Conformational flexibility in androgenic steroids - the structure of a new form of (+)-17-beta-hydroxy-19-nor-4-androsten-3-one (19-nortestosterone), c18h26o2

The structure and conformation of a second crystalline modification of 19-nortestosterone has been determined by X-ray methods. M r = 274, monoclinic P2 l, a=9.755(2), b= 11.467(3), c= 14.196(3)/L fl=101.07(2) ° , V=1558.4 (8) A 3, Z=4, Ox= I. 168 g cm -3, Mo Ka, 2 = 0.7107 ,/k, ~ = 0.80 cm -l, F(000) = 600, T= 300 K. R = 0.060 for 2158 observed reflections. The two molecules in the asymmetric unit show significant differences in the A-ring conformation from that of the previously reported form of the title compound [Precigoux, Busetta, Courseille & Hospital (1975). Acta Cryst. B31, 1527-1532]. The l a,2fl-half-chair conformation of the A ring increases its conformational freedom compared with testosterone.

Conformational flexibility of DNA: polymorphism and handedness

It is shown that left-handed duplexes are possible for A, B, and D forms of DNA. These duplexes are stereochemically satisfactory and are consistent with the observed x-ray intensity data. On scrutiny the refined right-handed models of B and D DNA by Arnott and coworkers are found to be stereochemically unacceptable. It was possible to formulate a stereochemical guideline for molecular model building based on theory and analysis of single-crystal structure data of dinucleoside monophosphate and higher oligomers. This led to both right- and left-handed DNA duplexes. The right-handed B and D DNA duplexes so obtained are stereochemically superior to earlier models and agree well with the observed x-ray intensity data. The observation that DNA can exist in either handedness for all the polymorphous forms of DNA at once explained A in equilibrium B and B in equilibrium D transitions. Hence it is confirmed that polymorphism of DNA is a reflection on the conformational flexibility inherent in DNA, the same cause that ultimately allows DNA in either handedness. The possibility of various types of right- and left-handed duplexes generated by using dinucleoside monophosphate and trinucleoside diphosphate as repeating units resulted in a variety of models, called RL models. All these models have alternating right and left helical segments and inverted stacking at the bend region as suggested by us earlier. It turns out that the B-Z DNA model of Wang et al. is only an example of RL models.

Preferred conformations and flexibility of aminoacyl side chain of penicillins

Possible conformations of penicillin G; d and l isomers of ampicillin; α-amino-α-methyl-benzyl penicillins and 3- pyridyl methyl penicillin have been studied by an energy minimization procedure using empirical potential functions. The preferred conformations of these antibiotics have been correlated with their biological activity. The conformational requirement of the antibiotic to be active against Gram-positive and Gram-negative (β-lactamase-negative) bacterial strains seems to be the same. The reduced activity of penicillin G against Gram-negative bacteria has been attributed to its lower ability to permeate the outer membrane. The flexibility of the sidechains of these antibiotics is also shown to be important for the desired biological activity.

Structures of isopropylidene nucleoside derivatives - implications for ribose ring flexibility under external cyclic constraints

Crystal structures of six isopropylidene nucleoside derivatives are described. The results show that, under external cyclic constraints, the ribose assumes a variety of unusual conformations. In those compounds which possess a base-to-sugar cyclization through the C(4′) atom, the furanose pucker is predominantly C(4′)-endo, O(4′)-exo. The possible relevance of the sulphur geometry in two of the compounds to certain structural aspects of the action of the enzyme thymidylate synthetase is also pointed out.

Flexibility of the furanose ring in nucleic acids: a compilation of crystal data

A compilation of crystal structure data on deoxyribo- and ribonucleosides and their higher derivatives is presented. The aim of this paper is to highlight the flexibility of deoxyribose and ribose rings. So far, the conformational parameters of nucleic acids constituents of ribose and deoxyribose have not been analysed separately. This paper aims to correlate the conformational parameters with the nature and puckering of the sugar. Deoxyribose puckering occurs in the C2′ endo region while ribose puckering is observed both in the C3′ endo and C2′ endo regions. A few endocyclic and exocyclic bond angles depend on the puckering and the nature of the sugar. The majority of structures have an anti conformation about the glycosyl bond. There appears to be a puckering dependence on the torsion angle about the C4′---C5′ bonds. Such stereochemical information is useful in model building studies of polynucleotides and nucleic acids.

