The intrinsic molecular potential of glyceryl monooleate layers and its effect on the conformation and orientation of an inserted molecule: example of gramicidin A.

It is shown by explicit calculation that the distribution of the atomic charges in the constituent molecules of a lipid monolayer or bilayer of glyceryl monooleate creates an intrinsic potential difference between the head region and the hydrocarbon region which tends to repel positive charges towards the exterior and attract negative charges to the interior. The analogies and differences between a bilayer and a monolayer are analyzed. The possible consequences of the intrinsic potential gradient in a lipid layer on the preferred orientation and conformation of a polar neutral molecule are illustrated on the case of a gramicidin A monomer.     

3D Hydrophobic Moment Vectors as a Tool to Characterize the Surface Polarity of Amphiphilic Peptides

The interaction of membranes with peptides and proteins is largely determined by their amphiphilic character. Hydrophobic moments of helical segments are commonly derived from their two-dimensional helical wheel projections, and the same is true for β-sheets. However, to the best of our knowledge, there exists no method to describe structures in three dimensions or molecules with irregular shape. Here, we define the hydrophobic moment of a molecule as a vector in three dimensions by evaluating the surface distribution of all hydrophilic and lipophilic regions over any given shape. The electrostatic potential on the molecular surface is calculated based on the atomic point charges. The resulting hydrophobic moment vector is specific for the instantaneous conformation, and it takes into account all structural characteristics of the molecule, e.g., partial unfolding, bending, and side-chain torsion angles. Extended all-atom molecular dynamics simulations are then used to calculate the equilibrium hydrophobic moments for two antimicrobial peptides, gramicidin S and PGLa, under different conditions. We show that their effective hydrophobic moment vectors reflect the distribution of polar and nonpolar patches on the molecular surface and the calculated electrostatic surface potential. A comparison of simulations in solution and in lipid membranes shows how the peptides undergo internal conformational rearrangement upon binding to the bilayer surface. A good correlation with solid-state NMR data indicates that the hydrophobic moment vector can be used to predict the membrane binding geometry of peptides. This method is available as a web application on http://www.ibg.kit.edu/HM/.     

Conformational characteristics of peptides and unanticipated results from crystal structure analyses

Preferred conformation and types of molecular folding are some of the topics that can be addressed by structure analysis using x-ray diffraction of single crystals. The conformations of small linear peptide molecules with 2-6 residues are affected by polarity of solvent, presence of water molecules, hydrogen bonding with neighboring molecules, and other packing forces. Larger peptides, both cyclic and linear, have many intramolecular hydrogen bonds, the effect of which outweighs any intermolecular attractions. Numerous polymorphs of decapeptides grown from a variety of solvents, with different cocrystallized solvents, show a constant conformation for each peptide. Large conformational changes occur, however, upon complexation with metal ions. A new form of free valinomycin grown from DMSO exhibits near three-fold symmetry with only three intramolecular hydrogen bonds. The peptide is in the form of a shallow bowl with a hydrophobic exterior. Near the bottom of the interior of the bowl are three carbonyl oxygens, spaced and directed so that they are in position to form three ligands to a K+, e.g., complexation can be completed by the three lobes containing the beta-bends closing over and encapsulating the K+ ion. In another example, free antamanide and the biologically inactive perhydro analogue, in which four phenyl groups become cyclic hexyl groups, have essentially the same folding of backbone and side chains. The conformation changes drastically upon complexation with Li+ or Na+. However, the metal ion complex of natural antamanide has a hydrophobic globlar form whereas the metal ion complex of the inactive perhydro analogue has a polar band around the middle. The structure results indicate that the antamanide molecule is in a complexed form during its biological activity. Single crystal x-ray diffraction structure analyses have identified the manner in which water molecules are essential to creating minipolar areas on apolar helices. Completely apolar peptides, such as membrane-active peptides, can acquire amphiphilic character by insertion of a water molecule into the helical backbone of Boc-Aib-Ala-Leu-Aib-Ala-Leu-Aib-Ala-Leu-Aib-OMe, for example. The C-terminal half assumes an alpha-helix conformation, whereas the N-terminal half is distorted by an insertion of a water molecule W(1) between N(Ala5) and O(Ala2), forming hydrogen bonds N(5)H...W(1) and W(1)...O(2). The distortion of the helix exposes C = O(Aib1) and C = O(Aib4) to the outside environment with the consequence of attracting additional water molecules. The leucyl side chains are on the other side of the molecule. Thus a helix with an apolar sequence can mimic an amphiphilic helix.     

