Mononitrosated and phenylamido substituted chelate linkage isomers of quadridentate Schiff base copper(II) complexes

The nitrosation of monophenylamido substituted quadridentate Schiff base complexes of copper(II) are observed to adopt N-bonded isonitroso coordination whereas the phenylisocyanation of the corresponding mononitrosated quadridentate complexes are found to prefer O-bonded isonitroso coordination.

Biaxial-load effects on isopachics for a crack at the interface of dissimilar media

The plane problem of two dissimilar materials, bonded together and containing a crack along their common interface, which were subjected to a biaxial load at infinity, is examined by giving a closed-form expression for the first stress invariant of the normal stresses, which is equally valid everywhere, near to, and far from, the crack-tip region. This exact expression for the first-stress invariant is compared by constructing the respective isopachic-fringe patterns, to the approximate expression with non-singular terms, due to the biaxiality factor, for the same quantity. Significant differences between respective isopachic-patterns were found and their dependence on the elastic properties of both materials and the applied loads was demonstrated. The relative errors between the computedK I - andK II -components by using the approximate expression for the first stress-invariant and the accurate one, derived from closed-form solution along either isopachic-fringes or along circles and radii from the crack-tip have been given, indicating in some cases large discrepancies between exact and approximate solutions.

Tautomeric forms of hydroxycyclotriphosphazatrienes; X-ray crystal structure of N3P3Ph2(OMe)3OH

The prefered tautomer(s) of hydroxycyclotriphosphazatrienes and prototropic exchange in solution have been established by 31P n.m.r. spectroscopy, thus confirming predictions deduced from basicity calculations; the X-ray structure of N3P3Ph2(OMe)3OH shows that it exists as the hydrogen-bonded dimer of the oxophosphazadiene tautomer in which a proton is adjacent to the PPh2 group.

X-Ray Studies On Crystalline Complexes Involving Amino-Acids And Peptides .11. Peptide Aggregation And Water-Structure In The Crystals Of A Hydrated 1-1 Complex Between L-Histidyl-L-Serine And Glycyl-L-Glutamic Acid

The hexahydrate of a 1:1 complex between L-histidyl-L-serine and glycyl-L-glutamic acid crystallizes in space group P1 with a = 4.706(1), b= 8.578(2), c= 16.521(3) ÅA; α= 85.9(1), β= 89.7(1)°, = 77.4(1). The crystal structure, solved by direct methods, has been refined to an R value of 0.046 for 2150 observed reflections. The two peptide molecules in the structure have somewhat extended conformations. The unlike molecules aggregate into separate alternating layers. Each layer is stabilized by hydrogen bonded head-to-tail sequences as well as sequences of hydrogen bonds involving peptide groups. The arrangement of molecules in each layer is similar to one of the plausible idealized arrangements of L-alanyl-L-alanine worked out from simple geometrical considerations. Adjacent layers in the structure are held together by interactions involving side chains as well as water molecules. The water structure observed in the complex provides a good model, at atomic resolution, for that in protein crystals. An interesting feature of the crystal structure is the existence of two water channels in the interfaces between adjacent peptide layers.

A hypothesis on the role of hydroxyproline in stabilizing collagen structure

The possibility of hydroxyproline residues stabilizing the collagen triple-helical structure by the formation of additional hydrogen bonds through their γ-hydroxyl group has been studied from structural considerations. It is not possible for this hydroxyl group to form a direct hydrogen bond with a suitable group in a neighbouring chain of the triple-helical protofibril. However, in the modified one-bonded structure, which is stabilized by additional hydrogen bonds being formed through water molecules as intermediaries (put forward in 1968 by Ramachandran, G. N. and Chandrasekharan, R.), it is found that the γ-hydroxyl group of hydroxyproline can form a good hydrogen bond with the water oxygen as acceptor, the hydrogen bond length being 2.82 Å. It is proposed that, in addition to stabilizing the collagen triple-helical structure due to the stereochemical properties of the pyrrolidine ring, hydroxyproline gives added stability by the formation of an extra hydrogen bond. Experimental studies on the determination of shrinkage and denaturation temperatures of native collagen and its synthetic analogues, as a function of their hydroxyproline content, are being undertaken to test this hypothesis.

Phase transition in malonic acid: An infrared study

Malonic acid is shown to undergo an interesting phase transition at 360 K when the two non-equivalent cyclic hydrogen-bonded dimers present in the low-temperature phase become equivalent.

