1,721,104 research outputs found
Progetto PRIN biennale: 'Legami ad Idrogeno Intelligenti in Natura e nei Materiali Funzionali'
No full textLa caratteristica peculiare del LI è che la sua forza non può essere direttamente valutata dalla natura degli atomi interagenti come dimostrato, per esempio, dal legame O-H..O che è noto variare la sua energia di legame nell'incredibile intervallo 0.2-31 kcal/mol. Il maggior contributo dato dal nostro gruppo in questo campo riguarda la scoperta che l’energia del LI può essere prevista a livello semiquantitativo mediante l'uso dei 'chemical leitmotifs' (CL), che sono i cinque motivi molecolari in grado di dar origine a legami forti (quattro casi) o di forza modetrata (un caso), tutti gli altri casi di LI essendo deboli. Questi cinque motivi sono stati indicati dagli acronimi (+/-)CAHB, (-)CAHB, (+)CAHB, RAHB e PAHB (CAHB = charge-assisted, RAHB = resonance-assisted, PAHB= polarization-assisted HB; (+)= positive, (-)= negative).
L'identificazione dei cinque CL ci ha permesso di mettere a punto un nuovo metodo di indagine capace di prevedere la forza dei LI che si formeranno sulla base delle sole caratteristiche strutturali delle molecole che partecipano all’associazione. Questa aumentata capacità di previsione rappresenta un sensibile vantaggio nella progettazione mirata di cristalli molecolari aventi specifiche proprietà fisiche (ingegneria cristallina) e nell'interpretazione delle complesse strutture biomacromolecolari e, in generale, offre la possibilità di identificare i modi più efficaci di interazione e riconoscimento molecolare 'su un foglio di carta ', vale a dire da un disegno schematico dei frammenti molecolari interagenti.
Questo nuovo strumento di indagine è stato da noi utilizzato per analizzare diversi sistemi molecolari e biomolecolari campione dove si ipotizzava che LI forti fossero coinvolti nel meccanismo d'azione di particolare fenomeni chimici (reattività, tautomeria, riconoscimento, meccanismi dei materiali funzionali,..) o biochimici (catalisi enzimatica, fenomeni di trasporto, binding recettoriale,..). Un’analisi preliminare ha stabilito che lo spettro dei fenomeni coinvolti è molto vasto, potendo includere la tautomeria cheto-enolica, le proprietà bistato dei cristalli ferroelettrici, il trasferimento protonico allo stato eccitato, la formazione di αlfa-eliche, l'accoppiamento delle basi nel DNA, la catalisi enzimatica nella chetosteroide-isomerasi e nelle proteasi a serina ed aspartico, oltre che la formazione di LI forti nei minerali e la trasmissione protonica in acqua e nei canali transmembrana della gramicidina A e delle acquaporine. Nell’ambito del presente progetto, questi LI particolarmente forti che, a causa della loro forza intrinseca, sono capaci di svolgere funzioni di controllo in sistemi complessi sono stati chiamati 'LI funzionali', perché capaci di svolgere un ruolo funzionale, o 'LI intelligenti (smart HB)', perché, visti dall’esterno, appaiono agire in modo intelligente
Modern Hydrogen Bonding Theory
No full textThe H-bond was discovered in 1920 by W.M. Latimer and W.H. Rodebush [1] with the collaboration of M.L Huggins [2], three young men working in the laboratory of G.N. Lewis who gave of it a definition based on the Lewis electron-dot formalism which appears to be quite lucid and accurate even in modern terms. By the time that L. Pauling wrote his famous book “The Nature of the Chemical Bond” (1939-1940) [3], the H-bond had received complete systematization within the scheme of the newly developing VB theory, including the distinction between weak electrostatic and strong covalent H-bonds which was successively given VB theoretical dignity by Coulson and Danielsson (1954) [4]. This line of thought was accepted during the 1957 Ljubljana Conference [5] (the first H-bond meeting) and in “The Hydrogen Bond” by Pimentel and McClellan (1960) [6] (the first H-bond book).
This unified approach did not survive the division of sciences in more specialized branches occurred in the post-war period. The accumulation of ever new thermodynamic, spectroscopic and structural data, together with the underlying battle between VB and MO methods, lead to a period of general confusion, summarized in the Hopfinger’s (1973) statement “The only one definite fact about H-bonds is that there does not appear to be any definite rules which govern their geometry” [7]. It became clear, however, that the main point of the discussion was centered on the H-bond nature itself, that is on whether the H-bond was electrostatic, covalent, or both, a subject on which the most imaginative positions became allowed.