The electrostatic potential for the phosphodiester group determined from X-ray diffraction.

The charge density distribution in the crystal structure of ammonium dimethylphosphate at 123 K has been determined from x-ray diffraction data (MoK alpha) using 8437 reflections with sin theta/lambda less than 1.33 A-1 [NH4+.(CH3)2PO4-, M(r) = 143.08, monoclinic, P2(1)/c, a = 10.007(1), b = 6.926(1), c = 9.599(2) A, beta = 105.40(1) degrees, V = 641.4(3) A3, Z = 4, F000 = 304, Dx = 1.4815 g.cm-3, mu = 3.726 cm-1]. Least-squares structure refinement assuming Stewart's rigid pseudoatom model (variables including Slater-type radial exponents and electron populations for multipole terms extending to octapoles for C, N, O, and P, and dipoles for H) gave R(F2) = 0.039 for all reflections. The dimethylphosphate anion is in the gauche-gauche conformation and has approximate twofold symmetry. One phosphoryl O atom forms three hydrogen bonds and the other forms one. Neither of the ester O atoms is hydrogen bonded. For the dimethylphosphate anion isolated from the crystal structure, a map of the electrostatic potential obtained using the pseudoatom charge parameters shows that the phosphoryl O atoms are considerably more electronegative than the ester O atoms. The electrostatic potential distribution obtained in this way has been fitted by least squares to a system of atom-centered point charges. The potential calculated from these point charges agrees with the experimental result. It also agrees reasonably well with potentials obtained from three other systems of point charges that are widely used as part of the semiempirical force field for molecular mechanics and molecular dynamics calculations involving nucleic acids.     

Properties of lipid complexes with amphipathic helix-forming peptides. Role of distribution of peptide charges

The peptides [Glu1,8,Leu11,17] 18A and [Glu4,9,Leu11,17] reverse-18A are 18-residue peptides designed to form amphipathic helices with opposite charge distribution; [Glu1,8,Leu11,17] 18A having positively charged residues at the hydrophobic/hydrophilic interface. Both [Glu1,8,Leu11,17] 18A and [Glu4,9,Leu11,17] reverse-18A strongly disrupt the bilayer structure as indicated by the relatively narrow lipid 1H and 31P NMR peaks. In addition, the 1H chemical shift of the quaternary ammonium methyl groups indicates that [Glu1,8,Leu11,17] 18A forms smaller lipoprotein particles with dimyristoylphosphatidylcholine (DMPC) than does [Glu4,9,Leu11,17] reverse-18A. However, motional properties of the lipid head group indicate that no specific salt bridges are formed between the phospholipid head group and the side chains of polar amino acids of either of the two peptides. In addition, the acyl chain conformation for the DMPC complexes with [Glu1,8,Leu11,17] 18A and with [Glu4,9,Leu11,17] reverse-18A are indistinguishable by the criterion of IR spectroscopy. The 2H linewidth of the solvent 2H2O remains narrower in frozen solutions of the DMPC-[Glu1,8,Leu11,17] 18A complexes suggesting the presence of more unfrozen bound water in this case. The two peptides exhibit many similarities in their interaction with lipids. However, [Glu1,8,Leu11,17] 18A can more readily lyse vesicles and activate lecithin:cholesterol acyltransferase. These differences do not appear to result from differences in specific charge interactions between the lipid and peptide but may be manifested through differences in hydration properties.     

