A molecular dynamics study of a PbO.SiO2 glass and melt

The structures of a PbO.SiO2 glass and melt have been studied using molecular dynamics simulation employing Born-Mayer-Huggins pair potentials. Various pair distribution functions are presented and discussed. Pb-Pb correlations persist in the melt, in agreement with experimental observations. The calculated and experimental radial distribution functions are compared.

Structural effects on the vaporization of high molecular weight esters

To understand the effect of molecular weight and branching on the heats of vaporization (AH,) and their flow behavior, AH, and viscosity (7) were measured at different temperatures in the high molecular weight ester series: linear flexible di-n-alkyl sebacates and compact branched triglycerides with molecular weight ranging from 300 to 900. AHv" values (AHv corrected to 298 K) have been obtained with experimental AH, and also computed according to the group additivity method; a smaller-CH,- group value of 3.8 kJ mol-' compared to the normal value of 5.0 kJ mol-' is found to give good agreement with the experimental data (within 2-5% error). Both ester series have the same AH," irrespective of their molecular features, namely,shape, flexibility, and polarity, suggesting the coiling of the molecules during vaporization. The segmental motion of these ester series during their flow and its dependence on their molecular features unlike AH,' are demonstrated by the correlation of the enthalpy of activation for viscous flow (AH*) and the ratio AE,/AH* = n (AE, is the energy of vaporization) with molecular weight.

Molecular modelling of saccharide-lipid interactions

Certain saccharides, including trehalose, sucrose and glucose, stabilize lipid bilayers against dehydration. It has been suggested that these saccharides replace waters of hydration as the system is dried, thereby maintaining the headgroups at their hydrated spacing. The lipid acyl chains consequently have sufficient free volume to remain in the liquid crystallines state, and the processes that disrupt membrane integrity are inhibited. Initial molecular graphic investigations of a model trehalose/DMPC system supported this idea (Chandrasekhar, I. and Gaber, B.P. (1988) J. Biomol. Stereodyn, 5, 1163–1171). We have extended these studies to glucose and sucrose. A set of AMBER potential parameters has been established that reproduce simple saccharide conformations, including the anomeric effect. Extensive energy minimizations have been conducted on all three systems. The saccharide-lipid interaction energies become less stable in the order trehalose

Isolation and characterization of a cytokinin-binding protein from cucumber cotyledons

A protein which binds specifically to [3H]-zeatin has been isolated from cucumber cotyledons by chromatographic techniques. Its binding to [3H]-zeatin was inhibited remarkably by the addition of non-radioactive cytokinins and the order of inhibition was zeatin > -zeatin riboside > N6-(Delta2-isopentenyl)adenine > N6-(Delta2-isopentenyl)adenosine > N6-benzyl-adenosine > kinetin riboside. This protein behaved as a soluble protein with an apparent molecular size of 43,000 daltons on gel filtration through calibrated Sephadex G-100 column. The dissociation constant, Kd, of the protein-zeatin complex was about 4 × 10–7 M.

Molecular profiling of indoor microbial communities in moisture damaged and non-damaged buildings