Spectroscopic and Coordination Behavior of α-Cyanothioacetamide

α-Cyanothioacetamide (CTAM) complexes of cuprous chloride CuCl–2CTAM and cuprous bromide CuBr–2CTAM have been prepared. The infrared spectra of CTAM and its complexes, and the laser Raman spectrum of CTAM have been recorded. Assignment of the frequencies of the ligand has been made on the basis of a normal coordinate analysis using the Urey-Bradley force field. The copper (I) complexes are inferred to have thiocarbonyl sulfur and amide nitrogen bonded CTAM as evidenced from infrared and electronic spectra.

Infrared studies on the conformation of synthetic alamethicin fragments and model peptides containing .alpha.-aminoisobutyric acid

ABSTRACT: Infrared studies of synthetic alamethicin fragments and model peptides containing a-aminoisobutyric acid (Aib) have been carried out in solution. Tripeptides and larger fragments exhibit a strong tendency to form /3 turns, stabilized by 4 - 1 10-atom hydrogen bonds. Dipeptides show less well-defined structures, though C5 and C7 conformations are detectable. Conformational restrictions imposed by Aib residues result in these peptides populating a limited range of states. Integrated intensities of the hydrogen-bonded N-H stretching band can be used to quantitate the number of intramolecular hydrogen bonds. Predictions made from infrared data are in excellent agreement with nuclear magnetic resonance and X-ray diffraction studies. Assignments of the urethane and tertiary amide carbonyl groups in the free state have been made in model peptides. Shifts to lower frequency on hydrogen bonding are observed for the carbonyl groups. The 1-6 segment of alamethicin is shown to adopt a 310 helical structure stabilized by four intramolecular hydrogen bonds. The fragments Boc-Leu-Aib-Pro-Val-Aib-OMe (1 2-1 6) and Boc-Gly-Leu-Aib-Pro-Val-Aib-OMe (1 1-1 6) possess structures involving 4 - 1 and 5 - 1 hydrogen bonds. Supporting evidence for these structures is obtained from proton nuclear magnetic resonance studies.

Conformation of the LL and LD hairpin bends with internal hydrogen bonds in proteins and peptides

The conformation of three linked peptide units having an internal 4 → 1 type of hydrogen bond has been studied in detail, and the low energy conformations are listed. These conformations all lead to the reversal of the chain direction, and may therefore be called as “hairpin bends” or “U-bends”. Since this bend can occur at the end of two chains hydrogen-bonded in the antiparallel β-conformation, it is also known as the “β-bend”. Two types of conformation are possible when the residues at the second and third Cα atoms are both of type L (the LL bend), while only one type is possible for the LD and the DL bend. The LL bend can also accommodate the sequences LG, GL, GG (G = glycine), while the LD bend can accommodate the sequences LG, GD and GG. The conformations for the sequences DD and DL are exact inverses (or mirror images) of those for the sequences LL and LD, respectively, and have dihedral angles (phi2, ψ2), (phi3, ψ3) of the same magnitudes, but of opposite signs as those for the former types, which are listed, along with the characteristics (length, angle and energy) of the hydrogen bonds. A comparison of the theoretical predictions with experimental data (from X-ray diffraction and NMR studies) on proteins and peptides, show reasonably good agreement. However, a systematic trend is observable in the experimental data, slightly deviating from theory, which indicates that some deformations occur in the shapes of the peptide units forming the bend, differing from that of the standard planar peptide unit.

The 310 Helical Conformation of the Amino Terminal Decapeptide of Suzukacillin. 270 MHz 'H NMR Evidence for Eight Intramolecular Hydrogen Bonds

The amino terminal suzukacillin decapeptide fragment, Boc-Aib-Pro-Val-Aib-Val-Ala-Aib-Ala-Aib-Aitbh-eO Me, two pentapeptides Boc-AibPrc-Val-AibVal-OMe and Boc-Ala-AibAla-AibAibOMe, and the tripeptide Boc-Ala-AibAibOMe have been studied by 270-MHz 'H NMR spectroscopy. By use of solvent dependence of chemical shifts in a CDC13-(CD3),S0 system and temperature dependence of amide NH chemical shifts in (CD3),S0, the intramolecularly hydrogen bonded NH groups in these peptides have been identified. The tripeptide possesses one hydrogen bond, both pentapeptides show evidence for three intramolecular hydrogen bonds, and the decapeptide has eight NH groups participating in hydrogen bonding. An Ala( 1)-Aib(2) @ turn is proposed for the tripeptide. Both pentapeptides favor 310 helical conformations composed of three consecutive B turns. The decapeptide adopts a 310 helical conformation with some flexibility at the Va1(5)-Ala(6) segment. The proposed conformations are consistent with the known stereochemical preferences of Aib residues.