In 1991, Jeffrey and Saenger published “Hydrogen Bonding in Biological Structures” [8] where, for the first time, the most reliance is placed on the restricted number of accurate neutron structures and, in their absence, on carefully selected X-rays ones. This marks a turning point in H-bond studies: we accept the idea that our previous theories may be in error because based on insufficiently accurate experimental data, suspend temporarily any judgment on them, and start again to collect the widest and most reliable set of H-bond data from which to infer the true nature of the H-bond and then to lay sound foundations for any further theoretical advance. In the last 15 years, this novel data-oriented method of dealing with the H-bond problem has involved many researchers worldwide who, taking advantage of the existing crystallographic (CSD) [9] and thermodynamic (NIST) databases, have produced substantial changes in our way of considering the H-bond phenomenon. These changes will be the object of the present lecture
STRUCTURE AND CRYSTAL PACKING OF THE ORGANIC SALT 2-(DIMETHYLAMINO)-5-METHYL-1,3-THIAZOLIUM 1,1,2,3,3-PENTACYANOPROPENIDE, C6H11N2S+.C8N5-
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Crystal and Molecular Structures of 2,6-cis-Dimethylpiperidyl-N-phenylacetamidine and 2,6-cis-Dimethylpiperidyl-N-phenyl-2,2-dimethylpropionamidine. An X-ray Crystallographic Investigation of the C(sp2)-N(piperidyl) Bond
No full textThe single crystal X-ray analyses of 2, 6-cis-dimethylpiperidyl-N-phenylacetamidine (MA) and 2, 6-cis-dimethylpiperidyl-N-phenyl-2, 2-dimethylpropionamidine (TBA) are described. MA crystallizes in the space group P2 1\c with four molecules in the unit cell of dimensions a = 10.238 (2), b = 10.189 (2), c = 12.875 (3) Å. and β = 95.82 (2)°. The structure was solved and refined from 1401 unique observed reflections collected on an automated four-circle diffractometer to final values of the discrepancy indices of R = 0.046 and Rw = 0.058. TBA crystallizes in the space group P21\c with eight molecules in the unit cell of dimensions a = 8.470 (2), b = 16.095 (3), c = 24.900 (4) Å, and β = 96.29 (2)°. From 2633 unique observed reflections similarly collected the structure was solved and refined to final values of the discrepancy indices of R = 0.061 and Rw = 0.074. The structure analyses show, in agreement with 13C NMR spectroscopic data, that ihe two molecules adopt different conformations around the C(sp2)-N(piperidyl) bond, the amidinic group and the piperidyl ring being approximately coplanar in MA and orthogonal in TBA, respectively. The comparison of the present data with the data in the literature, supported by nonbonded intramolecular potential energy calculations and IN DO calculations, allows clarification of the relationship among the torsion angle around the C-N bond, the bond distances in the amidinic group, the pyramidality of the N (piperidyl) atom and the conformation of the 2, 6-m-methyl groups in the piperidyl ring. © 1979, American Chemical Society. All rights reserved
Third Italian-Israeli Meeting on chemical crystallography: 'Crystal structure and molecular recognition'
No full textThird Italian-Israeli Meeting on chemical crystallography: 'Crystal structure and molecular recognition
24th European Crystallographic Meeting- 'Microsymposium: From molecular interactions to molecular assemblies'- Marrakech (Marocco), 22-27 August 2007
No full text'From molecular interactions to molecular assemblies
Hydrogen bond models and theories: The dual hydrogen bond model and its consequences
No full textThe H-bond can be reinterpreted starting from the dual H-bond model, for which any D–H···:A bond is not a bond donated by D–H to :A but rather consists of two bonds formed by the central proton with two adjacent acceptors. Analogously, the H-bond energy, E(HB), is not the D–H···:A dissociation energy but the smaller of two bond-dissociation energies, D0(D–H) and D0(H–A), by which −D: and :A are competitively bound to the proton. If one is stronger, the other is weaker, and weak the overall H-bond will be. Strong bonds occur when ΔD0 = D0(D–H) − D0(H–A) = 0 or, in terms of affinity for the proton (pa), when Δpa = pa(D−) − pa(A) = 0. H-bond properties are then function of two variables, pa(D−) and pa(A), or better of their linear combinations, Σpa = pa(D−) + pa(A) and Δpa = pa(D−) − pa(A), having respective meanings of mean donor/acceptor electronegativity and of energy difference, ΔrE, between tautomeric D–H···:A and (−)D:···H–A(+) forms. Two cases are studied. The case: ‘Σpa variable for Δpa = 0’ leads to quantitative relationships between D/A electronegativity and maximum energy, E(HB,MAX), achievable for each D/A electronegativity class, EC(D,A). The case: ‘Δpa variable for Σpa = constant’ leads to formulate three different but inter-consistent H-bond theories which are separately discussed. The last one, which is called ‘pKa equalization principle’ and where Δpa values are empirically estimated from the acid–base dissociation constants in water as ΔpKa = pKAH(D–H) − pKBH+(A–H+), is shown to be a powerful method of large applicability for predicting the H-bond strengths from thermodynamic parameters
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