Library

feed icon rss

Your email was sent successfully. Check your inbox.

An error occurred while sending the email. Please try again.

Proceed reservation?

Export
  • 1
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 2
    ISSN: 1399-0047
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A knowledge-based approach to crystal structure determination is presented. The approach integrates direct-methods and artificial-intelligence strategies to rephrase the structure determination process as an exercise in scene analysis. A general joint probability distribution framework, which allows the incorporation of isomorphous replacement, anomalous scattering and a priori structural information, forms the basis of the direct-methods strategies. The accumulated knowledge on crystal and molecular structures is exploited through the use of artificial-intelligence strategies, which include techniques of knowledge representation, search and machine learning.
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 829-833 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is assumed that a crystal structure in P{\bar 1} is fixed and that the 15 non-negative numbers R1, R2, R3, R4, R5; R12, R13, R14, R15, R23, R24, R25, R34, R35, R45 are also specified. The random vector (h, k, l, m, n) is assumed to be uniformly distributed over the subset of the fivefold Cartesian product W × W × W × W × W of reciprocal space W defined by |Eh| = R1, |Ek| = R2, |El| = R3, |Em| = R4, |En| = R5; (1) |Eh + k| = R12, |Eh + l| = R13, |Eh + m| = R14, |Eh + n| = R15, |Ek + l| = R23 |Ek + m| = R24, |Ek + n| = R25, |El + m| = R34, |El + m| = R35, |Em + n| = R45; (2) and h + k + l + m + n = 0. (3) Then the structure invariant (φ = φh + φk + φl + φm + φn, (4) as a function of the primitive random variables h, k, l, m, n, is itself a random variable, and its conditional probability distribution, given (1) and (2), is derived. The distribution yields reliable estimates for large numbers of quintets φ in terms of the 15 magnitudes (1) and (2).
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 41 (1985), S. 571-577 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The probabilistic theory of the three-phase structure invariants for a pair of isomorphous structures [Hauptman (1982). Acta Cryst. A38, 289-294] is reexamined. The analysis leads to distributions capable of estimating cosine invariants in the full range of -1 to +1. In particular, it is shown that heavy-atom substructure information can be incorporated easily into the distributions. The initial applications, using calculated diffraction data from the protein cytochrome c550, MR ∼ 14 500, and its PtCl2-4 derivative show that a remarkable increase in accuracy results from the use of the revised distributions, particularly after the incorporation of heavy-atom substructure information. Finally, it is shown that in the individual phase determinations the redundant cosine invariants play a role identical to that of the multiple isomorphous derivatives and thus provide the basis for the solution of the phase problem in the single isomorphous replacement case.
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 45 (1989), S. 247-254 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: By characterizing isomorphism in reciprocal space [i.e. diffraction data sets are isomorphous if they have the same geometry (the same reciprocal-lattice unit cell) and the same symmetry] it is shown that the diffraction data of a native protein and of its heavy-atom derivatives, the calculated data of a partial structure and the observed data of its associated complete structure, and the Friedel-pair data of an anomalously scattering crystal structure all belong to the more general class of isomorphous data sets. Their joint probability distributions for two- and three- phase structure invariants are shown to be isomorphous: they have the same functional form and differ only in individual atomic scattering factors. General joint probability distributions, which can be used for any isomorphous data pairs, are presented.
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 646-651 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The probabilistic theory of the three-phase structure invariant for a triplet of isomorphous structures is worked out. In particular, when diffraction data are available for a native protein and two derivatives, the conditional distributions of the three-phase structure invariants, given the nine magnitudes in their first neighbourhoods, are derived for the special case that the heavy atoms of the two derivatives are located in different positions in the unit cell. The distributions have the form P(Ω) = (1/K) exp (A cos Ω), where the parameters K and A are functions of the nine magnitudes in the first neighborhood. In the favorable case that the variance of a distribution happens to be small, a reliable estimate, 0 or π, of the invariant is obtained. An example shows that these distributions, which employ simultaneously the diffraction data from a triple of isomorphous structures, yield more accurate estimates for the three-phase structure invariants than are obtainable from earlier distributions, which employ diffraction data from only a pair of isomorphous structures [Hauptman (1982). Acta. Cryst. A38, 289-294]. Unique origin and enantiomorph specification in direct-methods applications to all three structures is an advantage of the present approach.