Polymorphism and conformational flexibility of DNA: right and left handed duplexes

Left handed duplexes are shown to be in agreement with the X-ray intensity data of A-, B- and D-forms of DNA. The structures are stereochemically satisfactory because they were obtained following a stereochemical guideline derived from theory and single crystal structure data of nucleic acid components. The same stereochemical guideline also led to right handed duplexes for B- and D-forms of DNA which have stereochemically preferred conformation and hence are superior to those given by Arnott and coworkers.

Not in their genes: phenotypic flexibility, behavioural traditions and cultural evolution in wild bonnet macaques

Phenotypic flexibility, or the within-genotype, context-dependent, variation in behaviour expressed by single reproductively mature individuals during their lifetimes, often impart a selective advantage to organisms and profoundly influence their survival and reproduction. Another phenomenon apparently not under direct genetic control is behavioural inheritance whereby higher animals are able to acquire information from the behaviour of others by social learning, and, through their own modified behaviour, transmit such information between individuals and across generations. Behavioural information transfer of this nature thus represents another form of inheritance that operates in many animals in tandem with the more basic genetic system. This paper examines the impact that phenotypic flexibility, behavioural inheritance and socially transmitted cultural traditions may have in shaping the structure and dynamics of a primate society--that of the bonnet macaque (Macaca radiata), a primate species endemic to peninsular India. Three principal issues are considered: the role of phenotypic flexibility in shaping social behaviour, the occurrence of individual behavioural traits leading to the establishment of social traditions, and the appearance of cultural evolution amidst such social traditions. Although more prolonged observations are required, these initial findings suggest that phenotypic plasticity, behavioural inheritance and cultural traditions may be much more widespread among primates than have previously been assumed but may have escaped attention due to a preoccupation with genetic inheritance in zoological thinking.

Structural flexibility in the biocatalyst-mediated functionalization of ring `A' in salannin, a tetranortriterpene from Azadirachta indica

Nocardia sp. quantitatively converts salannin 1 and 3-de-O-acetylsalannin 2 (C-seco limonoids) into 3-deacetoxy-1-de[(E)-2-methylbut-2-enoyloxy]salannin-1-en-3-one 10, a novel and potentially bioactive compound with an alpha,beta-unsaturated ketone moiety in ring `A'. In order to establish the sequence of events involved in this transformation and the structural specificity of this bacterial system, several new derivatives of salannin 1 have been prepared. These studies have indicated that the transformation is initiated by deacetylation at C-3, followed by oxidation of the secondary hydroxy group to 3-keto, which appears to facilitate the elimination of the tigloyloxy/acetoxy group at C-1 with the formation of an olefinic linkage between C-1 and C-2. The organism very efficiently transforms some of the derivatives of salannin into their corresponding compounds possessing an enone systemin ring `A', an essential pre-requisite for various biological activities. Some of the C-seco limonoids prepared in the present study, viz. 10, 1,2-didehydro-1,3-dideoxy-3-oxosalannic acid 18, 3-deacetoxy-1-de[(E)-2-methylbut-2-enoyloxy]-20,21,22,23-tetrahydrosal annin-1-en-3-one 15 and 1,2-didehydro-1,3-dideoxy-3-oxosalannol 23 were hitherto not known.

New Solid State Forms of the Anti-HIV Drug Efavirenz. Conformational Flexibility and High Z ` Issues

Structural information on the solid forms of efavirenz, a non-nucleoside reverse transcriptase inhibitor, is limited, although various polymorphic forms of this drug have been patented. We report here structural studies of four new crystal forms a pure form, a cyclohexane solvate, and cocrystals with 1,4-cyclohexanedione and 4,4'-bipyridine. Temperature dependent single-crystal to single-crystal phase transitions are observed for the pure form and for the cyclohexane solvate with an increase in the number of symmetry independent molecules, Z', upon a lowering of temperature. Other issues related to these solid forms, such as thermal stability, conformational flexibility, and high Z' occurrences, are addressed by using a combined experimental and computational approach.

