The interaction between hydrogen and platinum is studied in n-and p-type silicon using deep-level transient spectroscopy. Hydrogen is introduced by wet-chemical etching or during crystal growth. In both cases we find that hydrogen forms only electrically active complexes with platinum. Four platinum-hydrogen related deep levels are identified: E͑90͒ at E C Ϫ0.18 eV, E͑250͒ at E C Ϫ0.50 eV, H͑150͒ at E V ϩ0.30 eV, and H͑210͒ at E V ϩ0.40 eV. These levels belong to at least three different platinum-hydrogen complexes. Level E͑250͒ is identical to the so-called midgap level in Pt-doped Si, which is believed to control the minority-carrier lifetime in Pt-doped silicon. Level H͑150͒ is an acceptor and is present both in n-and p-type samples after hydrogenation. It belongs to a platinum-hydrogen complex which contains more hydrogen atoms than the complexes responsible for the other hydrogen-related levels. Annealing at temperatures above 600 K results in a complete dissociation of all the platinum-hydrogen related defects and the substitutional platinum concentration is fully restored.

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The creation of defects by hydrogen in silver-doped silicon crystals is investigated by deep-level transient spectroscopy. The electrical activity of the substitutional silver impurities can be totally removed due to defect formation with hydrogen atoms. However, this process includes the creation of intermediate electrically active silver-hydrogen complexes. One of the defects, Ag-H 1 , contains one hydrogen atom and introduces three levels in the energy gap. Another electrically active complex is formed by addition of a second hydrogen atom to the Ag-H 1 defect. The Ag-H complexes are stable up to 300-350°C. The electrically inactive complex includes at least three hydrogen atoms and anneals out at ϳ450°C. The kinetics of the defect transformations are studied in detail, and the distance of silver-hydrogen interaction is estimated to be very close to the lattice parameter. ͓S0163-1829͑99͒02308-5͔

The influence of the myelin proteolipid apoprotein on lipid chain order and dynamics was studied by 2H NMR of membranes reconstituted with specifically deuterated dimyristoyl phosphatidylcholines. Quadrupolar echo and saturation recovery experiments were fitted by numerical solution of the stochastic Liouville equation, using a model that includes both inter-and intramolecular motions [Meier, P., Ohmes, E. & Kothe, G. (1986) J. Chem. Phys. 85,[3598][3599][3600][3601][3602][3603][3604][3605][3606][3607][3608][3609][3610][3611][3612][3613][3614]. Combined simulations of both the relaxation times and the quadrupolar echo line shapes as a function of pulse spacing allowed unambiguous assignment of the various motional modes and a consistent interpretation of data from lipids labeled on the C-6, C-13, and C-14 positions of the sn-2 chain. In the fluid phase, the protein has little influence on either the chain order or the population of gauche rotational isomers but strongly retards the chain dynamics. Lipid-protein interactions are important determinants of biological membrane structure and function and, for this reason, have been the subject of intensive study by physicochemical methods. Magnetic resonance spectroscopy has made major contributions in this area because of its unique sensitivity to anisotropic molecular motion. and 3).The complexity of the systems involved dictates that a detailed description of the effects of integral membrane proteins on lipid chain dynamics can best be achieved by a combination of multipulse NMR experiments with comprehensive theoretical simulations. Such an analysis is currently lacking. In the present work, we have investigated myelin proteolipid apoprotein reconstituted with specifically deuterated dimyristoyl phosphatidylcholine ([Myr2]PtdCho) as a model system for lipid-protein interactions in biological membranes. A motional model has been employed that includes both inter-and intramolecular motion (i.e., both long-axis motion and trans-gauche isomerization) and which is valid in both fast and slow motional regimes (4,5). The simulation of quadrupole echo spectra as a function of pulse spacing, together with measurements of spin-lattice relaxation time (Tz), allows discrimination of the different motional modes, leading to an unambiguous description of the molecular dynamics. A consistent interpretation is obtained of data from reconstitutions with [Myr2]PtdCho labeled at the C-6, -13, and -14 atoms of the sn-2 chain. It is found that the protein has very little effect on either the degree of order of the lipid chain or the population of gauche rotational isomers but increases the rotational correlation times for chain fluctuation, chain rotation, and trans-gauche isomerism by a factor of 10 or more. These results are fully consistent with those obtained from ESR spectroscopy of spin-labeled lipids (6, 7). 3704The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordan...