Showing papers on "Hydrogen bond published in 1994"

Low-barrier hydrogen bonds and enzymic catalysis

Abstract: Formation of a short (less than 2.5 angstroms), very strong, low-barrier hydrogen bond in the transition state, or in an enzyme-intermediate complex, can be an important contribution to enzymic catalysis. Formation of such a bond can supply 10 to 20 kilocalories per mole and thus facilitate difficult reactions such as enolization of carboxylate groups. Because low-barrier hydrogen bonds form only when the pKa's (negative logarithm of the acid constant) of the oxygens or nitrogens sharing the hydrogen are similar, a weak hydrogen bond in the enzyme-substrate complex in which the pKa's do not match can become a strong, low-barrier one if the pKa's become matched in the transition state or enzyme-intermediate complex. Several examples of enzymatic reactions that appear to use this principle are presented.

Crystal and molecular structure of a collagen-like peptide at 1.9 A resolution.

Abstract: The structure of a protein triple helix has been determined at 1.9 angstrom resolution by x-ray crystallographic studies of a collagen-like peptide containing a single substitution of the consensus sequence. This peptide adopts a triple-helical structure that confirms the basic features determined from fiber diffraction studies on collagen: supercoiling of polyproline II helices and interchain hydrogen bonding that follows the model II of Rich and Crick. In addition, the structure provides new information concerning the nature of this protein fold. Each triple helix is surrounded by a cylinder of hydration, with an extensive hydrogen bonding network between water molecules and peptide acceptor groups. Hydroxyproline residues have a critical role in this water network. The interaxial spacing of triple helices in the crystal is similar to that in collagen fibrils, and the water networks linking adjacent triple helices in the crystal structure are likely to be present in connective tissues. The breaking of the repeating (X-Y-Gly)n pattern by a Gly-->Ala substitution results in a subtle alteration of the conformation, with a local untwisting of the triple helix. At the substitution site, direct interchain hydrogen bonds are replaced with interstitial water bridges between the peptide groups. Similar conformational changes may occur in Gly-->X mutated collagens responsible for the diseases osteogenesis imperfecta, chondrodysplasias, and Ehlers-Danlos syndrome IV.

Evidence for resonance-assisted hydrogen bonding. 4. Covalent nature of the strong homonuclear hydrogen bond. Study of the O-H--O system by crystal structure correlation methods

Abstract: All cases of strong (2.50≤d(O--O)≤2.65 A) and very strong (d(O--O) <2.50 A) O-H---O hydrogen bonds whose geometries are known from accurate neutron or X-ray diffraction studies are reviewed and classified in chemical classes belonging to three fundamental types: (A) -O-H---O-, or negative charge assisted hydrogen bonding, (-)CAHB; (B)=O---H + --O=, or positive charge assisted hydrogen bonding, (+)CAHB; and (C) -O-H---O=, where the two oxygens are interconnected by a system of π-conjugated double bonds, or resonance-assisted hydrogen bonding, RAHB

The development of a simple empirical scoring function to estimate the binding constant for a protein-ligand complex of known three-dimensional structure

Abstract: A new simple empirical function has been developed that estimates the free energy of binding for a given protein-ligand complex of known 3D structure. The function takes into account hydrogen bonds, ionic interactions, the lipophilic protein-ligand contact surface and the number of rotatable bonds in the ligand. The dataset for the calibration of the function consists of 45 protein-ligand complexes. The new energy function reproduces the binding constants (ranging from 2.5.10(-2) to 4.10(-14) M, corresponding to binding energies between -9 and -76 kJ/mol) of the dataset with a standard deviation of 7.9 kJ/mol, corresponding to 1.4 orders of magnitude in binding affinity. The individual contributions to protein-ligand binding obtained from the scoring function are: ideal neutral hydrogen bond: -4.7 kJ/mol; ideal ionic interaction: -8.3 kJ/mol; lipophilic contact: -0.17 kJ/mol A2; one rotatable bond in the ligand: +1.4 kJ/mol. The function also contains a constant contribution (+5.4 kJ/mol) which may be rationalized as loss of translational and rotational entropy. The function can be evaluated very fast and is therefore also suitable for application in a 3D database search or de novo ligand design program such as LUDI.

