Left-handed Chains Research Articles

Refinement of the Crystal Structures of Forms I and II of Isotactic Polybutene-1 and a Proposal of Phase Transition Mechanism between Them

The new crystal structure models of forms I and II of isotactic polybutene-1 have been proposed by analyzing the 2-dimensional X-ray diffraction data measured for the highly oriented samples of almost pure crystal forms. The crystal form I was found to take the hexagonal packing structure of the (3/1) helices with the space group P3, different from the previously reported R3c or R3c models. The right- and left-handed chains are packed alternately with the random directionality along the chain axis. The crystal form II was concluded to take the tetragonal unit cell of the (11/3) helical chains, the space group of which is P4b2. The right-handed (left-handed) chains are positioned at one site with the statistical disorder of upward and downward directionality along the chain axis. The time-dependent electron diffraction measurement showed that the crystal lattices of forms I and II are related to each other with the common 110 plane boundary, as already reported by the other researchers. By referring to ...

Structures and luminescent properties of a series of Ln–Ag heterometallic coordination polymers

Eight new 4f–4d heterometallic coordination polymers have been synthesized using pyridine-2,6-dicarboxylic acid (H2PDA) as ligand under hydrothermal conditions. {[PrAg(PDA)2(H2O)3]·2.5H2O}n (1) comprises a 2D grid-like net, and the grid structure is assembled through a Pr4Ag2 motif as the building block. The subsequent six complexes [LnAg(PDA)2(H2O)3]·3H2O}n (Ln = Ce3+, 2; Nd3+, 3; Sm3+, 4; Eu3+, 5; Tb3+, 6; Dy3+, 7) are 1D double-chain coordination polymers. In 2–7, the Ln3+ and Ag+ ions are linked by carboxylic O bridges to form the helical chains, while the right-handed and left-handed chains are linked through carboxylic O bridges to form the double-helical chain. Additionally, the lattice H2O molecules linked by hydrogen bonds construct the helical water chain. {[CeAg2(PDA)3(Py)4]·H2O}n (8) (Py = pyridine) contains one Ce4+ ion and displays a 1D wave-like chain, in which partial H2PDA ligands were decomposed to pyridine. Luminescent studies reveal that the emission intensity of 7 significantly increases with the introduction of Mg2+ ions into the system of 7, whereas the addition of other metal ions causes emission intensity to remain the same or even decrease. Therefore, complex 7 may serve as a luminescent probe of Mg2+.

A single-helix coordination polymer with mixed ligands [Zn 2(phen) 2( e, a- cis-1,4-chdc) 2(H 2O) 2] n (phen=1,10-phenanthroline; chdc=cyclohexanedicarboxylate)

A coordination polymer with mixed ligands [Zn 2(phen) 2( e, a- cis-1,4-chdc) 2(H 2O) 2] n (chdc=cyclohexanedicarboxylic acid; phen=1,10-phenanthroline) was prepared under hydrothermal conditions and characterized by elemental analyses, IR spectra, TG analysis, and single-crystal X-ray diffraction analysis. X-ray crystal structural analyses reveal that 1 and 2 are isomorphic and belong to the monoclinic system. C 40H 36Zn 2N 4O 10, P2 1/c , a=10.084(2) Å, b=8.9072(18) Å, c=20.276(4) Å, β=99.92(3)°, V=1793.9(6) Å 3, Z=1. In the structures of 1, the 1,4-chdc ligand possesses only one kind of e, a- cis-conformation although there are both cis- and trans-conformations in the raw material. Two oxygen atoms of one carboxyl in 1,4-chdc ligand and another oxygen atom of contraposition carboxyl link adjacent Zn atoms into an infinite 1D zigzag chain. The most attractive structural feature of 1 is that it exhibits an infinite chiral chain-like structure with 2 1 helices along the b-axis. In addition, the right- and left-handed chains are alternate. Meanwhile, the adjacent chains of 1 is linked via hydrogen bonds into 2D network structures, which further form 3D frameworks via π–π interactions of 1,10-phen.

A new type of single-helix coordination polymer with mixed ligands [M2(phen)2(e,a-cis-1,4-chdc)2(H2O)2]n (M=Co and Ni; phen=1,10-phenanthroline; chdc=cyclohexanedicarboxylate).