Epidemiological studies have shown an elevation in the incidence of asthma, allergic symptoms and respiratory infections among people living or working in buildings with moisture and mould problems. Microbial growth is suspected to have a key role, since the severity of microbial contamination and symptoms show a positive correlation, while the removal of contaminated materials relieves the symptoms. However, the cause-and-effect relationship has not been well established and knowledge of the causative agents is incomplete. The present consensus of indoor microbes relies on culture-based methods. Microbial cultivation and identification is known to provide qualitatively and quantitatively biased results, which is suspected to be one of the reasons behind the often inconsistent findings between objectively measured microbiological attributes and health. In the present study the indoor microbial communities were assessed using culture-independent, DNA based methods. Fungal and bacterial diversity was determined by amplifying and sequencing the nucITS- and16S-gene regions, correspondingly. In addition, the cell equivalent numbers of 69 mould species or groups were determined by quantitative PCR (qPCR). The results from molecular analyses were compared with results obtained using traditional plate cultivation for fungi. Using DNA-based tools, the indoor microbial diversity was found to be consistently higher and taxonomically wider than viable diversity. The dominant sequence types of fungi, and also of bacteria were mainly affiliated with well-known microbial species. However, in each building they were accompanied by various rare, uncultivable and unknown species. In both moisture-damaged and undamaged buildings the dominant fungal sequence phylotypes were affiliated with the classes Dothideomycetes (mould-like filamentous ascomycetes); Agaricomycetes (mushroom- and polypore-like filamentous basidiomycetes); Urediniomycetes (rust-like basidiomycetes); Tremellomycetes and the family Malasseziales (both yeast-like basidiomycetes). The most probable source for the majority of fungal types was the outdoor environment. In contrast, the dominant bacterial phylotypes in both damaged and undamaged buildings were affiliated with human-associated members within the phyla Actinobacteria and Firmicutes. Indications of elevated fungal diversity within potentially moisture-damage-associated fungal groups were recorded in two of the damaged buildings, while one of the buildings was characterized by an abundance of members of the Penicillium chrysogenum and P. commune species complexes. However, due to the small sample number and strong normal variation firm conclusions concerning the effect of moisture damage on the species diversity could not be made. The fungal communities in dust samples showed seasonal variation, which reflected the seasonal fluctuation of outdoor fungi. Seasonal variation of bacterial communities was less clear but to some extent attributable to the outdoor sources as well. The comparison of methods showed that clone library sequencing was a feasible method for describing the total microbial diversity, indicated a moderate quantitative correlation between sequencing and qPCR results and confirmed that culture based methods give both a qualitative and quantitative underestimate of microbial diversity in the indoor environment. However, certain important indoor fungi such as Penicillium spp. were clearly underrepresented in the sequence material, probably due to their physiological and genetic properties. Species specific qPCR was a more efficient and sensitive method for detecting and quantitating individual species than sequencing, but in order to exploit the full advantage of the method in building investigations more information is needed about the microbial species growing on damaged materials. In the present study, a new method was also developed for enhanced screening of the marker gene clone libraries. The suitability of the screening method to different kinds of microbial environments including biowaste compost material and indoor settled dusts was evaluated. The usability was found to be restricted to environments that support the growth and subsequent dominance of a small number microbial species, such as compost material.

Effects of molecular size in solvation dynamics

The effects of molecular size on the dynamics of polar solvation are studied by using a microscopic theory which includes the translational relaxation modes of the solvent consistently. It is shown that while in the absence of the translational contribution the solvation rate increases with the size of the solute (in agreement with the conclusions of the nonequilibrium MSA theory),a complete reversal of the solute size dependence occurs when translational modes make a significant contribution to the solvent polarization relaxation.

Conformational Variability in Modified Nucleosides. Crystal and Molecular Structure of 2',3'-O-Isopropylidene Inosine

The crystal structure of 2',3'-O-isopropylidene inosine shows a number of interesting features. The four independent molecules in the asymmetric unit exhibit significant conformational variations. Ribose puckers fall in the O(4')-exo region, unfavourable in unsubstituted nucleosides. Hypoxanthine bases show base-pairing (I.I) in a manner analogous to the guanine self pairs (G.G) in 2',3'-O-isopropylidene guanosine but with a C(2)-H…O(6) hydrogen bond instead of N(2)-H…O(6).

Molecular dynamics simulations on parallel and antiparallel C.G*G triplexes

Molecular dynamics (MD) studies have been carried out on the Hoogsteen hydrogen bonded parallel and the reverse Hoogsteen hydrogen banded antiparallel C.G*G triplexes. Earlier, the molecular mechanics studies had shown that the parallel structure was energetically more favourable than the antiparallel structure. To characterize the structural stability of the two triplexes and to investigate whether the antiparallel structure can transit to an energetically more favourable structure, due to the local fluctuations in the structure during the MD simulation, the two structures were subjected to 200ps of constant temperature vacuum MD simulations at 300K. Initially no constraints were applied to the structures and it was observed that for the antiparallel tripler, the structure showed a large root mean square deviation from the starting structure within the first 12ps and the N4-H41-O6 hydrogen bond in the WC duplex got distorted due to a high propeller twist and a moderate increase in the opening angle in the basepairs. Starting from an initial value of 30 degrees, helical twist of the average structure from this simulation had a value of 36 degrees, while the parallel structure stabilized at a twist of 33 degrees. In spite of the hydrogen bond distortions in the antiparallel tripler, it was energetically comparable to the parallel tripler. To examine the structural characteristics of an undistorted structure, another MD simulation was performed on the antiparallel tripler by constraining all the hydrogen bonds. This structure stabilized at an average twist of 33 degrees. In the course of the dynamics though the energy of the molecule - compared to the initial structure - improved, it did not become comparable to the parallel structure. Energy minimization studies performed in the presence of explicit water and counterions also showed the two structures to be equally favourable energetically Together these results indicate that the parallel C.G*G tripler with Hoogsteen hydrogen bonds also represents a stereochemically and energetically favourable structure for this class of triplexes.

Sequence Independent Recombination Triple Helices: A Molecular Dynamics Study

Recent experimental studies have shown that the Rec-A mediated homologous recombination reaction involves a triple helical intermediate, in which the third strand base forms hydrogen bonds with both the bases in the major groove of the Watson-Crick duplex. Such 'mixed' hydrogen bonds allow formation of sequence independent triplexes. DNA triple helices involving 'mixed' hydrogen bonds have been studied, using model building, molecular mechanics (MM) and molecular dynamics (MD). Models were built for a tripler comprising all four possible triplets viz., G.C*C, C.G*G, A.T*T and T.A*A. To check the stability of all the 'mixed' hydrogen bonds in such triplexes and the conformational preferences of such tripler structures, MD studies were carried out starting from two structures with 30 degrees and 36 degrees twist between the basepairs. It was observed that though the two triplexes converged towards a similar structure, the various hydrogen bonds between the WC duplex and the third strand showed differential stabilities. An MD simulation with restrained hydrogen bonds showed that the resulting structure was stable and remained close to the starting structure. These studies help us in defining stable hydrogen bond geometries involving the third strand and the WC duplex. It was observed that in the C.G*G triplets the N7 atom of the second strand is always involved in hydrogen bonding. In the G.C*C triplets, either N3 or O2 in the third strand cytosine can interchangeably act as a hydrogen bond acceptor.

Molecular Mechanics Studies of Homopolymeric and Mixed Sequence Py.Pu*Pu Triple Helices,

DNA triple helices containing two purine strands and one pyrimidine strand (C.G*G and T.A*A) have been studied, using model building followed by energy minimisation, for different orientations of the third strand resulting from variation in the hydrogen bonding between the Watson-Crick duplex and the third strand and the glycosidic torsion angle in the third strand. Our results show that in the C.G*G case the structure with a parallel orientation of the third strand, resulting from Hoogsteen hydrogen bonds between the third strand and the Watson-Crick duplex, is energetically the most favourable while in the T.A*A case the antiparallel orientation of the third strand, resulting from reverse Hoogsteen hydrogen bonds, is energetically the most favourable. These studies when extended to the mixed sequence triplexes, in which the second strand is a mixture of G and A, correspondingly the third strand is a mixture of G and APT, show that though the parallel orientation is still energetically more favourable, the antiparallel orientation becomes energetically comparable with an increasing number of thymines in the third strand. Structurally, for the mixed triplexes containing G and T in the third strand, it is seen that the basepair non-isomorphism between the C.G*G and the T.A*T triplets can be overcome with some changes in the base pair parameters without much distortion of either the backbone or the hydrogen bonds.

Molecular Modeling Studies on Amphotericin B and its Complex with Phospholipid,

Molecular dynamics simulation studies on polyene antifungal antibiotic amphotericin B, its head-to-tail dimeric structure and lipid - amphotericin B complex demonstrate interesting features of the flexibilities within the molecule and define the optimal interactions for the formation of a stable dimeric structure and complex with phospholipid.

