First measurement of the polarisation transfer on the proton in the reactionsH(\vec e,e'\vec p) andD(\vec e,e'\vec p)

The measurement of the polarisation transfer to the proton in the reactions $H(\vec e,e'\vec p)$ and $D(\vec e,e'\vec p)$ performed with longitudinally polarised electrons in quasi-free kinematics is presented. The coincidence measurement was executed atQ ²≈8fm ^?2 using the 855 MeV, c.w. beam of the Mainz Microtron MAMI. The recoil polarisation was determined by means of a carbon analyser. The experiment shows that the binding of the nucleon does not modify the polarisationP _x of the recoil proton within an error ofΔ P _x/P_x≈10%. The measured polarisation agrees with recent theoretical predictions. Implications for the measurement of the electric form factor of the neutron using the $D(\vec e,e'\vec n)$ reaction are discussed.     

VUV-transitions in ionic rare-gas alkali molecules

Mixtures of rare-gases (Rg) and alkali vapors (A) were excited by helium and argon ions. VUV transitions observed between 131.1 nm and 189.9 nm were ascribed to the decay of (RgA)⁺ ions where Rg=Kr, Xe and A=Cs, Rb, K, Na, Li. In the (ArA)⁺ systems (A=Rb, K, Na, Li) transitions appeared between 114.1 nm and 124.7 nm. Since the final state is only weakly bound, the (RgA)⁺ ions seem to be promising candidates for storage media of VUV lasers.     

Synthesis and NMD A Antagonistic Properties of the Enantiomers of 4-(3-phosphonopropyl)piperazine-2-carboxylic acid (CPP) and of the unsaturated analogue (E)-4-(3-phosphonoprop-2-enyl)piperazine-2-carboxylic acid (CPP-ene)

The (R)- and (s)-enantiomers of 4-(3-phosphonopropyl)piperazine-2-carboxylic acid (D - and L -CPP, resp.; 15 and 16 , resp.), and of its unsaturated analogue (E)-4-(3-phosphonoprop-2-enyl)piperazine-2-carboxylic acid (D - and L -CPP-ene, resp.; 13 and 14 , resp.) were prepared. The absolute configuration of the enantiomers was determined by a chemical correlation of the menthyl ester 7 with D -asparagine. The affinity of these derivatives for the NMDA receptor was determined by displacement of [³H]CPP in rat cerebral cortical membranes. In two functional tests (the frog hemisected spinal cord preparation and the sodium efflux test from rat brain slices), D -CPP-ene appears to be the most potent, enantiomerically pure, competitive NMDA antagonist known to date.     

Probing the Watson-Crick, wobble, and sugar-edge hydrogen bond sites of uracil and thymine

The nucleobases uracil (U) and thymine (T) offer three hydrogen-bonding sites for double H-bond formation via neighboring N-H and C=O groups, giving rise to the Watson-Crick, wobble and sugar-edge hydrogen bond isomers. We probe the hydrogen bond properties of all three sites by forming hydrogen bonded dimers of U, 1-methyluracil (1MU), 3-methyluracil (3MU), and T with 2-pyridone (2PY). The mass- and isomer-specific S1 <-- S0 vibronic spectra of 2PY.U, 2PY.3MU, 2PY.1MU, and 2PY.T were measured using UV laser resonant two-photon ionization (R2PI). The spectra of the Watson-Crick and wobble isomers of 2PY.1MU were separated using UV-UV spectral hole-burning. We identify the different isomers by combining three different diagnostic tools: (1) Selective methylation of the uracil N3-H group, which allows formation of the sugar-edge isomer only, and methylation of the N1-H group, which leads to formation of the Watson-Crick and wobble isomers. (2) The experimental S1 <-- S0 origins exhibit large spectral blue shifts relative to the 2PY monomer. Ab initio CIS calculations of the spectral shifts of the different hydrogen-bonded dimers show a linear correlation with experiment. This correlation allows us to identify the R2PI spectra of the weakly populated Watson-Crick and wobble isomers of both 2PY.U and 2PY.T. (3) PW91 density functional calculation of the ground-state binding and dissociation energies De and D0 are in agreement with the assignment of the dominant hydrogen bond isomers of 2PY.U, 2PY.3MU and 2PY.T as the sugar-edge form. For 2PY.U, 2PY.T and 2PY.1MU the measured wobble:Watson-Crick:sugar-edge isomer ratios are in good agreement with the calculated ratios, based on the ab initio dissociation energies and gas-phase statistical mechanics. The Watson-Crick and wobble isomers are thereby determined to be several kcal/mol less strongly bound than the sugar-edge isomers. The 36 observed intermolecular frequencies of the nine different H-bonded isomers give detailed insight into the intermolecular force field.