Surface Vibrational Spectroscopic Studies of Hydrogen Bonding and Hydrophobicity

Abstract: Surface vibrational spectroscopy by sum-frequency generation was used to study hydrophobicity at the molecular level at various interfaces: water-surfactant-coated quartz, water-hexane, and water-air. In all cases, hydrophobicity was characterized by the appearance of dangling hydroxyl bonds on 25 percent of the surface water molecules. At the water-quartz interface, packing restrictions force the water surface layer to have a more ordered, ice-like structure. A partly wettable water-quartz interface was also studied.

Crystal field effects on the topological properties of the electron density in molecular crystals: The case of urea

Abstract: The Quantum Theory of Atoms in Molecules, due to Bader, is applied to periodic systems. Results for molecular and crystalline urea are presented. Changes in both bond critical points and atomic properties due to changes of chemical environment are described. A rationale for the different lengths of the in‐plane and out‐of‐plane hydrogen bonds and for the lengthening of the CO bond in bulk urea is provided in terms of the properties of the Laplacian of the oxygen atom electron density distribution. An evaluation of molecular and atomic volume changes indicates that the decrease of molecular volume upon change of phase from gas to solid originates primarily from a contraction of the atomic basins directly involved in hydrogen bonds. Other atoms show a small expansion. The considerable decrease of oxygen and hydrogen atomic volumes is related to the mutual penetration of their van der Waals envelopes following hydrogen bond formation. The results confirm that urea is more polar in the solid phase.

A low-barrier hydrogen bond in the catalytic triad of serine proteases

Abstract: Spectroscopic properties of chymotrypsin and model compounds indicate that a low-barrier hydrogen bond participates in the mechanism of serine protease action. A low-barrier hydrogen bond between N delta 1 of His57 and the beta-carboxyl group of Asp102 in chymotrypsin can facilitate the formation of the tetrahedral adduct, and the nuclear magnetic resonance properties of this proton indicate that it is a low-barrier hydrogen bond. These conclusions are supported by the chemical shift of this proton, the deuterium isotope effect on the chemical shift, and the properties of hydrogen-bonded model compounds in organic solvents, including the hydrogen bond in cis-urocanic acid, in which the imidazole ring is internally hydrogen-bonded to the carboxyl group.

Do salt bridges stabilize proteins? A continuum electrostatic analysis

Abstract: The electrostatic contribution to the free energy of folding was calculated for 21 salt bridges in 9 protein X-ray crystal structures using a continuum electrostatic approach with the DELPHI computer-program package. The majority (17) were found to be electrostatically destabilizing; the average free energy change, which is analogous to mutation of salt bridging side chains to hydrophobic isosteres, was calculated to be 3.5 kcal/mol. This is fundamentally different from stability measurements using pKa shifts, which effectively measure the strength of a salt bridge relative to 1 or more charged hydrogen bonds. The calculated effect was due to a large, unfavorable desolvation contribution that was not fully compensated by favorable interactions within the salt bridge and between salt-bridge partners and other polar and charged groups in the folded protein. Some of the salt bridges were studied in further detail to determine the effect of the choice of values for atomic radii, internal protein dielectric constant, and ionic strength used in the calculations. Increased ionic strength resulted in little or no change in calculated stability for 3 of 4 salt bridges over a range of 0.1-0.9 M. The results suggest that mutation of salt bridges, particularly those that are buried, to "hydrophobic bridges" (that pack at least as well as wild type) can result in proteins with increased stability. Due to the large penalty for burying uncompensated ionizable groups, salt bridges could help to limit the number of low free energy conformations of a molecule or complex and thus play a role in determining specificity (i.e., the uniqueness of a protein fold or protein-ligand binding geometry).

Vibrational spectra of water molecules at quartz/water interfaces.

Abstract: Optical sum-frequency generation was used to study OH stretch vibrations of water molecules at fused quartz-water interfaces. The results indicate that orientations and bond ordering of interfacial water molecules are strongly affected by electrostatic interaction and hydrogen bonding of the molecules with the quartz surface.

Natural energy decomposition analysis: An energy partitioning procedure for molecular interactions with application to weak hydrogen bonding, strong ionic, and moderate donor–acceptor interactions

Abstract: We present a procedure for partitioning the Hartree–Fock self‐consistent‐field (SCF) interaction energy into electrostatic, charge transfer, and deformation components. The natural bond orbital (NBO) approach of Weinhold and co‐workers is employed to construct intermediate supermolecule and fragment wave functions that satisfy the Pauli exclusion principle, thereby avoiding the principal deficiency of the popular Kitaura–Morokuma energy decomposition scheme. The function counterpoise method of Boys and Bernardi enters the procedure naturally, providing an estimate of basis set superposition error (BSSE). We find that the energy components exhibit little basis set dependence when BSSE is small. Applications are presented for several representative molecular and ion complexes: the weak hydrogen bond of the water dimer, the strong ionic interaction of the alkali metal hydrides, and the moderate donor–acceptor interactions of BH3NH3 and BH3CO. Electrostatic interaction dominates the long‐range region of the p...

