Proton polarizability of poly(L-tyrosine)–hydrogen phosphate hydrogen bonds as a function of alkali cations

We studied films of poly(L -tyrosine) with hydrogen phosphate (residue/phosphate, 1:1) by ir spectroscopy. The influences of the alkali cations (Li⁺, Na⁺, K⁺) and of the degree of hydration were clarified. If Li⁺ ions are present, the OH ?^?OP hydrogen bonds formed in the dried films between the tyrosine OH groups and hydrogen phosphate are asymmetrical. The formation of hydrogen phosphate–hydrogen phosphate hydrogen bonds is prevented by the presence of the Li⁺ ions. With an increase in the degree of hydration, the tyrosine–phosphate bonds are not broken but become slightly stronger. Completely different behaviour is found if K⁺ ions are present. In dry films, the OH ?^?OP ? O^? ?HOP hydrogen bonds formed between tyrosine and hydrogen phosphate show large proton polarizability. The tyrosine proton has a noticeable residence time at the acceptor O atom of the phosphate. The difference in the behaviour of the system with K⁺ ions when compared to the system with Li⁺ ions can be explained, since the hydrogen acceptor O atom of phosphate ions is more negatively charged due to the weaker influence of the K⁺ ions. Furthermore, POH ?^?OP hydrogen bonds between hydrogen phosphate molecules are formed. With an increase in the degree of hydration, the tyrosine–hydrogen phosphate hydrogen bonds are broken, all tyrosine protons are found at the tyrosine residues, and the -PO₃^? groupings are in a symmetrical environment, indicating that the K⁺ ions are removed from these groupings. If the degree of hydration increases further, hydrogen-bonded systems such as hydrogen phosphate–water–hydrogen phosphate are formed that show large proton polarizability due to collective proton motion. When Na⁺ ions are present, the OH ?^?OP ? O^? ?HOP hydrogen bonds formed in dry films still show proton polarizability, but the residence time of the tyrosine proton at the phosphate is very short.     

Monensin A methyl ester complexes with Li+, Na+, and K+ cations studied by ESI-MS, 1H- and 13C-NMR, FTIR, as well as PM5 semiempirical method

Monensin A methyl ester (MON1) was synthesized by a new method and its ability to form complexes with Li+, Na+, and K+ cations was studied by electrospray ionization-mass spectroscopy (ESI-MS), 1H and 13C nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR), and PM5 semiempirical methods. It is shown that MON1 with monovalent metal cations forms stable complexes of 1:1 stoichiometry. The structures of the complexes are stabilized by intramolecular hydrogen bonds in which the OH groups are always involved. In the structure of MON1, the oxygen atom of the C=O ester group is involved in very weak bifurcated intramolecular hydrogen bonds with two hydroxyl groups, whereas in the complexes of MON1 with monovalent metal cations the C=O ester group is not engaged in any intramolecular hydrogen bonds. Furthermore, it is demonstrated that the strongest intramolecular hydrogen bonds are formed within the MON1-Li+ complex structure. The structures of the MON1 and its complexes with Li+, Na+, and K+ cations are visualized and discussed in detail.     

Proton polarizability and proton transfer in histidine-phosphate hydrogen bonds as a function of cations present: ir investigations

(L -His)_n- dihydrogen phosphate systems are studied by ir spectroscopy in the presence of various cations and as a function of the degree of hydration. Ir continua indicate that (I) OH … N ? O^?…H⁺N (IIR) hydrogen bonds are formed and that these bonds show high proton polarizability, which increases from the Li⁺ to the K⁺ system. In the K⁺?system, His-P_i-P_i chains are formed, showing particularly high proton polarizability due to collective proton motion within both hydrogen bonds. The OH N ? O^?…H^?N equilibria are determined from ir bands. With the Li⁺ system, 55% of the protons are present at the histidine residues; this percentage is smaller with the Na⁺ system (41%), and amounts to only 32% with the K⁺ system. With the increasing degree of hydration the removal of the degeneracy of ν_as?PO^2?₃ vanishes, indicating loosening of the cations from the phosphates. Nevertheless, the hydrogen bond acceptor O atom becomes more negative; a shift of the equilibrium to the right is observed in the OH… N ? O^?…H⁺N bond. This is explained by the strong interaction of the dipole of the hydrogen bonds with the water molecules. All these results show that protons can be shifted easily in these hydrogen bonds due to their high proton polarizability. The transfer equilibria can be controlled easily by local electrical fields. In addition, these results may be of significance when phosphates interact with proteins.     

