The central event in thymic selection of T cells bearing alpha beta TCRs is their interaction with self-peptides bound to self-MHC molecules. With the use of transgenic mouse lines expressing a single peptide/MHC class II complex, we show that CD4+ T cells with the preferential usage of particular TCR V(alpha)s and V(beta)s were selected to mature on this complex in lines with the lower expression, whereas such CD4+ T cells were eliminated in the thymus in a line with the relatively high expression. When a low expressing line was crossed with a high expressing line, the frequency of CD4+ T cells selected by this complex markedly decreased. Thus, these results suggest that a single peptide/MHC class II complex, being affected by its cell surface density in the thymus, can serve as both positively and negatively selecting ligand in vivo.

Chemical mechanical polishing of ͑0001͒ GaN has been demonstrated with sodium-hypochlorite-based solutions. Slurries including alumina abrasive provide an efficient means of planarization for both the Ga-and N-face that does not induce significant crystalline damage. Removal rates were found to be ϳ50 nm/min and were equivalent for both polarities. Additionally, a fine polishing method was developed that includes abrasive-free solutions of either sodium hypochlorite for the N-face or a mixture of sodium hypochlorite and citric acid for the Ga-face. With this fine polishing step, scratch-free surfaces were achieved with a root-mean-square roughness of 0.5-0.6 nm.Gallium-nitride-based structures are recognized as one of the most promising materials for short wavelength optoelectronic devices and high-power, high-frequency electronic devices. However, the potential of this material has been limited by the lack of a suitable lattice matched substrate for the epitaxially grown device layers. This has led to the development of bulk GaN substrates. Homoepitaxial devices fabricated on bulk substrates can exhibit an order of magnitude lower dislocation density compared to heteroepitaxially grown devices and have been shown to exhibit superior performance. 1,2 With the development of these substrates, surface preparation techniques must also be investigated to provide atomically smooth, damage-free surfaces, such as chemical mechanical polishing ͑CMP͒. Additionally, alternative processes that may further expand GaN technologies, including wafer bonding and layer transfer techniques, often require planarization steps creating a need for a well-controlled GaN CMP process. [3][4][5] CMP uses a combination of chemical and mechanical reactions to remove material leaving a planarized, damage-free surface. Ideally, material removal is achieved by chemically altering the surface to a mechanically weaker form; this material is then abraded from the surface leaving the bulk undisturbed. Planarization occurs due to the acceleration of both mechanical grinding and chemical transformation at the high points. Previous GaN CMP studies experimented with sodium or potassium hydroxide with silica abrasive slurries. 6,7 Although these slurries were able to achieve smooth surfaces over small areas on the N-face, the Ga-face showed no alteration. This was attributed to the fact that the Ga-face is more chemically inert than the N-face, making material removal by CMP of this face difficult. Additionally, the damage induced to the surface of the material by the KOH or NaOH solutions was not addressed. In this study, planarization of bulk GaN is carried out with a sodiumhypochlorite-based slurry to obtain a damage-free process that can be used for either polarity. Sodium hypochlorite is a strongly oxidizing agent that has been successfully used to polish other III-V materials. 8-10 Experimental 1 ϫ 1 cm GaN substrates were provided by Kyma Technologies. As-received, the Ga-face was ground so that it was not specular and the N-face was epiready po...

To develop an improved synthetic route to [3(6)](1,2,3,4,5,6)cyclophane (CP) 2, a more practical synthetic route to [3(5)](1,2,3,4,5)CP 3 than the original one was developed, which started from [3(2)](1,3)CP 7 via [3(4)](1,2,4,5)CP 5. The fundamental structural parameters of [3(n)]CPs (n = 3-6) in the solid state were elucidated, and the observed structures were in good agreement with the most stable conformers in solution and those predicted by the theoretical calculations. In the case of [3(6)]CP 2, the most stable C(6)(h) structure was observed in the crystal structure of the 2-TCNQ-F(4) (1:1) complex, whereas the highly strained structure with a D(6)(h) symmetry was observed in the crystal structure of 2 and the 2:TCNQ:benzene (1:1:1) complex because of a severe disorder problem. [3(n)]CPs (n > 3) showed reversible redox processes, and 2 (+0.39 V vs F(c)/F(c)(+), Cl(2)CHCHCl(2)) showed the lowest first half-wave oxidation potential [E(1/2) (I)] in [3(n)]CPs. The E(1/2) (I) data support the strong donating ability of 2 and its lower homologues. This is attributed to their molecular structures where effective hyperconjugation between the benzyl hydrogens and benzene ring is possible. By taking advantage of the strong electron-donating ability of [3(n)]CPs, their CT complexes with TCNE, TCNQ, and TCNQ-F(4) were prepared, and their crystal structural properties were examined. The single-crystal conductivity data of the CT complexes indicated that the TCNQ-F(4) complexes showed higher conductivities than the corresponding TCNQ complexes mainly due to a larger charge separation. Among the [3(n)]CP-TCNQ complexes, the [3(3)](1,3,5)CP 6-TCNQ-F(4) (1:1) complex showed the highest conductivity (10(-)(4) S cm(-)(1)), and this was ascribed to the formation of an infinite column of partially overlapped acceptors with a short acceptor-acceptor distance, while the formation of such a column was not observed in the 2-TCNQ-F(4) complex. Although the conductivities of the cyclophane-CT complexes are much lower than those of the TTF related complexes, this study successfully provides the basic knowledge for understanding the CT interactions in the solid state.

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The T cell repertoire is shaped by positive and negative selection of thymocytes through the interaction of α/β-T cell receptors (TCR) with self-peptides bound to self-major histocompatibility complex (MHC) molecules. However, the involvement of specific TCR-peptide contacts in positive selection remains unclear. By fixing TCR-β chains with a single rearranged TCR-β irrelevant to the selecting ligand, we show here that T cells selected to mature on a single MHC–peptide complex express highly restricted TCR-α chains in terms of Vα usage and amino acid residue of their CDR3 loops, whereas such restriction was not observed with those selected by the same MHC with diverse sets of self-peptides including this peptide. Thus, we visualized the TCR structure required to survive positive selection directed by this single ligand. Our findings provide definitive evidence that specific recognition of self-peptides by TCR could be involved in positive selection of thymocytes.

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[structure: see text] Trifluoro- and hexafluoro[3(3)](1,3,5)cyclophanes 3 and 4 were synthesized with TosMIC coupling as a key reaction. The pi-pi absorption bands show blue shifts as the number of fluorine atoms is increased. In the crystalline state, characteristic stacking with the fluorinated benzene rings facing each other is observed in both cases.