Optimization of the binding properties of a synthetic anion receptor using rational and combinatorial strategies

The solution conformations of tetrameric and hexameric cyclopeptides containing alternating L-proline and 6-aminopicolinic acid subunits strongly depend on solvent polarity. Whereas in polar solvents, such as d6-DMSO, both peptides prefer on average symmetric conformations with converging NH groups, in less polar chloroform intramolecular hydrogen bonds to the peptide NH groups stabilize other, and in the case of the hexapeptide, non-symmetrical conformations. Independent of the solvent, both peptides interact with anions via their NH groups but whereas anion binding requires a cleavage of the intramolecular hydrogen bonds accompanied by a conformational reorganization in chloroform, in polar solvents the peptides are already well preorganized for anion complexation. Complex formation between anions and the cyclic hexapeptide was even detected in highly competitive D2O/CD3OD or H2O/CH3CN mixtures, which was attributed to the special sandwich-type structure of the complexes formed. Stabilizing these 2:1 aggregates by covalently linking two cyclopeptide rings together affords ditopic receptors with a high anion affinity in protic solvents. Complex stability depends on the structure of the linker with which the two receptor moieties are connected and even more potent anion receptors were obtained by a dynamic combinatorial optimization of this linking unit.     

The effect of hydration on the mobility of phospholipids in the gel state. A proton nuclear magnetic resonance spin echo study

Proton nuclear magnetic resonance (NMR) dipolar echo studies are presented for the gel state of dipalmitoylglycerophosphocholine (dipalmitoyl-GPC) — heavy water dispersions. The mobility and the mean order of the chains and the head group of dipalmitoyl-GPC were determined for different water concentrations and temperatures. For smaller than 5 mol D₂O per mol dipalmitoyl-GPC the molecule undergoes temperature- and hydration-dependent restricted rotational oscillations about the long axis of the molecule. For hydration numbers equal or larger than 5 mol D₂O per mol dipalmitoyl-GPC the molecules rotate effectively about their long axes and intermolecular dipolar interactions between proton groups of neighbour molecules are averaged. The onset of the lateral diffusion of dipalmitoyl-GPC is observed which averages out all intermolecular dipolar interactions. Deviations of the individual segments of the chains from the all-trans state have to be considered. The widely accepted model that the dipalmitoyl-GPC molecules rotate about their long axes with stiff all-trans chains should be modified. The polar head groups of dipalmitoyl-GPC effectively rotate about the bilayer-normal and restricted rotations about single bonds in the head group are allowed. An order parameter of about 0.6 for the head group was obtained for fully hydrated dipalmitoyl-GPC molecules at ambient temperature.     

Tryptophan-containing peptide helices: interactions involving the indole side chain.

Two designed peptide sequences containing Trp residues at positions i and i + 5 (Boc-Leu-Trp-Val-Ala-Aib-Leu-Trp-Val-OMe, 1) as well as i and i + 6 (Boc-Leu-Trp-Val-Aib-Ala-Aib-Leu-Trp-Val-OMe, 2) containing one and two centrally positioned Aib residues, respectively, for helix nucleation, have been shown to form stable helices in chloroform solutions. Structures derived from nuclear magnetic resonance (NMR) data reveal six and seven intramolecularly hydrogen-bonded NH groups in peptides 1 and 2, respectively. The helical conformation of octapeptide 1 has also been established in the solid state by X-ray diffraction. The crystal structure reveals an interesting packing motif in which helical columns are stabilized by side chain-backbone hydrogen bonding involving the indole Nepsilon1H of Trp(2) as donor, and an acceptor C=O group from Leu(6) of a neighboring molecule. Helical columns also associate laterally, and strong interactions are observed between the Trp(2) and Trp(7) residues on neighboring molecules. The edge-to-face aromatic interactions between the indoles suggest a potential C-H...pi interaction involving the Czeta3H of Trp(2). Concentration dependence of NMR chemical shifts provides evidence for peptide association in solution involving the Trp(2) Nepsilon1H protons, presumably in a manner similar to that observed in the crystal.     