A new family of single-stranded helices coordination polymers with mixed ligands, [M2(phen)2(e,a-cis-1,4-chdc)2(H2O)2]n (1, M=Co; 2, M=Ni; chdc=cyclohexanedicarboxylic acid; phen=1,10-phenanthroline), were prepared under hydrothermal conditions and characterized by elemental analyses, IR spectra, TG analysis, and single-crystal X-ray diffraction analysis. X-ray crystal structural analyses reveal that 1 and 2 are isomorphic and belong to the monoclinic system. C40H36Co2N4O10, P2(1)/c, a=10.0566(5) A, b=8.8843(5) A, c=20.2912(14) A, beta=100.052(3) degrees, Z=2 for 1; and C40H36Ni2N4O10, P2(1)/c, a=9.8921(6) A, b=9.0151(4) A, c=20.1628(17) A, beta=100.31(2) degrees, Z=2 for 2. In the structures of 1 and 2, the 1,4-chdc ligand possesses only one kind of e,a-cis-conformation although there are both cis- and trans-conformations in the raw material. Two oxygen atoms of one carboxyl in 1,4-chdc ligand and another oxygen atom of contraposition carboxyl link adjacent Co or Ni atoms into an infinite 1-D zigzag chain. The most attractive structural feature of 1 and 2 is that they both exhibit an infinite chiral chainlike structure with 2(1) helices along the b axis. In addition, the right-handed and the left-handed chains are alternate. Meanwhile, the adjacent chains of 1 and 2 are linked via hydrogen bonds into 2-D network structures, which further form 3-D frameworks via pi-pi interactions of 1,10-phen.

Identification of the 183RWTNNFREY191Region as a Critical Segment of Matrix Metalloproteinase 1 for the Expression of Collagenolytic Activity

Matrix metalloproteinase 1 (MMP-1) cleaves types I, II, and III collagen triple helices into (3/4) and (1/4) fragments. To understand the structural elements responsible for this activity, various lengths of MMP-1 segments have been introduced into MMP-3 (stromelysin 1) starting from the C-terminal end. MMP-3/MMP-1 chimeras and variants were overexpressed in Escherichia coli, folded from inclusion bodies, and isolated as zymogens. After activation, recombinant chimeras were tested for their ability to digest triple helical type I collagen at 25 degrees C. The results indicate that the nine residues (183)RWTNNFREY(191) located between the fifth beta-strand and the second alpha-helix in the catalytic domain of MMP-1 are critical for the expression of collagenolytic activity. Mutation of Tyr(191) of MMP-1 to Thr, the corresponding residue in MMP-3, reduced collagenolytic activity about 5-fold. Replacement of the nine residues with those of the MMP-3 sequence further decreased the activity 2-fold. Those variants exhibited significant changes in substrate specificity and activity against gelatin and synthetic substrates, further supporting the notion that this region plays a critical role in the expression of collagenolytic activity. However, introduction of this sequence into MMP-3 or a chimera consisting of the catalytic domain of MMP-3 with the hinge region and the C-terminal hemopexin domain of MMP-1 did not express any collagenolytic activity. It is therefore concluded that RWTNNFREY, together with the C-terminal hemopexin domain, is essential for collagenolytic activity but that additional structural elements in the catalytic domain are also required. These elements probably act in a concerted manner to cleave the collagen triple helix.

Analysis of modes of packing of the most stable form of syndiotactic polypropylene

Conformational energy calculations have been performed on the syndiotactic polypropylene having s( 2 1 )2 symmetry with the aim to find the best conformation in the crystalline field. A detailed packing analysis by molecular mechanics has been effected in order to clarify some features of the crystal structure of the most stable form. The close packing is obtained along the direction of the b axis with chains having opposite chirality. A statistical arrangement of right- and left-handed chains is found in each site along the direction of the a axis. The results are in agreement with experimental data reported in the literature, and provide a satisfactory explanation of the discrepancies present in previously proposed structures.

Fire and Ice ... and Worms?

In the Perspective "alpha-Helical coiled coils: More facts and better predictions" by C. Cohen and D. A. D. Parry (28 Jan., p. 488), the second sentence of the second paragraph of column 1 on page 489 should have read, "The structural motif is indeed a left-handed three-helix bundle with left-handed chain connectivity."