Spectroscopic, mass spectrometry, and semiempirical investigation of a new ester of Monensin A with ethylene glycol and its complexes with monovalent metal cations

A new ester of Monensin A with ethylene glycol (MON2) has been synthesized by a new method and its ability to form complexes with Li+, Na+, and K+ cations has been studied by ESI MS, 1H and 13C NMR, FT-IR, and PM5 semiempirical methods. It is demonstrated that MON2 forms stable complexes of 1:1 stoichiometry with monovalent metal cations. The structures of the complexes are stabilized by intramolecular hydrogen bonds in which the OH groups are always involved. In the structure of MON2 the oxygen atom of the C=O ester group is involved in very weak bifurcated intramolecular hydrogen bonds with two hydroxyl groups, whereas in the complexes of MON2 with monovalent metal cations the C=O ester group is not engaged in any intramolecular hydrogen bonds. The structures of the MON2 and its complexes with Li+, Na+, and K+ cations are visualized and discussed in detail.     

Hydrogen bonds between protein side chains and phosphates and their role in biological calcification

Poly(L-histidine)-phosphate (H2PO4-, HPO4(2-)) and poly(L-glutamate)-phosphate systems (residue/phosphate, 1:1) in the presence of Ca2+ are studied by infrared spectroscopy. In the poly(L-histidine)-phosphate systems N...HOP in equilibrium NH+...O-P hydrogen bonds are formed where most phosphate protons are found at the histidine ring. With an increase in the degree of hydration the proportion of the proton limiting structure NH+...O-P increases. In the poly(L-glutamate)-dihydrogen phosphate system most phosphate protons are found at the carboxylate groups. Different behavior is observed for poly(L-glutamate)-hydrogen phosphate mixtures, where the residence time of the phosphate proton at the hydrogen acceptor carboxylate group is very short. This residence time increases, however, with increasing humidity. All these results support the triphasic theory of biological calcification involving a tripartite protein-calcium-phosphate complex where these hydrogen bonds can be present. The behavior of these hydrogen bonds can also explain the formation of a nidus of calcium phosphate salts in calcium oxalate-containing urinary calculi.     

Infrared investigation of poly-L-histidine structure dependent on protonation

Poly-L -histidine (PLH) films at different degrees of protonation were produced mid subjected to infrared spectroscopic investigation (range 4000-650 cm^?1). In addition, the N-deuterated film spectra were plotted. The amide II and III bands show that the peptide group is present in the trans form. The amide I and II bands show that at 0% and 50% protonation the PLH occurs as an α-helix and at 100% protonation as a random coil with some ranges in β structure. At 0% and 50% protonation, no hydration water is bound to the backbone. At 0% protonation all NH groups are linked to each other or to water molecules via hydrogen bonds. At 50% protonation NH⁺?N bonds form between the imidazole rings. These protons are present in continuous energy level distribution. Such bonds with tunneling protons are extremely polarizable and between these bonds may act proton dispersion forces. The Cl^? ions are bonded to the NH groups of the imidazole groups. The hydration water is bonded to the Cl^?? ions and to the NH groups. At 100% protonation, hydration water is bonded also to the CO groups of the backbone. The NH groups of the backbone, like those of the rings, endeavor especially in the dry state to bond to the Cl^? ions. This leads to a strong steric constraint of the random coil.     

Bovine serum albumin observed by infrared spectrometry. II. Hydration mechanisms and interaction configurations of embedded H(2)O molecules

The hydration mechanism of bovine serum albumin (BSA) is studied, and we analyze (de)hydration spectra displayed previously. We first determine the three elementary (de)hydration spectra on which all these (de)hydration spectra can be decomposed. They correspond to three different hydration mechanisms for the protein, which we define after a quantitative analysis performed in a second step. The first mechanism, which involves ionization of carboxylic COOH groups, occurs at low hydration levels and rapidly reaches a plateau when the hygroscopy is increased. It is a mechanism that involves a single H(2)O molecule and consequently requires somewhat severe steric conditions. The second mechanism occurs at all hydration levels and, because it involves more H(2)O molecules, requires less severe steric conditions. It consists of the simultaneous hydration of one amide N--H group and one carbonyl-amide C=O group by four H(2)O molecules and one carboxyl COO(-) group by eight H(2)O molecules. The third mechanism is simpler and consists of the introduction of H(2)O molecules into the hydrogen-bond network of the hydrated protein. It becomes important at a high hydration level, when the presence of an appreciable number of H(2)O molecules makes this hydrogen-bond network well developed. This analysis also shows that 80 H(2)O molecules remain embedded in one dried protein made of 604 peptide units. They are held by hydrogen bonds established by N--H groups and at the same time they establish two hydrogen bonds on two carbonyl-amide C=O groups. The proportion of free N--H groups can be determined together with that of carbonyl-amide C=O groups accepting no hydrogen bonds and that of carbonyl-amide C=O groups accepting two hydrogen bonds. The proportion of N--H groups establishing one hydrogen bond directly on a carbonyl-amide C=O group is 65%, which is the proportion of peptide units found in alpha helices in BSA.     

