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21.
Ignacy Z. Siemion 《Magnetic resonance in chemistry : MRC》1971,3(5):545-550
The NMR-spectra of cis- and trans-isomers of proline-containing diketopiperazines were investigated. It was demonstrated that the differences in the spectra are connected to the different position (pseudoaxial or pseudoequatorial) of the substituent (α-hydrogen atom, α-methyl-group, α-isopropyl-group). 相似文献
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Hajós conjectured that everys-chromatic graph contains a subdivision ofK
s, the complete graph ons vertices. Catlin disproved this conjecture. We prove that almost all graphs are counter-examles in a very strong sense. 相似文献
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The conformation of the methionine side-chain in methanolic solution does not differ from that of norleucine; some differences, however, were observed for S-methylcysteine and norvaline derivatives. It was also shown that the temperature coefficients of the CH2 group resonances could be used for the assignment of the HR-β and HS-β signals. 相似文献
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Two cyclic peptide like compounds, cyclo-anthranoyl-L- prolyl (1) and cyclo-homoanthranoyl-L-prolyl (2) have been synthesized and investigated by UV spectroscopy and measurement of circular dichroism. Compound 1 of entirely rigid conformation with two N-CO groups conjugated to the aromatic ring, Ar-NH-CO left handed helical, Ar-CO-N right handed helical, shows a very strong positive Cotton effect centered around 250 nm and a strong negative one centered around 227 nm. Compound 2, which has only one conjugated skewed electron system, Ar-NH-CO, can exist in two different stable conformations, one of them being left handed, the other one right handed helical. It also shows a strong positive Cotton effect centered around 235 nm. Since from earlier 13C NMR data the latter was concluded to be the preferred conformation, a right handed helical sense of the amide bond-aromatic ring system is likely to correlate with a positive Cotton effect of this inherently dissymmetric chromophore. 相似文献