2O)4(alpha-2-P2W17O61)]7- species.

The interaction of MS-325 with human serum albumin and its effect on proton relaxation rates.

MS-325 is a novel blood pool contrast agent for magnetic resonance imaging currently undergoing clinical trials to assess blockage in arteries. MS-325 functions by binding to human serum albumin (HSA) in plasma. Binding to HSA serves to prolong plasma half-life, retain the agent in the blood pool, and increase the relaxation rate of water protons in plasma. Ultrafiltration studies with a 5 kDa molecular weight cutoff filter show that MS-325 binds to HSA with stepwise stoichiometric affinity constants (mM(-1)) of K(a1) = 11.0 +/- 2.7, K(a2) = 0.84 +/- 0.16, K(a3) = 0.26 +/- 0.14, and K(a4) = 0.43 +/- 0.24. Under the conditions 0.1 mM MS-325, 4.5% HSA, pH 7.4 (phosphate-buffered saline), and 37 degrees C, 88 +/- 2% of MS-325 is bound to albumin. Fluorescent probe displacement studies show that MS-325 can displace dansyl sarcosine and dansyl-L-asparagine from HSA with inhibition constants (K(i)) of 85 +/- 3 microM and 1500 +/- 850 microM, respectively; however, MS-325 is unable to displace warfarin. These results suggest that MS-325 binds primarily to site II on HSA. The relaxivity of MS-325 when bound to HSA is shown to be site dependent. The Eu(III) analogue of MS-325 is shown to contain one inner-sphere water molecule in the presence and in the absence of HSA. The synthesis of an MS-325 analogue, 5, containing no inner-sphere water molecules is described. Compound 5 is used to estimate the contribution to relaxivity from the outer-sphere water molecules surrounding MS-325. The high relaxivity of MS-325 bound to HSA is primarily because of a 60-100-fold increase in the rotational correlation time of the molecule upon binding (tau(R) = 10.1 +/- 2.6 ns bound vs 115 ps free). Analysis of the nuclear magnetic relaxation dispersion (T(1) and T(2)) profiles also suggests a decrease in the electronic relaxation rate (1/T(1e) at 20 MHz = 2.0 x 10(8) s(-1) bound vs 1.1 x 10(9) s(-1) free) and an increase in the inner-sphere water residency time (tau(m) = 170 +/- 40 ns bound vs 69 +/- 20 ns free).

A New Type of Heteropolyoxometalates formed from Lacunary Polyoxotungstate Ions and Europium or Yttrium Cations.

Four lacunary PW9 O349- ions (blue polyhedra) are held together by eight MIII (M = Y, Eu) ions and seven W atoms that form a [M8 W7 O30 ]6+ network (center) in the novel heteropolyoxometalates [(PM2 W10 O38 )4 (W3 O14 )]30- . In the solid state, the anions interact strongly with K+ countercations, which thus play a role in the formation of this particular structure.

Lanthanide Complexes of the alpha-1 Isomer of the [P(2)W(17)O(61)](10-) Heteropolytungstate: Preparation, Stoichiometry, and Structural Characterization by (183)W and (31)P NMR Spectroscopy and Europium(III) Luminescence Spectroscopy.

The alpha-1 and alpha-2 [P(2)W(17)O(61)](10)(-) isomers, derivatives of the Wells-Dawson molecule, [alpha-P(2)W(18)O(62)](6)(-), may be useful ligands for stabilizing high-valent metal ions and lanthanides and actinides. However, the potential utility of the [alpha1-P(2)W(17)O(61)](10)(-) ligand has not been realized. Specifically, for the lanthanides, the stoichiometry, structure, and purity of the lanthanide complexes of the [alpha1-P(2)W(17)O(61)](10)(-) isomer are ambiguous. We have prepared lanthanide (Ln) complexes of the [alpha1-P(2)W(17)O(61)](10)(-) isomer in >/=98% isomeric purity, according to (31)P NMR data. (183)W NMR data clearly showed, for the first time, that the C(1) symmetry of the [alpha1-P(2)W(17)O(61)](10)(-) lanthanide complexes was maintained in solution. We determined the stoichiometry of the lanthanide complexes of the [alpha1-P(2)W(17)O(61)](10)(-) isomer in solution by two different methods: a complexometric titration method and excited state lifetime measurements and luminescence titrations for the europium(III) analogue. All experiments show a 1:1 Ln:[alpha1-P(2)W(17)O(61)](10)(-) ratio. The (31)P NMR data showed that the lanthanides with smaller ionic radii (higher charge-size ratio) form stable complexes, even surviving crystallization from hot water. On the other hand, the lanthanum analogues were not stable in solutions of high lithium content. The tetrabutylammonium salt of the [Lu(alpha1-P(2)W(17)O(61))](7)(-) complex showed >/=98% isomeric purity and the C(1) symmetry required for a derivative of [alpha1-P(2)W(17)O(61)](10)(-). Also the tetrabutylammonium cation stabilized the [Lu(alpha1-P(2)W(17)O(61))](7)(-) complex; a mixed tetrabutylammonium, lithium salt was stable in water for weeks according to (31)P NMR spectroscopy.