Ab initio molecular orbital calculations on ion-molecule and ion pair-molecule complexes of the water-lithium cyanide system

Ab initio molecular orbital (MO) calculations with the 3-21G and 6-31G basis sets were performed on a series of ion-molecule and ion pair-molecule complexes for the H2O + LiCN system. Stabilisation energies (with counter-poise corrections), geometrical parameters, internal force constants and harmonic vibrational frequencies were evaluated for 16 structures of interest. Although the interaction energies are smaller, the geometries and relative stabilities of the monohydrated contact ion pair are reminiscent of those computed for the complexes of the individual ions. Thus, interaction of the oxygen lone pair with lithium leads to a highly stabilised C2v structure, while the coordination of water to the cyanide ion involves a slightly non-linear hydrogen bond. Symmetrical bifurcated structures are computed to be saddle points on the potential energy surface, and to have an imaginary frequency for the rocking mode of the water molecule. On optimisation the geometries of the solvent shared ion pair structures (e.g. Li+cdots, three dots, centered OH2cdots, three dots, centered CN−) revealed a proton transfer from the water molecule leading to hydrogen bonded forms such as Li-O-Hcdots, three dots, centered HCN. The variation in the force constants and harmonic frequencies in the various structures considered are discussed in terms of ion-molecular and ion pair-molecule interactions.

Molecular methods for the epidemiological analysis of methicillin-resistant Staphylococcus aureus (MRSA) and Streptococcus pneumoniae

Methicillin-resistant Staphylococcus aureus (MRSA) and Streptococcus pneumoniae are major health problems worldwide, both found in symptomless carriage but also causing even life-threatening infections. The aim of this thesis was to characterise MRSA and S. pneumoniae in detail by using several molecular typing methods for various epidemiological purposes: clonality analysis, epidemiological surveillance, outbreak investigation, and virulence factor analysis. The characteristics of MRSA isolates from the strain collection of the Finnish National Infectious Disease Register (NIDR) and pneumococcal isolates collected from military recruits and children with acute otitis media (AOM) were analysed using various typing techniques. Antimicrobial susceptibility testing, pulsed-field gel electrophoresis (PFGE), multilocus sequence typing (MLST), spa typing, staphylococcal cassette chromosome mec (SCCmec) typing, and the detection of Panton-Valentine leukocidin (PVL) genes were performed for MRSA isolates. Pneumococcal isolates were analysed using antimicrobial susceptibility testing, serotyping, MLST, and by detecting pilus islet 1 (PI-1) and 2 (PI-2) genes. Several international community- and hospital-associated MRSA clones were recognised in Finland. The genetic diversity among MRSA FIN-4 isolates and among FIN-16 isolates was low. Overall, MRSA blood isolates from 1997 to 2006 were genetically diverse. spa typing was found to be a highly discriminatory, rapid and accurate typing method and it also qualifies as the primary typing method in countries with a long history of PFGE-based MRSA strain nomenclature. However, additional typing by another method, e.g. PFGE, is needed in certain situations to be able to provide adequate discrimination for epidemiological surveillance and outbreak investigation. An outbreak of pneumonia was associated with one pneumococcal strain among military recruits, previously healthy young men living in a crowded setting. The pneumococcal carriage rate after the outbreak was found to be exceptionally high. PI-1 genes were detected at a rather low prevalence among pneumococcal isolates from children with AOM. However, the study demonstrated that PI-1 has existed among pneumococcal isolates prior to pneumococcal conjugate vaccine and the increased antimicrobial resistance era. Moreover, PI-1 was found to associate with the serotype rather than the genotype. This study adds to our understanding of the molecular epidemiology of MRSA strains in Finland and the importance of an appropriate genotyping method to be able to perform high-level laboratory-based surveillance of MRSA. Epidemiological and molecular analyses of S. pneumoniae add to our knowledge of the characteristics of pneumococcal strains in Finland.