Large Torsion Angle Research Articles

N-Benzyl-2,5-bis(2-thienyl)pyrrole.

The solid-state structure of the title compound, C19H15NS2, is unusual among substituted thiophene/pyrrole derivatives in that the molecular packing is dominated by pi-pi interactions between the benzyl substituents. This may be due to the large torsion angles observed between adjacent heterocycles. Torsion angles between adjacent rings in polypyrrole and polythiophene conducting polymers are related to conjugation length and the conductivity properties of the polymer materials. The title compound crystallizes in space group P21/c with two molecules in the asymmetric unit, both of which exhibit disorder in one of their thiophene rings.

Antiferromagnetic exchange in meta-phenylene bridged bis(tris-o-iminosemiquinonato)metal complexes

By reaction of the ligand N,N′ bis(3,5-di-tert-butyl-2-hydroxyphenyl)-1,3-phenylenediamine (1), with Fe, Co or Mn salts, three complexes were synthesized where the bis-bidentate ligand is in the bis-semiquinonato oxidation state. Although the m-phenylene linker is known to afford ferromagnetic coupling in diradicals, the antiferromagnetic interaction of intramolecular origin we observed is not unexpected, given the large torsion angles between the semiquinonato and the m-phenylene planes.

To What Extent Can Nine‐Membered Monocycles Be Aromatic?

AbstractAb initio and density functional computations have been used to characterize the heteronins. New targets for synthesis are suggested on the basis of the results. The aromaticity of the heteronins was evaluated via numerous criteria such as magnetic properties (NICS and δH), geometric indices (Bird, BDSHRT and HOMA), aromatic stabilization energies (ASE), and the barriers to planarity. Along with the experimentally characterized cyclononatetraenyl (1CH−) and azonide (1N−) anions, the phosphonide ion (1P−, which has not been reported before), favors planar (C2v) symmetry. All the other heteronins are nonplanar in order to alleviate ring strain. Comparisons of the neutral planar heteronins (constrained to C2v symmetry) show the extent to which heteroatoms like P and S, as well as N and O (in oxonin, 1O), can participate in the cyclic electron delocalization. Due to hyperconjugation, the C2v‐symmetric 9,9‐distannylcyclononatetraene 1C(SnH3)2 is moderately aromatic, in contrast to 1CH2, which is a nonaromatic, conjugated system. The planar C2v‐symmetric 1BH and 1AlH and 1CF2 are antiaromatic, like the planar form of 1CH+. Annelation with three‐membered rings resulted in planar heteronins. The sole exception was phosphonin (8PH), but the barrier to planarity decreased to 1.8 kcal/mol in 8PR, due to the bulky R: 2,6‐di‐tert‐butylphenyl substituent at the phosphorus atom. Among the 8π heteronins, only the previously reported 1CH+ is Möbius‐aromatic; 1BH, 1AlH and 1CF2 also have C2 structures but are nonaromatic due to large torsional angles which preclude cyclic delocalization. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Preparation and magnetic interaction of a polymer pendent nitroxide with a bulky protecting group

In order to clarify the origin of antiferromagnetic nature of polyphenylacetylene pendant polyradicals, a phenylacetylene-polymer I using a nitroxide with a bulky protecting group as a spin-building block was synthesized and its magnetic behavior was studied by means of SQUID and EPR. The antiferromagnetic behavior observed in both SQUID and EPR-matrix experiments indicates that antiferromagnetic nature is ascribed to the probable large torsion angle(s) of the polymer main chain.

Synthesis and Characterization of Nitrato-Triamine-Metal(II) Complexes. Conglomerate Crystallization Part 55. Crystal Structure of [Ni(dien)(O2NO)(ONO2)] (I), {[Cu(dien)-(μ-ONO2)]NO3}∞ (II), [Zn(dien)(O2NO)(ONO2)] (III), [Ni(Medpt)(O2NO)(ONO2)] (IV) and [Cu(Medpt)(ONO2)2] (V)