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 544-548 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Integrated direct methods- isomorphous probability distributions [Hauptman (1982). Acta Cryst. A38, 289-294] are interpreted in terms of the familiar parameters of the isomorphous replacement method, the diffraction ratio and the differences in the diffraction intensities of a native protein and its heavy-atom derivative. The analysis shows that the reliability of the phase estimates is a function of the degree of heavy-atom substitution in the derivative. It clearly pinpoints the most favorable conditions for retrieving phase information from the intensity data of an isomorphous pair of structures. Finally, it provides a means to determine a priori the overall reliability of the phase estimates and to design the calculations accordingly.
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 8
    Electronic Resource
    Electronic Resource
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 50 (1994), S. 155-166 
    ISSN: 1399-0047
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Methods to assist in the spatial and visual analysis of electron-density maps have been investigated as part of a project in molecular scene analysis [Fortier, Castleden, Glasgow, Conklin, Walmsley, Leherte & Allen (1993). Acta Cryst. D49, 168–178]. In particular, the usefulness of the topological approach for the segmentation of medium-resolution (3 Å) maps of proteins and their interpretation in terms of structural motifs has been assessed. The approach followed is that proposed by Johnson [Johnson (1977). ORCRIT. The Oak Ridge Critical Point Network Program. Chemistry Division, Oak Ridge National Laboratory, USA] which provides a global representation of the electron-density distribution through the location, identification and linkage of its critical points. In the first part of the study, the topological approach was applied to calculated maps of three proteins of small to medium size so as to develop a methodology that could then be used for analyzing maps of medium resolution. The methodology was then applied to both calculated and experimental maps of penicillopepsin at 3 Å resolution. The study shows that the networks of critical points can provide a useful segmentation of the maps, tracing the protein main chains and capturing their conformation. In addition, these networks can be parsed in terms of secondary-structure motifs, through a geometrical analysis of the critical points. The procedure adopted for secondary-structure recognition, which was phrased in terms of geometry-based rules, provides a basis for a further automated implementation of a more complete set of recognition operations through the use of artificial-intelligence techniques.
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 9
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 572-575 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is assumed that a crystal structure in P1 is fixed and that the random variables (vectors) h, k, l, m, n are uniformly and independently distributed over the subset of reciprocal space defined by h + k + l + m + n = 0. Then the 15 structure factors Eh, Ek, El, Em, En, Eh + k, Eh + l, Eh + m, Eh + n, Ek + l, Ek + m, Ek + n, El + m, El + n, Em + n as functions of the primitive random variables h, k, l, m, n, are themselves random variables, and their joint probability distribution is found. This distribution plays the central role in the theory and estimation of the five-phase structure invariants φh + φk + φl + φm + φn.
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 10
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 575-580 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is assumed that a crystal structure in P1 is fixed and that the 15 non-negative numbers R1, R2, R3, R4, R5; R12, R13, R14, R15, R23, R24, R25, R34, R35, R45 are also specified. The random variables (vectors) h, k, l, m, n are assumed to be uniformly and independently distributed in the regions of reciprocal space defined by |Eh| = R1, |Ek| = R2, |El| = R3, |Em| = R4, |En| = R5; (1) |Eh + k| = R12, |Eh + l| = R13, |Eh + m| = R14, |Eh + n| = R15, |Ek + l| = R23, |Ek + m| = R24, |Ek + n| = R25, |El + m| = R34, |El + n| = R35, |Em + n| = R45: (2) and h + k + l + m + n = 0. (3) Then the structure invariant φ = φh + φk + φl + φm + φn, as a function of the primitive random variables h, k, l, m, n, is itself a random variable, and its conditional probability distribution, given (1) and (2), is derived. The distribution leads to estimates for cos φ in terms of the 15 magnitudes (1) and (2).
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
Close ⊗
This website uses cookies and the analysis tool Matomo. More information can be found here...