Reversed hexagonal phase formation in lecithin-alkane-water systems with different acyl chain unsaturation and alkane length

Investigations of lipid-alkane systems are important for an understanding of the interactions between lipids and hydrophobic/amphiphilic peptides or other hydrophobic biological molecules. A study of the formation of nonlamellar phases in several phosphatidylcholine (PC)-alkane-2H2O systems has been performed. The PC molecules chosen in this work are dipalmitoyl-PC (DPPC), 1-palmitoyl-2-oleoyl-PC (POPC), dioleoyl-PC (DOPC), and dilinoleoyl-PC (DLiPC), lipids that in excess water form just a lamellar liquid-crystalline phase up to at least 90 degrees C. The addition of n-alkanes (C8-C20) to these PC-2H2O systems induces the formation of reversed hexagonal (HII) and isotropic phases. The water and dodecane concentrations required to form these phases depend on the degree of acyl chain unsaturation of the PC molecules and increase in the order DLiPC approximately DOPC less than POPC less than DPPC. The most likely explanation to this result is that the diameter of the lipid-water cylinders in the HII phase grows gradually larger with increased acyl chain saturation and more water and dodecane are consequently needed to fill the water cylinders and the void volumes between the cylinders, respectively. The ability of the alkanes to promote the formation of an HII phase is strongly chain length dependent. Although the number of alkane carbon atoms added per DOPC molecule in the DOPC-n-alkane-2H2O mixtures was kept constant, this ability decreased on going from octane to eicosane. The thermal history of a DPPC-n-dodecane-2H2O sample was important for its phase behavior.(ABSTRACT TRUNCATED AT 250 WORDS)     

Statistical analysis of protein structures suggests that buried ionizable residues in proteins are hydrogen bonded or form salt bridges

It is well known that nonpolar residues are largely buried in the interior of proteins, whereas polar and ionizable residues tend to be more localized on the protein surface where they are solvent exposed. Such a distribution of residues between surface and interior is well understood from a thermodynamic point: nonpolar side chains are excluded from the contact with the solvent water, whereas polar and ionizable groups have favorable interactions with the water and thus are preferred at the protein surface. However, there is an increasing amount of information suggesting that polar and ionizable residues do occur in the protein core, including at positions that have no known functional importance. This is inconsistent with the observations that dehydration of polar and in particular ionizable groups is very energetically unfavorable. To resolve this, we performed a detailed analysis of the distribution of fractional burial of polar and ionizable residues using a large set of ?2600 nonhomologous protein structures. We show that when ionizable residues are fully buried, the vast majority of them form hydrogen bonds and/or salt bridges with other polar/ionizable groups. This observation resolves an apparent contradiction: the energetic penalty of dehydration of polar/ionizable groups is paid off by favorable energy of hydrogen bonding and/or salt bridge formation in the protein interior. Our conclusion agrees well with the previous findings based on the continuum models for electrostatic interactions in proteins. Proteins 2011; © 2011 Wiley‐Liss, Inc.     

Interactions of cationic-hydrophobic peptides with lipid bilayers: a Monte Carlo simulation method