Layer-by-layer self-assembly of modified hyaluronic acid/chitosan based on hydrogen bonding

The fabrication of hydrogen bonded polymer self-assembly for drug delivery has been accomplished via layer-by-layer sequential assembly from aqueous solution. In this study, the self-assembly was constructed based on hydrogen bonding between DNA base (adenine and thymine) pairs substituted on the backbone of chitosan and hyaluronic acid. Chitosan was modified with adenine, whereas hyaluronic acid was modified with thymine. Subsequently, these two polymers were sequentially absorbed on flat substrate by taking advantage of interactions of DNA base pairs via hydrogen bonding. Interlayer hydrogen bonding of these two polymers produces stable multilayer film without using any cross-linking agent. Thin film formation on quartz substrate has been monitored with UV-vis spectra and an AFM study. Formation of multilayer hydrogen-bonded thin film has been further confirmed with SEM. Encapsulation and release behavior of the therapeutic drug from the multilayer thin film at different conditions has been illustrated using UV-vis spectra. Cell viability of modified polymers using MTT assay confirmed no cytotoxic effect.

Mononitrosalted and phenylamido substituted chelate linkage isomers of quadridentate Schiff base copper(II) complexes

The nitrosation of monophenylamido substututed quadridentate Schiff base complexes of copper(II) are observed to adopt N-bonded isonitroso coordination whereas the phenylisocyanation of the corresponding mononitrosated quadridentate complexes are found to prefer O-bonded isonitroso coordination.

Structural Studies of Analgesics and Their Interactions. VIII.* Rotational Isomerism and Disorder in the Crystal Structure of Meelofenamie Acid

Meclofenamic acid, C I4HIICI2NO2, probably the most potent among analgesic fenamates, crystallizes in the triclinic space group P1, with a = 8.569 (5), b = 8.954(8), c -- 9.371 (4) A, ct = 103.0 (2), fl -- 103.5 (2), y = 92.4 (2) ° , Z = 2, D m = 1.43 (4), D c = 1.41 Mg m -3. The structure was solved by direct methods and refined to R = 0.135 for 1062 observed reflections. The anthranilic acid moiety in the molecule is nearly planar and is nearly perpendicular to the 2,6-dichloro-3-methylphenyl group. The molecules, which exist as hydrogen-bonded dimers, have an internal hydrogen bond involving the imino and the carboxyl groups. The methyl group is disordered and occupies two positions with unequal occupancies. The disorder can be satisfactorily explained in terms of the rotational isomerism of the 2,6-dichloro-3-methylphenyl group about the bond which connects it to the anthranilic acid moiety and the observed occupancies on the basis of packing considerations.

Studies on CNTs/DNA composite

The electrical and optical properties of MWCNTs/DNA composite were studied. Electrical conductivity studies reveal that, the increase in CNTs concentration in DNA increases the conductivity. Fourier transformed Infrared (FTIR) spectrum shows that the CNTs are bonded to DNA covalently at the ends and defects sites and the wrapping of DNA on the CNTs is due to van der Waals force.

The Structure of Monosodium Phosphoenolpyruvate

CDH406P-.Na +.H20 , M r = 208.0, is monoclinic, Cc, a = 11.423 (2), b = 23.253 (5), c - 6.604 (1) A, fl = 123.63 (1) °, U = 1460.6 A 3, D x =. 1.89 Mg m -a, Z = 8, 2(Mo Ka) = 0.7107 A, p(Mo Ka) = 0.44 mm -~, F(000) = 840. Final R = 0.063 for 1697 reflections.The two crystallographically independent molecules of phosphoenolpyruvate (PEP) (A and B) are almost mirror images of each other, the mirror being the planar enolpyruvate group. The torsion angle C(3)-C(2)- O(1)-P(1) is 122.6 in A and -112.0 ° in B, in contrast to -209.1 ° in PEP.K. The enolic C(2)-O(1) has a partial double-bond character [1.401 (A), 1.386A (B)]. The high-energy P~O bond (1.595 and 1.610A) is comparable to that in PEP.K (1.612 A). Na(1) has six nearest neighbours while Na(2) has only five. The Na + ions are involved in binding only the phosphates of different molecules, in contrast to the K ÷ ion in PEP. K, which binds to both the phosphate and carboxyl ends of the same molecule. The planar carboxyl groups stack on each other at an average distance of 3.2 A instead of forming hydrogen-bonded dimers usually found in carboxylate structures.