Effect of hydrogen bonds on the thermodynamic behavior of liquid water.

Abstract: We propose an extension of the van der Waals equation which is designed to incorporate, in an approximate fashion, the effects of the network of hydrogen bonds that exist in liquid water. The resulting model qualitatively predicts the unique thermodynamic properties of water, including those of the deeply supercooled states. It also reconciles two proposals for the phase behavior of supercooled and stretched water and provides a thermodynamic origin for the observed polymorphism of the amorphous solid form of water.

Layered Materials by Molecular Design: Structural Enforcement by Hydrogen Bonding in Guanidinium Alkane- and Arenesulfonates

Abstract: Single crystal X-ray structures of a series of guanidinium alkane- and arensulfonates C(NH 2 ) 3 + RSO 3 - (R=(CH 2 ) x CH 3 (x=0-3), (1S)-(+)-10-camphor, benzene, 1-naphthalene, and 2-naphthalene) reveal self-assembly of the ions into unique two-dimensional hydrogen-bonded sheets directed by hydrogen bonds betwen the six guanidinium protons and the six lone electron pairts of the sulfonate oxygen atoms. The sheets assemble in the third dimension as either single layers or bilayers with interpenetrating R groups, depending upon the steric requirements of the R groups

The active center of a mammalian alpha-amylase. Structure of the complex of a pancreatic alpha-amylase with a carbohydrate inhibitor refined to 2.2-A resolution.

Abstract: An X-ray structure analysis of a crystal of pig pancreatic alpha-amylase (EC 3.2.1.1) that was soaked with acarbose (a pseudotetrasaccharide alpha-amylase inhibitor) showed electron density corresponding to five fully occupied subsites in the active site. The crystal structure was refined to an R-factor of 15.3%, with a root mean square deviation in bond distances of 0.015 A. The model includes all 496 residues of the enzyme, one calcium ion, one chloride ion, 393 water molecules, and five bound sugar rings. The pseudodisaccharide acarviosine that is the essential structural unit responsible for the activity of all inhibitors of the acarbose type was located at the catalytic center. The carboxylic oxygens of the catalytically competent residues Glu233 and Asp300 form hydrogen bonds with the "glycosidic" NH group of the acarviosine group. The third residue of the catalytic triad Asp197 is located on the opposite side of the inhibitor binding cleft with one of its carbonyl oxygens at a 3.3-A distance from the anomeric carbon C-1 of the inhibitor center. Binding of inhibitor induces structural changes at the active site of the enzyme. A loop region between residues 304 and 309 moves in toward the bound saccharide, the resulting maximal mainchain movement being 5 A for His305. The side chain of residue Asp300 rotates upon inhibitor binding and makes strong van der Waals contacts with the imidazole ring of His299. Four histidine residues (His101, His201, His299, and His305) are found to be hydrogen-bonded with the inhibitor. Many protein-inhibitor hydrogen bond interactions are observed in the complex structure, as is clear hydrophobic stacking of aromatic residues with the inhibitor surface. The chloride activator ion and structural calcium ion are hydrogen-bonded via their ligands and water molecules to the catalytic residues.

Molecular dynamics simulation of the gramicidin channel in a phospholipid bilayer

Abstract: A molecular dynamics simulation of the gramicidin A channel in an explicit dimyristoyl phosphatidylcholine bilayer was generated to study the details of lipid-protein interactions at the microscopic level. Solid-state NMR properties of the channel averaged over the 500-psec trajectory are in excellent agreement with available experimental data. In contrast with the assumptions of macroscopic models, the membrane/solution interface region is found to be at least 12 A thick. The tryptophan side chains, located within the interface, are found to form hydrogen bonds with the ester carbonyl groups of the lipids and with water, suggesting their important contribution to the stability of membrane proteins. Individual lipid-protein interactions are seen to vary from near 0 to -50 kcal/mol. The most strongly interacting conformations are short-lived and have a nearly equal contribution from both van der Waals and electrostatic energies. This approach for performing molecular dynamics simulations of membrane proteins in explicit phospholipid bilayers should help in studying the structure, dynamics, and energetics of lipid-protein interactions.