We present a computational model of the interaction between hydrophobic cations, such as the antimicrobial peptide, Magainin2, and membranes that include anionic lipids. The peptide's amino acids were represented as two interaction sites: one corresponds to the backbone alpha-carbon and the other to the side chain. The membrane was represented as a hydrophobic profile, and its anionic nature was represented by a surface of smeared charges. Thus, the Coulombic interactions between the peptide and the membrane were calculated using the Gouy-Chapman theory that describes the electrostatic potential in the aqueous phase near the membrane. Peptide conformations and locations near the membrane, and changes in the membrane width, were sampled at random, using the Metropolis criterion, taking into account the underlying energetics. Simulations of the interactions of heptalysine and the hydrophobic-cationic peptide, Magainin2, with acidic membranes were used to calibrate the model. The calibrated model reproduced structural data and the membrane-association free energies that were measured also for other basic and hydrophobic-cationic peptides. Interestingly, amphipathic peptides, such as Magainin2, were found to adopt two main membrane-associated states. In the first, the peptide resided mostly outside the polar headgroups region. In the second, which was energetically more favorable, the peptide assumed an amphipathic-helix conformation, where its hydrophobic face was immersed in the hydrocarbon region of the membrane and the charged residues were in contact with the surface of smeared charges. This dual behavior provides a molecular interpretation of the available experimental data.     

Calculation of the energy difference between the quaternary structures of deoxy- and oxyhemoglobin.

The energy difference between the quaternary structures of deoxy- and oxyhemoglobin is evaluated on the basis of the atomic coordinates determined by X-ray diffraction analysis. Calculation of the van der Waals interaction between subunits shows that in a hemoglobin molecule as a whole, the interaction is more attractive in the oxy form than in the deoxy form by about 8 kcal/mol, and that in each pair of two subunits except the pair alpha1alpha2, the interaction energy varies by about 15 kcal/mol. The electrostatic interactions originating in the partial charges on all constituent atoms of hemoglobin and in the polar residues on the surface of hemoglobin make only a small contribution to the energy difference between the quaternary structures of deoxy- and oxyhemoglobin. Thus, the contribution of the clusters of the polar residues in the internal cavity between like subunits and also of the freedom of rotation of the C-terminal of each subunit in oxyhemoglobin may be important energetically in the transition from deoxy to oxy quaternary structure. In this point, the present calculation supports Perutz' model, but suggests necessity of further investigations on the transitional characteristics of the quaternary structure in the intermediate steps of oxygenation. The discussion on the transitional characteristics is given in the last section.     

The Hofmeister effect and the behaviour of water at interfaces

Starting from known properties of non-specific salt effects on the surface tension at an air-water interface, we propose the first general, detailed qualitative molecular mechanism for the origins of ion-specific (Hofmeister) effects on the surface potential difference at an air-water interface; this mechanism suggests a simple model for the behaviour of water at all interfaces (including water-solute interfaces), regardless of whether the non-aqueous component is neutral or charged, polar or non-polar. Specifically, water near an isolated interface is conceptually divided into three layers, each layer being I water-molecule thick. We propose that the solute determines the behaviour of the adjacent first interfacial water layer (I1); that the bulk solution determines the behaviour of the third interfacial water layer (I3), and that both I1 and I3 compete for hydrogen-bonding interactions with the intervening water layer (I2), which can be thought of as a transition layer. The model requires that a polar kosmotrope (polar water-structure maker) interact with I1 more strongly than would bulk water in its place; that a chaotrope (water-structure breaker) interact with I1 somewhat less strongly than would bulk water in its place; and that a non-polar kosmotrope (non-polar water-structure maker) interact with I1 much less strongly than would bulk water in its place. We introduce two simple new postulates to describe the behaviour of I1 water molecules in aqueous solution. The first, the 'relative competition' postulate, states that an I1 water molecule, in maximizing its free energy (--delta G), will favour those of its highly directional polar (hydrogen-bonding) interactions with its immediate neighbours for which the maximum pairwise enthalpy of interaction (--delta H) is greatest; that is, it will favour the strongest interactions. We describe such behaviour as 'compliant', since an I1 water molecule will continually adjust its position to maximize these strong interactions. Its behaviour towards its remaining immediate neighbours, with whom it interacts relatively weakly (but still favourably), we describe as 'recalcitrant', since it will be unable to adjust its position to maximize simultaneously these interactions.(ABSTRACT TRUNCATED AT 400 WORDS)     

Molecular dynamics studies of the interface between a model membrane and an aqueous solution.