Study of conformation structure and molecular interactions of rosamycin by infrared spectrum analysis

IR spectra of rosamycin and its solutions in inert (CCl4 and C2Cl4), proton acceptor (tetrahydrofuran, hexametapol and diethylamine) and proton donor (CHCl3 and CH3OD) solvents were studied at various concentrations (0.1 to 0.001 mol/l) and temperatures (20 to 100 degrees C) in the region of the vC = O and vOH absorption bands (1600-1800 and 3200 3650 sm 1). It was found that the absorption bands at 3480 and 3560 sm-1 observed in the spectra of rosamycin diluted solutions in the inert solvents referred to variations of vOH...N of the aminosugar fragment and to vOH...O = C of the ester group of the macrocycle. Bands at 1697 and 1717 sm-1 referred to vC = O of the ketone and aldehyde carbonyl groups and band at 1728 sm-1 referred to vC = O of the ester group whose carbonyl was involved in the C = H...HO intramolecular hydrogen bond. Intensity of vC = O band (1745 sm-1) of the free ester group was nought. However, it increased with using the proton acceptor solvents. OH...N and OH...O = C intramolecular hydrogen bonds stabilized rosamycin molecule conformation. Mechanism of rosamycin interaction with the proton donor and acceptor molecules was elucidated. It was shown that tertiary nitrogen was the center of rosamycin molecule protonation.     

Intramolecular hydrogen bonds and conformation of the lincomycin molecule in organic solvents

IR spectra (1600-1800 and 3000-3650 cm-1) of lincomycin base solutions in inert (CCl4 and C2Cl4), proton acceptor (dioxane, dimethylsulfoxide and triethyl amine) and proton donor (CHCl3, CD3OD and D2O) solvents were studied. Analysis of the concentration and temperature changes in the spectra revealed that association in lincomycin in the inert solvents was due to intramolecular hydrogen linkage involving amide and hydroxyl groups. Disintegration of the associates after the solution dilution and temperature rise was accompanied by formation of intramolecular bonds stabilizing the stable conformation structure of the lincomycin molecule. The following hydrogen linkage in the conformation was realized: NH...N (band v NH...N at 3340 cm-1), OH...O involving the hydroxyl at C-7 and O atoms in the D-galactose ring (band v OH...O at 3548 cm-1), a chain of the hydrogen bonds OH...OH...OH in the lincomycin carbohydrate moiety (band v OH...O at 3593 cm-1 and v OH of the end hydroxyl group at 3625 cm-1). Bonds NH and C-O of the amide group were located in transconformation. Group C-O did not participate in the intramolecular hydrogen linkage.     

Proline-containing beta-turns. IV. Crystal and solution conformations of tert.-butyloxycarbonyl-L-prolyl-D-alanine and tert.-butyloxycarbonyl-L-prolyl-D-alanyl-L-alanine

The conformations of the dipeptide t-Boc-Pro-DAla-OH and the tripeptide t-Boc-Pro-DAla-Ala-OH have been determined in the crystalline state by X-ray diffraction and in solution by CD, n.m.r. and i.r. techniques. The unit cell of the dipeptide crystal contains two independent molecules connected by intermolecular hydrogen bonds. The urethane-proline peptide bond is in the cis orientation in both the molecular forms while the peptide bond between Pro and DAla is in the trans orientation. The single dipeptide molecule exhibits a "bent" structure which approximates to a partial beta-turn. The tripeptide adopts the 4----1 hydrogen-bonded type II beta-turn with all trans peptide bonds. In solution, the CD and i.r. data on the dipeptide indicate an ordered conformation with an intramolecular hydrogen bond. N.m.r. data indicate a significant proportion of the conformer with a trans orientation at the urethane-proline peptide bond. The temperature coefficient of the amide proton of this conformer in DMSO-d6 points to a 3----1 intramolecular hydrogen bond. Taken together, the data on the dipeptide in solution indicate the presence (in addition to the cis conformer) of a C7 conformation which is absent in the crystalline state. The spectral data on the tripeptide indicate the presence of the type II beta-turn in solution in addition to the nonhydrogen-bonded conformer with the cis peptide bond between the urethane and proline residues. The relevance of these data to studies on the substrate specificity of collagen prolylhydroxylase is pointed out.     