In absolute ethanol and in the presence of triethylorthoformate, reactions of metal(II) nitrates with linear tridentate amines afforded metal complexes of the formula M(NNN)(NO3)2, where M = Ni2+, Cu2+ and Zn2+, and NNN = dien and Medpt. The compounds fall into three categories in accordance with their stereochemistry and mode of binding of the nitrato ligands. Compounds I, [Ni(dien)(O2NO)(ONO2)] and III, [Zn(dien)(O2NO)(ONO2)] are isomorphous and isostructural. They crystallize in the monoclinic space group P21/n with nearly identical cell constants. The stereochemistry of these two compounds is such that the terdentate dien ligand forms a fac MN3 moiety with the two oxygens of the bidentate nitrato ligand trans to the terminal NH2. These ligands form the base of the octahedral arrangement in which the sixth position, trans to the secondary nitrogen of the dien, is an oxygen of the monodentate nitrato ligand. Compound IV, [Ni(Medpt)(O2NO)(ONO2)] falls into the same category as I and III despite the fact that the two rings in the Ni-Medpt moiety are six-membered rings, unlike those in compounds I and III which are five-membered rings. Nevertheless, the nickel-amine arrangement is fac. The bidentate nitrato-oxygens are trans to the terminal NH2 of the amine ligand, and the oxygen of the monodentate nitrato ligand is trans to the tertiary amine-nitrogen. Such stereochemistry is prevalent for nickel and zinc compounds. Interestingly, compound IV crystallizes as a conglomerate (space group P212121). Compound II, {[Cu(dien)(μ-ONO2)]NO3}∞ belongs to the second category and has a polymeric structure. The repeating fragment in the polymeric chain is a Cu(dien)-O fragment with the monodentate nitrato ligand occupying an equatorial position of the base. A second oxygen of the equatorial nitrate becomes an axial ligand for an adjacent Cu-N3O fragment. In this way the substance propagates into an infinite chain. The repeating unit has an effective square pyramidal, five-coordinate, configuration. Finally, the compound crystallizes as a racemate. The second nitrate necessary for charge compensation of this copper(II) compound is ionic and its function is to hold the infinite chains of the lattice. The third category represented by compound V, [Cu(Medpt)(ONO2)2] contains two molecules in the asymmetric unit of the racemic lattice (monoclinic, space group P21/a). The structure of Cu-Medpt is unlike that of IV in that both species present in the asymmetric unit have the amine ligand in a mer configuration which together with a monodentate oxygen of a nitrato ligand form a base plane of a square pyramid. The fifth ligand of both Cu2+ ions is a second monodentate nitrato ligand. The stereochemical differences between the two Cu2+ ions are insignificant for the Cu-Medpt fragment, which share the same conformation and configuration. The major difference between the two species is the torsional angles defined by the Cu-O-N-O angles. The difference arises from variation in the hydrogens of the primary amine moieties selected by nitrato-oxygens to form intramolecular hydrogen bonds. Finally, there is a little variation in the equatorial Cu-ONO2 stereochemistry because of steric hindrance, imposed by the Medpt, preventing large torsional angles by these nitrato ligands. This is evident by comparing the two copper species shown in Finally, nitrate-to-Br ligand exchange was found to take place when KBr pellets are prepared for IR spectral measurements.

Structure of an Early Intermediate in the M-State Phase of the Bacteriorhodopsin Photocycle

The structure of an early M-intermediate of the wild-type bacteriorhodopsin photocycle formed by actinic illumination at 230 K has been determined by x-ray crystallography to a resolution of 2.0 Å. Three-dimensional crystals were trapped by illuminating with actinic light at 230 K, followed by quenching in liquid nitrogen. Amide I, amide II, and other infrared absorption bands, recorded from single bacteriorhodopsin crystals, confirm that the M-substate formed represents a structure that occurs early after deprotonation of the Schiff base. Rotation about the retinal C13—C14 double bond appears to be complete, but a relatively large torsion angle of 26° is still seen for the C14—C15 bond. The intramolecular stress associated with the isomerization of retinal and the subsequent deprotonation of the Schiff base generates numerous small but experimentally measurable structural changes within the protein. Many of the residues that are displaced during the formation of the late M (M N) substate formed by three-dimensional crystals of the D96N mutant (Luecke et al., 1999b) are positioned, in early M, between their resting-state locations and the ones which they will adopt at the end of the M phase. The relatively small magnitude of atomic displacements observed in this intermediate, and the well-defined positions adopted by nearly all of the atoms in the structure, may make the formation of this structure favorable to model (simulate) by molecular dynamics.