Displacement of Inner-Sphere Water Molecules from Eu(3+) Analogues of Gd(3+) MRI Contrast Agents by Carbonate and Phosphate Anions: Dissociation Constants from Luminescence Data in the Rapid-Exchange Limit.

Europium(III) (7)F(0) --> (5)D(0) excitation spectroscopy is used to determine if the anions carbonate and phosphate present in physiological fluids are able to displace water molecules from the first coordination sphere of Eu(3+) analogues of Gd(3+) MRI contrast agents. A lengthening of the Eu(3+) excited state lifetime in the presence of millimolar concentrations of carbonate or phosphate indicates that water molecules are displaced by an anion. Only those metal complexes that contain negatively charged ligands and more than one water molecule in the first coordination sphere of Eu(3+) have their water molecules displaced by saturating concentrations of carbonate or phosphate. Conditional dissociation constants, K(d)'s, for Eu(3+)-ligand complexes with phosphate or carbonate are determined from titrations wherein the Eu(3+) excited state lifetimes are monitored. For phosphate, K(d)'s lie in the range 1.2-90 mM, whereas for carbonate, the range is 35-200 mM. The titrations also indicate that only a single anion binds to a metal chelate complex and that the single anion may, under saturating anion concentrations, displace on average more than one, but not all, first coordination sphere water molecules. Eu(3+ 7)F(0) --> (5)D(0) excitation spectra indicate that, in some cases, many different Eu(3+)-containing species are in fast exchange in the presence of added anion, presumably involving different numbers of first coordination sphere water molecules. Our results show that, under physiological conditions, phosphate and carbonate will, on average, displace less than half of a water molecule from the first coordination sphere of a typical contrast agent and suggest that the effect on proton spin relaxation is likely to be minimal.

Effect of Mixed Pendent Groups on the Solution and Catalytic Properties of Europium(III) Macrocyclic Complexes: Bifunctional and Monofunctional Amide and Alcohol Pendents in Septadentate or Octadentate Ligands.

Five new Eu(III) macrocyclic complexes have been prepared and their solution and catalytic properties studied. The Eu(III) complexes with septadentate ligands TRED and NB-TRED dissociate rapidly at pH 7.4, 37 degrees C (TRED = 1,4,7-tris(hydroxyethyl)-1,4,7,10-tetraazacyclododecane, NB-TRED = 1-(nitrobenzyl)-4,7,10-tris(hydroxyethyl)-1,4,7,10-tetraazacyclododecane). Dissociation rates as determined in the presence and absence of strongly binding competing ligands suggest that under most conditions the Eu(III) complexes of ATHC, ABHC, and CNPHC are more kinetically inert to dissociation than is the Eu(III) complex containing all hydroxyethyl groups (ATHC = 1-(carbamoylmethyl)-4,7,10-tris(hydroxyethyl)-1,4,7,10-tetraazacyclododecane, ABHC = 1,7-bis(carbamoylmethyl)-4,10-bis(hydroxyethyl)-1,4,7,10-tetraazacyclododecane, CNPHC = 1-(1-carboxamido-3-(4-nitrophenyl)propyl)-4,7,10-tris(2-hydroxyethyl)-1,4,7,10-tetraazacyclododecane). Laser-induced luminescence excitation spectra of Eu(III) complexes of ABHC, ATHC, CNPHC, THED, and S-THP suggest that there is a single major species in solution at pH 6.3 and a second species that appears at more basic pH values (THED = 1,4,7,10-tetrakis(hydroxyethyl)-1,4,7,10-tetraazacyclododecane, S-THP = 1S,4S,7S,10S-tetrakis(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane). The species present at basic pH is proposed to be an alkoxide or hydroxide complex; pK(a) values as determined by potentiometric titrations are 7.5 and 8.1 for Eu(CNPHC)(3+) and Eu(ABHC)(3+), respectively. Eu(CNPHC)(3+), Eu(ATHC)(3+), and Eu(ABHC)(3+) promote transesterification of the hydroxypropyl ester of 4-nitrophenyl phosphate with pseudo-first-order rate constants at pH 7.3, 37 degrees C, and 1.00 mM complex of 1.4 x 10(-)(5), 9.3 x 10(-)(6), and 1.0 x 10(-)(6) s(-)(1), respectively. Both Eu(CNPHC)(3+)and Eu(ABHC)(3+) promote attack of an hydroxyethyl group of the macrocycle on bis(4-nitrophenyl) phosphate with pseudo-first-order rate constants at pH 7.3, 37 degrees C, and 1.00 mM complex of 1.5 x 10(-)(4) and 3.5 x 10(-)(5) s(-)(1), respectively. In general, an increase in the number of amide groups on the macrocycle of the Eu(III) complex decreases the rate of both intramolecular or intermolecular phosphate diester transesterification reactions.