Molecular Dynamics (MD) computer simulation studies are reported for a system consisting of two model membranes in contact with an aqueous solution. The influence of the membrane on the adjacent liquid is of main interest in the present study. It is therefore attempted to make the system sufficiently large to encompass the entire region between bulk liquid and the membranes. The latter are modeled by two-dimensional arrays of COO- groups with rotational and translational degrees of freedom. The water molecules are represented by the well-tested TIP4P model. The intermolecular potentials are parametrized in terms of Coulomb interactions between partial charges on the molecular frames and empirical, mostly Lennard-Jones (12-6), interactions centered at the atomic positions. A strong layering of the liquid accompanied by an increase in average water density is found in the vicinity of the membrane. The structural perturbation reaches approximately 8 A into the liquid. We discuss the static structure in these layers in terms of atom-atom distance distribution functions and study the average orientation of the water molecule dipoles with respect to the membrane. From the distribution of the ions, we find that less than 50% of the surface charge of the membrane is neutralized by Na+ ions in the first layer above the membrane. A simplified model of the adsorption site of the ion on the membrane is developed from the distance distributions. Finally the hydration of the Na+ in the first adsorbed layer is discussed.     

Methylation of the imidazole side chains of the Alzheimer disease amyloid-beta peptide results in abolition of superoxide dismutase-like structures and inhibition of neurotoxicity

The toxicity of the amyloid-beta peptide (Abeta) is thought to be responsible for the neurodegeneration associated with Alzheimer disease. Generation of hydrogen peroxide has been implicated as a key step in the toxic pathway. Abeta coordinates the redox active metal ion Cu2+ to catalytically generate H2O2. Structural studies on the interaction of Abeta with Cu have suggested that the coordination sphere about the Cu2+ resembles the active site of superoxide dismutase 1. To investigate the potential role for such structures in the toxicity of Abeta, two novel Abeta40 peptides, Abeta40(HistauMe) and Abeta40(HispiMe), have been prepared, in which the histidine residues 6, 13, and 14 have been substituted with modified histidines where either the pi- or tau-nitrogen of the imidazole side chain is methylated to prevent the formation of bridging histidine moieties. These modifications did not inhibit the ability of these peptides to form fibrils. However, the modified peptides were four times more effective at generating H2O2 than the native sequence. Despite the ability to generate more H2O2, these peptides were not neurotoxic. Whereas the modifications to the peptide altered the metal binding properties, they also inhibited the interaction between the peptides and cell surface membranes. This is consistent with the notion that Abeta-membrane interactions are important for neurotoxicity and that inhibiting these interactions has therapeutic potential.     

Optimization of binding electrostatics: charge complementarity in the barnase-barstar protein complex

Theoretical and experimental studies have shown that the large desolvation penalty required for polar and charged groups frequently precludes their involvement in electrostatic interactions that contribute strongly to net stability in the folding or binding of proteins in aqueous solution near room temperature. We have previously developed a theoretical framework for computing optimized electrostatic interactions and illustrated use of the algorithm with simplified geometries. Given a receptor and model assumptions, the method computes the ligand-charge distribution that provides the most favorable balance of desolvation and interaction effects on binding. In this paper the method has been extended to treat complexes using actual molecular shapes. The barnase-barstar protein complex was investigated with barnase treated as a target receptor. The atomic point charges of barstar were varied to optimize the electrostatic binding free energy. Barnase and natural barstar form a tight complex (K(d) approximately 10(-14) M) with many charged and polar groups near the interface that make this a particularly relevant system for investigating the role of electrostatic effects on binding. The results show that sets of barstar charges (resulting from optimization with different constraints) can be found that give rise to relatively large predicted improvements in electrostatic binding free energy. Principles for enhancing the effect of electrostatic interactions in molecular binding in aqueous environments are discussed in light of the optima. Our findings suggest that, in general, the enhancements in electrostatic binding free energy resulting from modification of polar and charged groups can be substantial. Moreover, a recently proposed definition of electrostatic complementarity is shown to be a useful tool for examining binding interfaces. Finally, calculational results suggest that wild-type barstar is closer to being affinity optimized than is barnase for their mutual binding, consistent with the known roles of these proteins.     