New Insight into the Nature of Electron Delocalization: the Driving Forces for Distorting the Geometry of Stilbene-Like Species

To understand the nature of electron delocalization while questioning the abnormally large torsional angle θ of N-phenylmethylene-3-pyridineamine (6), we greatly improved our new program for energy partitioning. Meanwhile, the crystal structures of N-phenylmethylene-2-thiazoleamine (1a) and N-(4-nitro-phenyl)methylene-2-thiazoleamine (1b) were determined using X-ray diffraction. As shown by the optimized geometries of the molecules, such as 1a, 1b, (4-NO2-Ph)-CHN-2-pyrimidyl, (4-NO2-Ph)-CHN-2-pyridyl, and 6 with HF, DFT, MP2, and AM1, the results that d2(Ee(θ))/d|θ|2 > 0.0 for total electronic energy and d2(EN(θ))/d|θ|2 < 0.0 for nuclear repulsion and d(Ee(θ=42°))/d|θ|2 = 0.0 are not an artifact of a given optimized method, nor a distinct feature of a special molecule. As shown by the energy partitions, the π−π, π−σ, and nonbonded σ−σ interactions between fragments are always destabilization, and it is the nonbonded σ−σ interaction, rather than the π−π interaction, that distorts stilbene-like species away from their planar geometry. The destabilizing EX interactions between fragments is basically stabilization as far as its total effect on whole electronic state is considered. Correspondingly, the stabilizing CT interaction is practically destabilization. Thus, at the planar geometry, it is due to d(CT)/d(r) < 0.0, d(EX)/d(r) <0.0, or their sum d(CT)/d(r) + d(EX)/d(r) < 0.0, where r = rab or r14, to shorten the length r14 of the bond C1−N4 as well as to reduce the distance rab between fragments. A stilbene-like species has to distort itself away from its planar geometry in order to maintain its lowest total electronic energy Ee as far as possible when the attractive force d(Ee(θ))/d(rab) > 0.0 is not large enough to balance the resistance force d(Vab)/d(rab) < 0.0. Resistances to the distortion arise from the destabilizing π−σ interaction and from the dEN(θ)/d|θ| > 0.0. At a geometry with about θ = 52°, d(ΔE(θ))/d|θ| = 0 is a compromise between the nonbonded σ−σ and π−σ interactions, and it is approximately in accord with the d(Ee(θ=42°))/d|θ| = 0.0 obtained from standard Gaussian 98 program.

Electronic structure of some conjugated polymers for electron transport

The chemical and electronic structure of three different, strictly alternating copolymers, poly(2,5-diheptyl-1,4-phenylene-alt-1,4-naphthylene) (P14NHP), poly(2,5-diheptyl-1,4-phenylene-alt-2,6-naphthylene) (P26NHP) and poly(2,5-diheptyl-1,4-phenylene-alt-9,10-anthrylene) (P910AHP), have been studied by photoelectron spectroscopy and optical absorption spectroscopy. The experimental results have been analyzed using the results of quantum chemical calculations. In the geometrical structure of all three of the polymers there are large torsion angles between the phenylene unit and the naphthylene or anthrylene units. These large torsion angles lead to localization of the π-electron wave functions, and minimal conjugation along the polymer backbone. For all three polymers, the highest occupied molecular orbital is completely localized to the naphthylene or anthrylene unit. The frontier molecular orbital wave functions are very reminiscent of the highest occupied orbitals of the isolated naphthalene or anthracene molecules. The optical absorption spectra of all three polymers verify the existence of large optical band gaps, consistent with the large torsion angels. The first several optical transitions in the polymers are also very reminiscent of the transitions in single naphthalene and anthracene molecules.

The morph server: a standardized system for analyzing and visualizing macromolecular motions in a database framework.

The number of solved structures of macromolecules that have the same fold and thus exhibit some degree of conformational variability is rapidly increasing. It is consequently advantageous to develop a standardized terminology for describing this variability and automated systems for processing protein structures in different conformations. We have developed such a system as a 'front-end' server to our database of macromolecular motions. Our system attempts to describe a protein motion as a rigid-body rotation of a small 'core' relative to a larger one, using a set of hinges. The motion is placed in a standardized coordinate system so that all statistics between any two motions are directly comparable. We find that while this model can accommodate most protein motions, it cannot accommodate all; the degree to which a motion can be accommodated provides an aid in classifying it. Furthermore, we perform an adiabatic mapping (a restrained interpolation) between every two conformations. This gives some indication of the extent of the energetic barriers that need to be surmounted in the motion, and as a by-product results in a 'morph movie'. We make these movies available over the Web to aid in visualization. Many instances of conformational variability occur between proteins with somewhat different sequences. We can accommodate these differences in a rough fashion, generating an 'evolutionary morph'. Users have already submitted hundreds of examples of protein motions to our server, producing a comprehensive set of statistics. So far the statistics show that the median submitted motion has a rotation of approximately 10 degrees and a maximum Calpha displacement of 17 A. Almost all involve at least one large torsion angle change of >140 degrees. The server is accessible at http://bioinfo.mbb.yale. edu/MolMovDB