Evidence for hydrogen bonding in solutions of 1-ethyl-3-methylimidazolium halides, and its implications for room-temperature halogenoaluminate(III) ionic liquids

Abstract: Multinuclear NMR spectroscopy and conductivity measurements showed that the 1-ethyl-3-methyl-imidazolium cation, [emim]+, not only forms strong hydrogen bonds (using all three ring protons H2, H4 and H5) with halide ions in polar molecular solvents (e.g. ethanenitrile) and ionic liquids, but that it exists in a quasi-molecular state, [emim]X, in non-polar solvents (e.g. trichloro- and dichloro-methane), showing a conventional aromatic stacking phenomenon.

Unusually stable β-sheet formation in an ionic self-complementary oligopeptide

Abstract: A 16-residue amphiphilic oligopeptide (EAK16) with every other residue alanine and also containing glutamic acid and lysine (Ac-NH-AEAEAKAKAEAEAKAK-CONH2) is able to form an unusually stable beta-sheet structure. The beta-sheet structure is stable at very low concentrations in water and at high temperatures. Various pH changes at 1.5, 3, 7, and 11 had little effect on the stability of the beta-sheet structure. The beta-sheet structure was not altered significantly even in the presence of 0.1% SDS, 7 molar guanidine hydrochloride, or 8 molar urea. One of the structural characteristics of the EAK16 is its ionic self-complementarity in that ionic bonds and hydrogen bonds between Glu and Lys can form readily between two oligopeptide beta-sheet structures. This structural feature is probably one of the factors that promotes its extreme stability. This is the first example of such an extended ionic self-complementarity in a protein structure. EAK16 and its related peptides may have applications as useful biomaterials. It also offers a good model for studying the mechanism of beta-sheet formation. Because the oligopeptide can self-assemble to form a membranous structure, it may have relevance to origin of life research.

Intramolecular Hydrogen Bonding in Derivatives of .beta.-Alanine and .gamma.-Amino Butyric Acid; Model Studies for the Folding of Unnatural Polypeptide Backbones

Abstract: We have examined the intramolecular hydrogen bonding behavior of simple β- and γ-amino acid derivatives as a prelude to efforts to design unnatural polyamides that will adopt compact and specific folding patterns analogous to those of α-amino acid polymers (proteins). We postulate that the desired folding behavior will be most likely if intramolecular hydrogen bonds are unfavorable between nearest neighbor amide groups on the polymer backbone. Previous work from other laboratories and our own has shown that this criterion applies to α-amino acid polymers. Variable-temperature FT-IR data reported here for α-alanine derivatives in methylene chloride indicate that nearest neighbor hydrogen bonding is unfavorable for this type of residue as well

Enzyme crystal structure in a neat organic solvent

Abstract: The crystal structure of the serine protease subtilisin Carlsberg in anhydrous acetonitrile was determined at 2.3 A resolution. It was found to be essentially identical to the three-dimensional structure of the enzyme in water; the differences observed were smaller than those between two independently determined structures in aqueous solution. The hydrogen bond system of the catalytic triad is intact in acetonitrile. The majority (99 of 119) of enzyme-bound, structural water molecules have such a great affinity to subtilisin that they are not displaced even in anhydrous acetonitrile. Of the 12 enzyme-bound acetonitrile molecules, 4 displace water molecules and 8 bind where no water had been observed before. One-third of all subtilisin-bound acetonitrile molecules reside in the active center, occupying the same region (P1, P2, and P3 binding sites) as the specific protein inhibitor eglin c.

Specific alteration of the oxidation potential of the electron donor in reaction centers from Rhodobacter sphaeroides.

Abstract: The effects of multiple changes in hydrogen bond interactions between the electron donor, a bacteriochlorophyll dimer, and histidine residues in the reaction center from Rhodobacter sphaeroides have been investigated. Site-directed mutations were designed to add or remove hydrogen bonds between the 2-acetyl groups of the dimer and histidine residues at the symmetry-related sites His-L168 and Phe-M197, and between the 9-keto groups and Leu-L131 and Leu-M160. The addition of a hydrogen bond was correlated with an increase in the dimer midpoint potential. Measurements on double and triple mutants showed that changes in the midpoint potential due to alterations at the individual sites were additive. Midpoint potentials ranging from 410 to 765 mV, compared with 505 mV for wild type, were achieved by various combinations of mutations. The optical absorption spectra of the reaction centers showed relatively minor changes in the position of the donor absorption band, indicating that the addition of hydrogen bonds to histidines primarily destabilized the oxidized state of the donor and had little effect on the excited state relative to the ground state. Despite the change in energy of the charge-separated states by up to 260 meV, the mutant reaction centers were still capable of electron transfer to the primary quinone. The increase in midpoint potential was correlated with an increase in the rate of charge recombination from the primary quinone, and a fit of these data using the Marcus equation indicated that the reorganization energy for this reaction is approximately 400 meV higher than the change in free energy in wild type. The mutants were still capable of photosynthetic growth, although at reduced rates relative to the wild type. These results suggest a role for protein-cofactor interactions--in particular, histidine-donor interactions--in establishing the redox potentials needed for electron transfer in biological systems.