过氧化氢酶在植物胁迫响应中的功能研究进展

作为信号分子的过氧化氢是植物复杂信号传导中的一个重要组成部分,它介导了多种植物胁迫反应并对其平衡的维持和调控起到了非常重要的作用。越来越多的研究证实了胁迫反应中过氧化氢酶与过氧化氢含量的变化有一定的关系,同时又和其它信号因子存在着交互作用。本文综述了近年来过氧化氢酶在植物遭受病害、水分、盐渍、光等胁迫反应中的调控作用,以及在这些反应中过氧化氢酶、过氧化氢、蛋白激酶、转录因子与其它信号分子所构成的可能信号网络和过氧化氢的限速步骤方面的研究进展。     

Electrostatic interactions of charged groups in an aqueous medium and their effect on the formation of the secondary structures of polypeptide chains

Analysis of experimental data shows that interaction of charges in an aqueous medium is satisfactorily evaluated by macroscopic dielectric water permeability even at small (approximately 3--4 A) distances. It has been shown that a non-polar molecule located between the charges weakens their interaction. At the same time interaction between charges situated on the non-polar molecules somewhat increases. An estimate is made of the free energy loss of alpha- and beta-structures of polypeptides, resulting from the insertion of charged groups as well as of the free energy of interactions involving charged groups in different secondary structures.     

Conformational properties of trans Ac-Asn-Pro-Tyr-NHMe and trans Ac-Tyr-Pro-Asn-NHMe in dimethylsulfoxide and in water determined by multinuclear n.m.r. spectroscopy

Vicinal coupling constants between various nuclei provide backbone and side-chain conformational information for a series of asparagine- and tyrosine-containing peptides in DMSO and in H2O. By enriching Tyr of Ac-Asn-Pro-Tyr-NHMe with 15N, it has been possible to distinguish between the resonances of the two side-chain beta protons of Tyr. Analysis of the coupling constants in terms of the distributions of side-chain conformations in these peptides indicates that the addition of Asn to the Pro-Tyr sequence leads to a less random conformational distribution. When compared to the side-chain rotamer distribution of Ac-Asn-NHMe and Ac-Tyr-NHMe, particular Asn and Tyr side-chain conformations of Ac-Asn-Pro-Tyr-NHMe are stabilized in dimethylsulfoxide solution. The interaction(s) which stabilize a unique Tyr side-chain conformation of Ac-Asn-Pro-Tyr-NHMe in dimethylsulfoxide are not present in Ac-Ala-Pro-Tyr-NHMe and are unaffected by the addition of Val-Pro to the C-terminus of Asn-Pro-Tyr. In water, a preferential stabilization of one Asn side-chain conformation of Ac-Asn-Pro-Tyr-NHMe is also observed, while the Tyr side-chain rotamer distribution is similar to that of Ac-Tyr-NHMe. An interaction between the Asn side chain and the Pro-Tyr-NHMe backbone was previously shown to stabilize a beta-bend conformation at Pro-Tyr in water. Data are also presented for Ac-Tyr-Pro-Asn-NHMe, for which local interactions do not stabilize particular backbone conformations in dimethylsulfoxide or in water. The conformations of the peptides studied here are relatively insensitive to temperatures between 27 degrees and 62 degrees, both in dimethylsulfoxide and in water. The sequences Asn-Pro-Tyr and Tyr-Pro-Asn occur in ribonuclease A, and these tripeptides serve as models for the interactions involved in the folding of this protein.