Molecular and crystal structure of hexa(phenylthio)-1,3-butadiene at 87 K

Abstract The crystal structure of hexa(phenylthio)-l,3-butadiene, C40H30S6, was studied by X-ray diffraction at 87 K. The space group is monoclinic, Ia, a = 13.947(2) Å, b = 22.254(4) Å, c = 11.610(2) Å, β = 106.54(1)° and Z = 4. Intensity data were collected with MoKα radiation to (sin θ)/λ &lt; 0.705 Å−1. The structure was solved by direct methods. Refinement based on 4920 |F o| converged at Rw = 0.032. The central bond of the butadienyl core, 1.472(4) Å, is comparable to the single bond of unconjugated 1,3-butadiene, whereas the two double bonds, 1.353(5) Å and 1.371(5) Å, are longer than expected for a C(sp 2) = C(sp 2) bond. This may be attributed in part to hybridization changes caused by the S atoms. The large torsion angle over the central bond C1–C2–C3–C4 = −66.8(4)°, reflects the accommodation of this fragment to a highly crowded intramolecular environment. All phenyl rings have intramolecular contacts within van der Waals radii with at least one other ring and one of the butadiene carbon atoms. The sulfur atoms are engaged in an inner loop of short 1,n S···S contacts in the range 3.070(3) Å −3.429(4) Å for n ≥ 3. Averaged deviations from 120° in the ipso, 0.5(1)°, and ortho, −0.5(1)°, positions of the six phenyl rings show that the S atoms serve as electronegative substituents attracting electron density from the rings.

Synthesis, characterization and crystal structure of ruthenium(II) polypyridyl complexes

[Ru(bipy)2(5-R-phen)](ClO4)2 complexes have been prepared, where bipy=2,2′- bipyridine, phen=1,10-phenanthroline, and R=H, Me, NO2 or NH2. The influence of the different 5- substituted phen on the electron delocalization and ligand-ligand interactions have been investigated by solution n.m.r.. The ligand- ligand interaction has also been observed in the solid state by determining the single crystal structure of [Ru(bipy)2(5- NO2-phen)](ClO4)2. The strong steric interaction between the polypyridyl ligands was relieved neither by the elongation of Ru–N bonds (2.055–2.086A) nor by increase of N–Ru–N bite angles (77.8–79.4°), but rather by distortion of the coordination sphere by forming specific angles (δ=2.0, 3.2 and 4.2°) between the polypyridyl ligand planes and coordination planes (N–Ru–N), and larger torsion angles (5.8 and 8.2°) between the two pyridine rings for each bipy.

Inhomogeneity of Spin-Coated and Cast Non-Regioregular Poly(3-hexylthiophene) Films. Structures and Electrical and Photophysical Properties

We report structures and electrical and photophysical properties of spin-coated and cast non-regioregular poly(3-hexylthiophene) (P3HT) films. Large differences of the properties are found in these films prepared from different conditions. Results of optical absorption, electron spin resonance, and X-ray diffraction measurements show that the extent of conjugation of P3HT is homogeneous in the spin-coated film, whereas it is inhomogeneous in the cast film. The existence of relatively strong distorted segments with large torsion angles in the thiophene backbone is also suggested, especially in the cast film. Carrier transfer phenomena and unrelaxed exciton diffusion to an emissive singlet polaron exciton are discussed on the basis of proposed structure models of the spin-coated and the cast films.

Synthesis and Characterization of Axially Chiral Molecules Containing Dendritic Substituents

Enantiomerically pure, axially chiral (S)-1,1′-bi-2-naphthol has been used as a core material to which Frechet-type dendritic wedges of the zeroth up to the fourth generation were attached, yielding the first axially chiral dendrimers 1−5. The chiroptical features of these compounds were studied and revealed an increasing molar optical activity for higher generations of dendrimers. This effect can be explained by a larger torsional angle between the naphthyl units, caused by steric repulsions between the dendritic wedges. However, the effect is marginal, indicating a high degree of flexibility present in the axially chiral dendrimers.

