Home About us Contact
|
Home About us Contact | ||
|
|
||
Planar Sheets (planar + sheet)
Selected AbstractsTwo modes of O,H...O hydrogen bonding utilized in dimorphs of racemic 6- O -acryloyl-2- O -benzoyl- myo -inositol 1,3,5-orthoformateACTA CRYSTALLOGRAPHICA SECTION C, Issue 2 2009Shobhana Krishnaswamy The title compound, C17H16O8, yields conformational dimorphs [forms (I) and (II)] at room temperature, separately or concomitantly, depending on the solvent of crystallization. The yield of crystals of form (I) is always much more than that of crystals of form (II). The molecule has one donor ,OH group that can make intermolecular O,H...O hydrogen bonds with one of the two acceptor C=O groups, as well as with the hydroxyl O atom; interestingly, each of the options is utilized separately in the dimorphs. The crystal structure of form (I) contains one molecule in the asymmetric unit and is organized as a planar sheet of centrosymmetric dimers via O,H...O hydrogen bonds involving the OH group and the carbonyl O atom of the acryloyl group. In the crystal structure of form (II), which contains two independent molecules in the asymmetric unit, two different O,H...O hydrogen bonds, viz. hydroxyl,hydroxyl and hydroxyl,carbonyl (benzoyl), connect the molecules in a layered arrangement. Another notable feature is the transformation of form (II) to form (I) via melt crystallization upon heating to 411,K. The higher yield of form (I) during crystallization and the thermal transition of form (II) to form (I) suggest that the association in form (I) is more highly favoured than that in form (II), which is valuable in understanding the priorities of molecular aggregation during nucleation of various polymorphs. [source] In-situ high-pressure study of the ordered phase of ethyl propionateACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2007Roman Gajda Ethyl propionate, C5H10O2 (m.p. 199,K), has been in-situ pressure-frozen and its structure determined at 1.34, 1.98 and 2.45,GPa. The crystal structure of the new high-pressure phase (denoted ,) is different from phase , obtained by lowering the temperature. The freezing pressure of ethyl propionate at 296,K is 1.03,GPa. The molecule assumes an extended chain s-trans,trans,trans conformation, only slightly distorted from planarity. The closest intermolecular contacts in both phases are formed between carbonyl O and methyl H atoms; however, the ethyl-group H atoms in phase , form no contacts shorter than 2.58,Å. A considerable molecular volume difference of 24.2,Å3 between phases , and , can be rationalized in terms of degrees of freedom of molecules arranged into closely packed structures: the three degrees of freedom allowed for rearrangements of molecules confined to planar sheets in phase ,, but are not sufficient for obtaining a densely packed pattern. [source] An unusual syn conformation of 5-formyluracil stabilized by supramolecular interactionsACTA CRYSTALLOGRAPHICA SECTION C, Issue 11 2007Gustavo Portalone The asymmetric unit of the amino,oxo tautomer of 5-formyluracil (systematic name: 2,4-dioxo-1,2,3,4-tetrahydropyrimidine-5-carbaldehyde), C5H4N2O3, comprises one planar amino,oxo tautomer, as every atom in the structure lies on a crystallographic mirror plane. At variance with all the previously reported small-molecule crystal structures containing the 5-formyluracil residue, the formyl substituent in the title compound exhibits an unusual syn conformation. The molecules are linked into planar sheets parallel to the bc plane by a combination of six N,H...O and C,H...O hydrogen bonds. Four of the hydrogen bonds are utilized to stabilize the formyl group in the syn conformation. [source] RAPID EFFECT OF PROGESTERONE ON TRANSEPITHELIAL RESISTANCE OF HUMAN FETAL MEMBRANES: EVIDENCE FOR NON-GENOMIC ACTIONCLINICAL AND EXPERIMENTAL PHARMACOLOGY AND PHYSIOLOGY, Issue 2 2008CH Verikouki SUMMARY 1The factors that regulate human fetal membrane transport mechanisms are unknown. The aim of the present study was to investigate the effect of progesterone on transepithelial electrical resistance (RTE) in the human amniochorion. 2Fetal membranes from uncomplicated term pregnancies were obtained immediately after vaginal or Caesarean deliveries. Intact pieces were mounted as planar sheets separating an Ussing chamber. Progesterone (10,4 to 10,7 mol/L), mifepristone (10,4 to 10,8 mol/L) and combinations of progesterone plus mifepristone were applied to the chambers facing the fetal or maternal sides of the membrane. The RTE was measured before and 1, 5, 10, 15, 20, 25, 30, 45 and 60 min after each solution was added (at 37°C). The RTE was calculated in ,.cm2, according to Ohm's law. 3The mean (±SEM) basal value of RTE before the application of any substance in all experiments was 29.1 ± 0.4 ,.cm2., The net change in the RTE (,RTE) in relation to the basal value was calculated in each experiment. Progesterone, mifepristone and the combination of progesterone and mifepristone induced a rapid, surge-type increase in RTE during the 1st min on both sides of the membrane. The combination of progesterone plus mifepristone exerted a synergistic action. The effect was stronger on the fetal side than on the maternal side for all substances tested (P < 0.05). The highest ,RTE during the 1st min on the fetal side was seen with the combination of progesterone plus mifepristone (4.0 ± 0.3 ,.cm2) and the lowest ,RTE occurred with mifepristone (1.5 ± 0.1 ,.cm2). 4The present results demonstrated that the RTE of human fetal membranes increases rapidly in response to progesterone. It is possible that changes in RTE play a role in the control of membrane permeability during pregnancy. [source] | ||||||||||