Molecular and crystal structure of the anhydrous form of chitosan

Abstract: The molecular and crystal structure of the anhydrous form of chitosan, a (1→4)-linked 2-amino-2-deoxy-β-D-glucan, has been determined by combined X-ray diffraction analysis and stereochemical model refinement. Chitosan chains crystallize in an orthorhombic unit cell with dimensions a= 0.828 nm, b= 0.862 nm, and c (fiber axis)= 1.043 nm. Systematic absences are consistent with the space group P2 1 2 1 2 1 . The unit cell comprises four glucosamine residues, giving a density of 1.52 g/cm 3 . The X-ray diffraction pattern was recorded on the imaging plate, and an intensity of each reflection spot was estimated by two-dimensional measurement and subsequent background removal. Chain conformation is an extended 2-fold helix stabilized by intramolecular O3...O5 hydrogen bonds, and an O6 atom is rotated at near gt position. Two chains pass through the unit cell with an antiparallel packing arrangement. Intermolecular N2...O6 hydrogen bonds contribute to the three-dimensional stabilizing of the crystal structure. The reliability of the structure analysis is indicated by the X-ray residual R= 0.175 and R″= 0.178 against the observed 38 hkl's. Some significant similarities are observed between the anhydrous crystal structures of the present chitosan and (1→4)-β-D-mannan

Hydrogen bond networks

Abstract: Preface. Section A: Modelling and Hydrogen-Bond Structures. Hydrogen Bonding and the Fragility of Supercooled Liquids and Biopolymers C.A. Angell, C. Alba-Simionesco, J. Fan, J.L. Green. Hydrogen Bonding and Molecular Mobility in Aqueous Systems A. Geiger, T. Kowall. Structural and Dynamical Quantum Effects in Aqueous Solution P.J. Rossky. Novel Features in the Equation of State of Metastable Water P.H. Poole, F. Sciortino, U. Essmann, M. Hemmati, H.E. Stanley, C.A. Angell. The Role of H-Bonds in the Formation of Ices J-C Li, D.K. Ross. Radial Distribution Function of Heavy Water Steam A. Fontana, P. Postorino, M.A. Ricci, A.K. Soper. Molecular Dynamics on a Water Model with Polarizability and Hyperpolarizability G. Ruocco, M. Sampoli. M.D. Simulations of Stretched TIP4P--Water in the Supercooled Regime G. Ruocco, M. Sampoli, A. Torcini, R. Vallauri. Anomalous Sound Dispersion in Liquid Water U. Balucani, G. Ruocco, M. Sampoli, A. Torcini, R. Vallauri. Sound Propagation in Hydrogen Bonded Molecular Liquids: the Case of Liquid Water F. Sciortino, S. Sastry. Orientational Correlations in Hydrogen Bonded Networks A.K. Soper. Are Hydrogen Bonds Present in Hydrogen Halides Liquids Other than HF? C. Andreani, F. Menzinger, M. Nardone, F.P. Ricci, M.A. Ricci, A.K. Soper. Investigation of the Structure of Liquid Formic Acid I. Bako, P. Jedlovszky, G. Palinkas, J.C. Dore. Section B: Spectroscopic Studies, Complexes and Solutions. Incoherent Inelastic Neutron Scattering from Liquid Water: a Computer Simulation Study J.-C. Leicknam, M. Diraison, G. Tarjus, S. Bratos. The Observation of Different Strengths of H-Bonds in Ices J-C Li, D.K. Ross, M.H.B.Hayes, W.F. Sherman, M. Adams. IR Spectra and Dynamics of H2O (D2O, HDO) Molecules in a Still Poorly Known Liquid: Water Y. Marechal. Low Frequency Raman Spectra from Anhydrous Sulfuric and Chlorosulfonic Acids and Liquid Water, Disruption of Tetrahedral Hydrogen Bonding Relation to Water Structure Y.C. Chu, G.E. Walrafen. Picosecond Holeburning Spectroscopy in the Infrared of Water and Other Hydrogen-Bonded Systems A. Laubereau, H. Graener. Low Frequency Raman Spectra in Water by Normal Mode Analysis S. Sastry, H.E. Stanley, F. Sciortino. Light Scattering from Liquid Water M.A. Ricci. Dielectric Properties of Aqueous Solutions G. Salvetti, E. Tombari. Formic Acid, Ethanol in Vycor Glass and Water in Aluminosilicate Zeolites C.K. Loong, F. Trouw, L.E. Iton. Application of the Reactive Flux Formalism to Study Water Hydrogen Bond Dynamics A. Luzar, D. Chandler. Temperature Dependence of Ion Solvation Dynamics in Liquid Water H. Resat, F.O. Raineri, B-C. Perng, H.L. Friedman. Theoretical Simulation of OH and OD Stretching Bands of Isotopically Diluted HDO Molecules in Lithium Formate Solution M.J. Wojcik, K. Hermansson, J. Lindgren, L. Ojamae. Influence of Water Molecules on the Nucleation Rate of Polymorphic Complexes with Different Conformations in Solution S. Petit, G. Coquerel, G. Perez. Hydrogen-Bond Nature in Solids Based on Nuclear Quadrupole Resonance Spectroscopy Studies B. Nogaj. Simulation of Liquid Mixtures G. Palinkas, K. Heinzinger. Section C: Networks, Interfaces and Confined Geometry. Structure and Dynamics of Water in Confined Geometry S-H. Chen, M.-C. Bellissent-Funel. Structure and Dynamics of Water at Interfaces P.J. Rossky. Hydrogen B