Kinetics of Formation of Ca(2+) Complexes of Acyclic and Macrocyclic Poly(amino carboxylate) Ligands: Bimolecular Rate Constants for the Fully-Deprotonated Ligands Reveal the Effect of Macrocyclic Ligand Constraints on the Rate-Determining Conversions of Rapidly-Formed Intermediates to the Final Complexes.

The apparent bimolecular rate constants, k(1) (M(-)(1) s(-)(1)), for the formation of Ca(2+) complexes of a series of acyclic (edta, egta, cdta) and macrocyclic (dota, teta, do3a) poly(amino carboxylate) ligands were determined in the pH range 7-13 using the fluorescent ligand quin2 in a stopped-flow apparatus to monitor the ligand competition reaction. The k(1) values are observed to reach maximum constant values at high pH, characteristic of reactions involving the fully-deprotonated ligand species. Bimolecular formation constants k(Ca)(L), k(Ca)(HL), and k(Ca)(H)()2(L), characteristic of the reaction of the fully-deprotonated and mono- and diprotonated ligands, respectively, were derived from the pH dependence of the k(1) values. The k(Ca)(L) values of the acyclic ligands are edta, 4.1 x 10(9) M(-)(1) s(-)(1); egta, 2.1 x 10(9) M(-)(1) s(-)(1); and cdta, 2.3 x 10(9) M(-)(1) s(-)(1), while the corresponding values for the macrocyclic ligands are dota, 4.7 x 10(7) M(-)(1) s(-)(1); teta, 1.1 x 10(7) M(-)(1) s(-)(1); and do3a, 1.0 x 10(8) M(-)(1) s(-)(1). The smaller values for the macrocyclic ligands are consistent with ligand-dictated constraints imposed on the conversion of a stable intermediate to the final complex, a process which involves the simultaneous stripping of several water molecules from the first-coordination sphere of the Ca(2+) ion.

Kinetics of the Formation of Macrocyclic Polyaminocarboxylate Ligand Complexes: A Laser-Excited Luminescence Study of the Eu(3+)-dtpa-dien System.

Excitation spectroscopy of the (7)F(0) --> (5)D(0) transition of Eu(3+) is used to detect and characterize a kinetic intermediate in the formation of a complex between Eu(3+) and the macrocyclic ligand dtpa-dien (1,4,7-tris(carboxymethyl)-9,17-dioxo-1,4,7,10,13,16-hexaazacyclooctadecane). Both the long-lived intermediate and the final product, [Eu(Hdtpa-dien)(H(2)O)](+), are formed immediately upon mixing the components, as evidenced by separate peaks in the excitation spectrum. The transformation of the intermediate to the final product, monitored by excitation spectroscopy, occurs by both proton-assisted and non-proton-assisted pathways. It is proposed that the intermediate represents a "blind alley" in the pathway to a final nine-coordinate tricapped trigonal prismatic product. The intermediate with an "up, down, up" configuration of carboxylate arms must undergo some decoordination to form the final complex. Molecular mechanics calculations and excited state lifetimes suggest that the intermediate is eight-coordinate with one coordinated water molecule. The stability constant of the final complex is found to be log K = 14.11 +/- 0.05.