The mechanical properties of single-quasicrystals of decagonal AlCoCuSi have been studied for the first time by applying the Vickers indentation method. The hardness has been determined as H ~ 9.6 MPa. Estimates for the modulus of elasticity and the fracture toughness are given. The quasicrystals are highly strained internally. Scratching experiments show slight anisotropies in the abrasive friction coefficient. The dominant abrasive mechanism is microplowing.
Hydrazide-and amide-linked oligonucleoside analogues with integrated bases and backbone were designed to allow for a rapid synthesis of long and water-soluble oligomers. The uracil-, cytosine-, and adenine-derived hydrazide building blocks 13 -15 were synthesized by nucleophilic substitution with the hydrazine 23 of the halides 19, 28, and 34, derived from the alcohols 18, 27, and 33, respectively, while the uracil-, cytosine-, and adenine-derived amide building blocks 45 -47 were synthesized by a Curtius degradation of the carboxylic acids 51, 56, and 61. These acids were obtained by Wittig reaction of the aldehydes 49, 53, and 58. The guanine-derived monomers 44 and 48 were synthesized by reductive cyclisation of the nitroso amides 38 and 63, respectively, resulting from acylation of the known 2,6diamino-4-(benzyloxy)-5-nitrosopyrimidine (37).Stacking of the nucleobases and the right geometry for Watson -Crick base pairing required adjusting the torsion angles k, x, and z from À 50, 180, and À 90 to À 70, þ 70, and À 1008, respectively ( Fig. 1, b ! c). We assumed that changing k and z from their energetically ideal values will result in only little torsional strain, and that the energy to be paid would be overcompensated by the H-bonds between the nucleobases [11] [25], and by base stacking [26]. However, calculations 7 ) to evaluate the energy to be paid for the adjustment of the torsion angle x from 1808 to þ 708 gave contradictory results 8 ). It is thus not possible to specify an overall energy value to be paid for the conformational transition from conformer b to c in Fig. 1.Modelling studies of various dimers suggested that stacking interactions depend on the sequence, and decrease in the order UA % UU % AA > AU. To evaluate the formation of duplexes between longer oligonucleotide analogues, we minimised the conformational energy of the octamer U 4 A 4 (Fig. 3) 9 ). The model suggested Watson -Crick-type base pairing and base stacking, with the hydrazide linker adopting a conformation with torsion angles k -z as deduced for the cyclic duplex (Fig. 1, c). The Helvetica Chimica Acta -Vol. 92 (2009) 1138. Model of the hydrazide-linked octamer U 4 A 4 . a) Side view. b) Top view. 7 ) Ab initio calculations using Spartan 06 and the 6-31G* basis set. 8 ) The conformational energies of N-methyl-2-(6-methyluracil-1-yl-)acetamide and N-methyl-2-(8methyladenine-9-yl-)acetamide were minimised, resulting in an energy minimum of x ¼ À 708 (cf. experimentally favoured: x ¼ 1808). The value of x ¼ 708, required for duplex formation, corresponds to a calculated energy maximum. 9 ) Amber* force-field calculation using Macromodel 7.0, with the H-bonds between the nucleobases constrained.
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