Structure-activity relationships of penem antibiotics: Crystallographic structures and implications for their antimicrobial activities

Twelve closely related crystal structures of the penem derivatives revealed a characteristic short contact of the oxygen atom in the C2 side-chains with the S1 atom. The side-chain conformations of the crystal structures showed a good correlation with the antimicrobial activity. In particular, the penems which show high antimicrobial activity have similar torsion angles for S1-C2-C1′-C2′, suggesting that the disposition of the C2′ atom would be important for binding to penicillin-interacting enzymes. Two conformations of the C6 hydroxyethyl group were observed in the crystal structures. Of those two, the conformation with a larger torsion angle ( δ = 179.2°) is deduced to be the enzyme-bound conformation in the Michaelis complex.

Triphenylstannylferrocene and 1,1'-bis(triphenylstannyl)ferrocene

The coordination about the Sn atoms is tetrahedral for both triphenylstannylferrocene, [FeSn(C 5 H 4 )(C 5 H 5 )(C 6 H 5 ) 3 ] (1), and 1,1'-bis(triphenylstannyl)ferrocene, [FeSn 2 (C 5 H 4 ) 2 (C 6 H 5 ) 6 ] (2), with C-Sn-C angles ranging from 106.4 (1) to 111.3 (1) o . The cyclopentadienyl rings in the ferrocene moieties of both compounds, (1) and (2), are planar with dihedral angles of 1.7 (2) and 1.4 (3) o , respectively, between the rings. However, the rings in (1) are eclipsed whereas in (2) the presence of two bulky triphenyltin groups attached in a trans configuration causes the rings to be slightly staggered, the largest torsion angle about the centroids of the two cyclopentadienyl rings being 19.4 o

Relationships between nitro group reduction potentials and torsion angles in di-ortho-substituted nitrobenzenes; a crystallographic and oxygen-17 NMR study

A series of 3-nitro-4-alkylbenzamides has been prepared, and the effects of nitro group torsion angle on reduction potential studied. Nitro and carboxamide group torsion angles have been determined by 17O NMR spectroscopy and X-ray crystallography, and one-electron reduction potentials by pulse radiolysis. 17O Chemical shifts indicated similar amide torsion angles (from 35° to 45°) as the alkyl group varied from hydrogen to tert-butyl, but widely differing nitro group torsion angles; from 36°(hydrogen) to 92°(tert-butyl). Crystal structures of the isopropyl and tert-butyl derivatives indicate amide group torsion angles (50° and 64°) somewhat larger than those predicted by 17O NMR, and nitro group torsion angles (59° and 65° respectively) considerably smaller than those predicted by 17O NMR (75° and 92° respectively). These results support earlier data that 17O chemical shifts predict for erroneously large nitro group torsion angles in non-rigid but sterically crowded molecules, because of additional contributions to the shift from van der Waals repulsions. The drop in reduction potential of 90 mV between the unsubstituted and tert-butyl derivatives is too large to be accounted for by the electronic effects of the alkyl groups, and indicates that increasing the nitro group torsion angle significantly lowers reduction potential.

Domain Closure in Lactoferrin: Two Hinges Produce a See-saw Motion Between Alternative Close-packed Interfaces

Lactoferrin is an iron transport protein. Upon binding iron, the two domains in the N-terminal half of the molecule move together. Previous work has shown that this domain closure involves two hinges. Using the newly refined structure of the open form, the structural mechanism underlying this motion is analyzed here in detail. Upon closure the domains rotate 54° essentially as rigid bodies. The axis of rotation passes through the two β-strands linking the domains. These strands contain hinges in the sense that three large torsion angle changes are responsible for the bulk of the motion while smaller torsion angle changes in neighboring residues are responsible for the remainder of the motion. The rotation axes of these three torsion angle changes are nearly parallel to the axis of the overall 54° rotation, so the local motion in the hinges can be directly related to the overall motion. A crucial feature of the hinge residues is that they have very few packing constraints on their main-chain atoms. The domains make different packing contacts with each other in the open and closed forms. These contacts form two interdomain interfaces arranged on either side of the hinges. Pivoting about the hinges produces a see-saw motion between the two interfaces. That is, when the domains close down, residues in the interface on one side of the hinges become buried and close-packed and residues on the other side become exposed. The situation is reversed when the domains open up. Lactoferrin provides a particularly clear example of the general features of hinged domain motion. It is compared to other instances of hinged domain closure and contrasted with instances of shear domain closure, where the overall motion is a summation of many small sliding motions between close-packed segments of polypeptide.

