Structure of L-tyrosyl-L-leucine monohydrate

C15H22N204.H20 , Mr= 312.37, monoclinic,P21, a=5.577(2), b=8.686(2), c= 16.228 (2) A,fl=92.63(2) ° , V=785(1)A 3, Z=2, O =1.34,Dx= 1.32Mgm -3, CuKa, 2= 1.54184'~, /2=0.78 mm -I, F(000) = 320, T= 293 K. The final R value for 1607 observed reflections ll,,>_3tr(l,,)l is 0.039. The terminal N 1 is protonated and the dipeptide exists as a zwitterion. The crystal structure is stabilized by extensive hydrogen-bonding interactions involving N and O atoms, with N...O in the range 2.65 (1)-2.95 (1) ,/~ and O...O in the range 2.60 (1)-2.78 (1) A.

Structure of L-Arginyl-L-aspartie Acid Dihydrate

C~0H~gN5Os.2H20, Mr=325.32, monoclinic,P2~, a = 12.029 (2), b=4.904 (2), c=13.215 (2) A, fl= 107.68 (2) ° , F= 743 (1) A 3, Z= 2,D m = 1-45, D x = 1.45 Mg m -3, Cu Ka, 2 = 1.54184 A,fl= 1.01mm -1, F(000)=348, T=293K. The final R value for 1277 observed reflections 110 >_ 3tr(Io)l is 0.031. The dipeptide exists as a zwitterion. The arginyl side-chain conformation is similar to that found in arginyl-glutamic acid [Pandit, Seshadri & Viswamitra (1983). Acta Cryst. C39, 1669-16721. The guanidyl group forms a pair of hydrogen bonds with oxygen atoms of the backbone carboxyl group. The crystal structure is also stabilized by -bonding interactions involving both water molecules.

Structure of N2-carbamoyl-L-asparagine

CsH9N304, M r= 175.1, orthorhombic,P212~2 ~, a = 7.486 (1), b = 9.919 (2), c =20.279 (2) A, V= 1505.8 A 3, z = 8, D x = 1.54, D m = 1.60 Mg m -3, ~,(Cu Ka) = 1.5418 A, g = 1. I I mm -~, F(000) = 736, T = 300 K, final R = 0.032 for 1345 observed reflections. The two independent molecules in the asymmetric unit are related by a pseudo twofold axis, with the asparagine side chains having different conformations [X 2 being -132.1 (3) and 139.6 (2)°]. The crystal structure is stabilized by extensive hydrogen bonding, with a specific interaction between the carboxyl group of one molecule and the carbamoyl group of another forming hydrogen-bonded chains.

Structure-Reactivity Correlations of Benzoin Alkyl Ethers. Structures of 2-Methoxy-1,2-diphenylethanone (I) and 2-Isopropoxy-1,2-diphenylethanone (II)

(I): C15H1402, Mr---226.27, triclinic, Pi,a=8.441 (2), b= 10.276 (1), c= 15.342 (2)A, a=91.02 (2), ~ t= 79.26 (2), y= 105.88 (2) °, V=1256.8 (4)A 3, Z=4, D,,= 1.209 (flotation in KI),D x - 1.195 g cm -3, #(Mo, 2 = 0.7107/~) = 0.44 cm -~,F(000) = 480, T= 293 K, R -- 0.060 for 1793 significant reflections. (II): C~THlsO2, Mr= 254.83, orthorhombic, Pca21, a=8.476 (1); b= 16.098 (3), c=10.802(3)A, V=1473.9 (5) A s, Z=4, Dm=1.161 (flotation in KI), Dx= 1.148gem -3, /~(Mo, 2=0.7107 A) =0.41 cm -~, F(000) = 544, T= 293 K, R = 0.071 for 867 significant reflections. Both (I) and (II) crystallize in a cisoid conformation for the carbonyl group and alkoxy groups. Compounds (I) and (II) are photostable on irradiation in the solid state in spite of the favourable conformation of the functional groups for intramolecular H abstraction. Absence of photoreaction of (I)and (II) in the solid state is rationalized in the light of unfavourable intramolecular geometry.

Structure, stoichiometry and spectroscopy of oxide superconductors

In the new oxide superconductors, structure and oxygen stoichiometry play the most crucial role. Thus, all the high-temperature oxide superconductors are orthorhombic perovskites with low-dimensional features. Oxygen stoichiometry in YBa2Cu3O7-δ has an important bearing on the structure as well as superconductivity. This is equally true in the La3-xBa3+xCu 6O14+δ system of which only the 123 oxide (x = 1) with the orthorhombic structure shows high Tc. Orthorhombicity though not essential, is generally found ; it is necessary for the formation of twins. The nature of oxygen and copper in the cuprates has been examined by electron spectroscopy. Copper in these cuprates is only in 1 + and 2 + states. It seems likely that oxygen holes are responsible for superconductivity of the cuprates as well as Ba(Bi, Pb)O3. High Tc superconductivity is also found in oxides of the Bi-(Ca, Sr)-Cu-O and related oxides possessing Cu-O sheets.