Effect of coordinated ions on structure and flexibility of parallel G-quadruplexes: A molecular dynamics study

ingle tract guanine residues can associate to form stable parallel quadruplex structures in the presence of certain cations. Nanosecond scale molecular dynamics simulations have been performed on fully solvated fibre model of parallel d(G(7)) quadruplex structures with Na+ or K+ ions coordinated in the cavity formed by the O6 atoms of the guanine bases. The AMBER 4.1 force field and Particle Mesh Ewald technique for electrostatic interactions have been used in all simulations. There quadruplex structures are stable during the simulation, with the middle four base tetrads showing root mean square deviation values between 0.5 to 0.8 Angstrom from the initial structure as well the high resolution crystal structure. Even in the absence of any coordinated ion in the initial structure, the G-quadruplex structure remains intact throughout the simulation. During the 1.1 ns MD simulation, one Nai counter ion from the solvent as well as several water molecules enter the central cavity to occupy the empty coordination sites within the parallel quadruplex and help stabilize the structure. Hydrogen bonding pattern depends on the nature of the coordinated ion, with the G-tetrad undergoing local structural variation to accommodate cations of different sizes. in the absence of any coordinated ion. due to strong mutual repulsion, O6 atoms within G-tetrad are forced farther apart from each other, which leads to a considerably different hydrogen bonding scheme within the G-tetrads and very favourable interaction energy between the guanine bases constituting a G-tetrad. However, a coordinated ion between G-tetrads provides extra stacking energy for the G-tetrads and makes the quadruplex structure more rigid. Na+ ions, within the quadruplex cavity, are more mobile than coordinated K+ ions. A number of hydrogen bonded water molecules are observed within the grooves of all quadruplex structures.

Polymorphism in Opto-Electronic Materials with a Benzothiazole-fluorene Core: A Consequence of High Conformational Flexibility of pi-Conjugated Backbone and Alkyl Side Chains

Fluorene and its derivatives are well-known organic semiconducting materials in the field of opto-electronic devices because of their charge transport properties. Three new organic semiconducting materials, namely, 2,2'-((9,9-butyl-9H-fluorene-2,7-diyl)bis(4,1 phenylene))bisbenzod]thiazole, C4; 2,2'-((octyl-9H-fluorene-2,7-diyl)bis(4,1 phenylene))bisbenzod]thiazole, C8; and 2,2'-((9,9-dodecayl-9H-fluorene-2,7-diyl)bis(4,1 phenylene))bisbenzod]thiazole, C12 with a benzothiazole-fluorene backbone, were synthesized and characterized for their photophysical properties. A phenomenon of concomitant polymorphism has been investigated in the first two derivatives (C4 and C8) and has been analyzed systematically in terms of the packing characteristics involving pi ... pi interactions. The conformational flexibility of the pi-conjugated 2,2'-(fluorene-2,7-diyl)bis(4,1 phenylene)bisbenzod]thiazole backbone coupled with orientational freedom of the terminal alkyl chains were found to be the key factors responsible for these polymorphic modifications. Attempts to grow suitable crystals for single crystal X-ray diffraction of compound C12 were unsuccessful.

Synthesis, properties, and comparative analyses on the chain flexibility of an ultrastable vinyl polyperoxide

The most stable vinyl polyperoxide, namely, poly(indene peroxide) (PINDP), an alternating copolymer of indene and oxygen, was synthesized by the oxidative polymerization of indene. It was characterized by H-1-NMR and C-13-NMR, Fourier transform infrared, differential scanning calorimetry, direct pyrolysis mass spectrometry, and gas chromatography mass spectrometry studies. The overall activation energy for the degradation and the glass-transition temperature of PINDP were 47 kcal/mol and 327 K respectively, which were very high compared to those of other vinyl polyperoxides. (C) 2002 Wiley Periodicals, Inc.