Verdazyls. Part 33. EPR and ENDOR studies of 6-oxo- and 6-thioxoverdazyls. X-Ray molecular structure of 1,3,5-triphenyl-6-oxoverdazyl and 3-tert-butyl-1,5-diphenyl-6-thioxoverdazyl

A range of 6-oxoverdazyls 2b–h and 6-thioxoverdazyls 3a–c and 3e–h has been directly prepared by dehydrogenation of the corresponding 1, 4, 5, 6-tetrahydro-1, 2, 4, 5-tetrazin-3(2H)-ones 5b–h and thiones 7a–c and 7e–hwith lead dioxide, potassium hexacyanoferrate (III), or bis(4-methylphenyl)-aminyl. X-Ray analyses reveal a nearly planar verdazyl framework for the oxoverdazyl 2a, whereas the thioxoverdazyl 3ftakes on a flat boat conformation. In the latter, owing to the bulky sulfur the N-phenyl groups are considerably twisted out of the plane of the verdazyl ring. Electronic absorption spectra of the deeply coloured radicals exhibit characteristic bands in the visible region with λmax, 1 ranging from 445 to 608 nm. EPR, ENDOR and 2H NMR studies have led to a complete analysis and full assignment of all hyperfine coupling constants. The π-SOMO of 6-oxo-2 and 6-thioxo-verdazyls 3, having nodes at C(3) and C(6), is mainly confined to the nitrogens of the verdazyl ring. Spin delocalization into the N-phenyl groups is reduced, particularly in the 6-thioxoverdazyls, owing to the large torsion angle about the N–C(phenyl) bond.

Effect of volume loading, pressure loading, and inotropic stimulation on left ventricular torsion in humans.

The transmural distribution of fiber angles and the extent of shortening among obliquely oriented fibers are likely to be major determinants of the twisting motion that accompanies left ventricular (LV) ejection. As such, measurements of torsion may provide useful information about LV contractile function, but other factors, such as ventricular loading conditions, may also regulate this motion. Torsion angles (theta i) of midventricular and apical regions were measured relative to a reference minor axis near the base in seven human cardiac allografts from biplane radiographic images of metallic midwall markers. Pressure loading with methoxamine (5-10 muk/kg/min) increased LV end-systolic pressure by 41 +/- 14 mm Hg (p less than 0.0001). Volume loading with normal saline raised LV end-diastolic pressure from 9.9 +/- 5.2 to 19.6 +/- 4.9 mm Hg (p less than 0.0001). These alterations in LV loading conditions were associated with no change in theta i (difference not significant) for any marker site. Inotropic stimulation with dobutamine (5 micrograms/kg/min) increased values of theta i by as much as twofold (p less than 0.05); this response varied considerably depending on marker location, with the middle and apical inferior wall and the apical lateral wall being the most sensitive. When the marker site associated with the largest torsion angle (theta max) was considered in each patient, dobutamine increased theta max in all cases (25.2 +/- 10.5 degrees versus 15.8 +/- 7.7 degrees, p less than 0.001), whereas pressure and volume loading had negligible effects. This 59% increase in theta max was greater than that of conventional load-dependent indexes of LV systolic performance such as stroke volume (16%), ejection fraction (22%), and maximum rate of LV pressure rise (52%). This component of LV motion is relatively insensitive to alterations in preload and afterload, while changes in contractile state influence LV torsion in a regionally heterogeneous manner. Quantification of LV torsion may, therefore, provide a sensitive and relatively load-independent measure of contractile performance that may prove to be useful in the serial assessment of LV function.

Crystal structure of the ASP-199.fwdarw.asparagine mutant of chloramphenicol acetyltransferase to 2.35.ANG. resolution: structural consequences of disruption of a buried salt bridge

The crystal structure of the Asp-199----Asn mutant of chloramphenicol acetyltransferase (CAT) has been determined to 2.35-A resolution. In wild-type CAT Asp-199 is involved in a fully buried intrasubunit salt bridge with Arg-18, an interaction that is adjacent to the active site. Replacement of aspartate with asparagine by site-directed mutagenesis disrupts this salt bridge and causes extensive conformational changes within the active site. The imidazole group of the catalytically essential His-195 is reoriented, with the loss of interactions thought to stabilize the preferred tautomer of this residue. Arg-18 and Asn-199 form three new intersubunit interactions as a result of large side-chain torsion angle changes which cause the movement of two polypeptide loops, some residues of which are up to 20 A away from the site of the mutation. The new interactions of Arg-18 and Asn-199 compensate for the loss of the buried salt bridge and afford near-wild-type thermostability to Asn-199 CAT, albeit with a greatly reduced activity.