Design of Organic Structures in the Solid State: Molecular Tapes Based on the Network of Hydrogen Bonds Present in the Cyanuric Acid.cntdot.Melamine Complex

Abstract: This paper describes the single-crystal X-ray structures of I :l complexes between seven N,Nrbis(4-X- phenyl)melamines (X = H, F, Cl, Br, I, CH3, and CF3) and 5,5-diethylbarbituric acid. These complexes crystallize as infinite tapes having components joined by triads of hydrogen bonds; the tapes pack with their long axes parallel. The crystalline architecture of these complexes-parallel tapes-serves as a structurally constrained framework with which to study physical-organic relationships between the structures of the crystals and the molecules of which they are composed. The complex with X = Br exists in polymorphic forms. One of the polymorphs is isomorphous to the X = Cl complex; the other is related to the X = I complex.

Electrostatic interactions control the parallel and antiparallel orientation of alpha-helical chains in two-stranded alpha-helical coiled-coils.

Abstract: The role of interchain electrostatic interactions in orientating alpha-helical chains to form two-stranded parallel and antiparallel coiled-coils has been investigated. Four disulfide-bridged coiled-coils were designed: parallel coiled-coils with interchain electrostatic attractions (P/A) and repulsions (P/R) and antiparallel coiled-coils with interchain electrostatic attractions (AP/A) and repulsions (AP/R). These coiled-coils were made by air oxidation of two 35-residue peptides with the appropriate heptad repeat (LaEbAcLdEeGfKg or LaAbEcLdKeGfEg) to give the desired interchain electrostatic interactions, and the appropriate position of the cysteine residue (C2 or C33) to give the desired chain orientation. The coiled-coils were characterized by circular dichroism spectroscopy, and their stabilities were assessed by guanidine hydrochloride and urea denaturations. The results indicated that the favored chain orientation, that is, the major disulfide-bridged product formed under benign conditions, was the one that provides interchain electrostatic attractions between oppositely-charged amino acid residues in the e-g' and g-e' positions of the parallel coiled-coil and the g-g' and e-e' positions in the antiparallel coiled-coil. When the electrostatic interactions were similar, the antiparallel coiled-coils were more stable than the parallel coiled-coils. However, the overall stability of the coiled-coils was either increased by interchain electrostatic attractions or decreased by interchain electrostatic repulsions, as determined by urea denaturation. Thus, the order of overall stability of these coiled-coils was AP/A > P/A > AP/R > P/R. This study demonstrates the importance of interchain electrostatic interactions in determining the parallel or antiparallel orientation of alpha-helical chains in two-stranded coiled-coils.