Free‐energy function based on an all‐atom model for proteins

We have developed a free‐energy function based on an all‐atom model for proteins. It comprises two components, the hydration entropy (HE) and the total dehydration penalty (TDP). Upon a transition to a more compact structure, the number of accessible configurations arising from the translational displacement of water molecules in the system increases, leading to a water‐entropy gain. To fully account for this effect, the HE is calculated using a statistical‐mechanical theory applied to a molecular model for water. The TDP corresponds to the sum of the hydration energy and the protein intramolecular energy when a fully extended structure, which possesses the maximum number of hydrogen bonds with water molecules and no intramolecular hydrogen bonds, is chosen as the standard one. When a donor and an acceptor (e.g., N and O, respectively) are buried in the interior after the break of hydrogen bonds with water molecules, if they form an intramolecular hydrogen bond, no penalty is imposed. When a donor or an acceptor is buried with no intramolecular hydrogen bond formed, an energetic penalty is imposed. We examine all the donors and acceptors for backbone‐backbone, backbone‐side chain, and side chain‐side chain intramolecular hydrogen bonds and calculate the TDP. Our free‐energy function has been tested for three different decoy sets. It is better than any other physics‐based or knowledge‐based potential function in terms of the accuracy in discriminating the native fold from misfolded decoys and the achievement of high Z‐scores. Proteins 2009. © 2009 Wiley‐Liss, Inc.     

Conformation-Determining role for the N-acetyl group in the O-glycosidic linkage, α-GalNAc-Thr

An ¹H-nmr study of 2-acetamido-2-deoxy-3,4,6-tri-O-acetyl-D-galactopyranose (AcGalNAc) glycosylated Thr-containing tripeptides in Me₂SO-d₆ solution reveals two mutually exclusive intramolecular hydrogen bonds. In Z-Thr(AcGalNAc)-Ala-Ala-OMe, there is an intramolecular hydrogen bond between the Thr amide proton and the sugar N-acetyl carbonyl oxygen. The strength of this hydrogen bond will be dependent on the amino acid residues on the Thr C terminal side to some undetermined distance. In Ac-Thr(AcGalNAc)-Ala-Ala-OMe, a different intramolecular hydrogen bond between the sugar N-acetyl amide proton and the Thr carbonyl oxygen exists. The choice of hydrogen bonds seems dependent on the bulkiness of the residues on the Thr N terminal side. The consequence of such strong hydrogen bonds is a clearly defined orientation of the sugar moiety with respect to the peptide backbone. In the former, the plane of the sugar pyranose ring is roughly oriented perpendicularly to the peptide backbone. The latter orientation is where the plane of the sugar ring is roughly in line with the peptide backbone. In both orientations, the sugar moiety can increase the shielding of the neighboring amino acid residues from the solvent. The idea that the amino acid residues near the glycosylated Thr influence orientation of the sugar moiety with respect to the peptide backbone and in turn possibly hinder peptide backbone flexibility has interesting implications in the conformational as well as the biological role of O-glycoproteins.     

Peptide--water association in peptide crystals.

The structures of 37 peptide crystals, containing 78 water-peptide hydrogen bonds and 77 other hydrogen bonds involving water, were surveyed to identify the geometry of peptide backbone hydration. In the sample, hydration of peptide carbonyl occurred more frequently than hydration of peptide N--H. The most probable value of the C'=O ... O water angle was near 138 degrees, considerably greater than the 120 degrees to the axis of a lone electron pair on the carbonyl oxygen. Associated water oxygens tended to be in the plane of the peptide bond, bui--H and Ci+1=O atoms, was common in glycine-containing cyclic hexapeptides. The distribution of angles between two hydrogen bonds at a single water molecule, as defined by the three nonhydrogen atoms involved, was centered near the tetrahedral angle.     

N.m.r. study on conformation of Ac-Thr(alpha-GalNAc)-Ala-Ala-OMe as a model for mucin type glycoprotein

This study report on the results of high resolution 1H n.m.r. investigations on Ac-Thr(alpha-GalNAc)-Ala-Ala-OMe 1 as a mucin type model glycopeptide of antifreeze glycoprotein (AFGP) in both dimethyl sulfoxide (DMSO) and H2O. The temperature dependence of amide proton chemical shifts strongly suggested the presence of the intramolecular hydrogen bond between the amide proton of GalNAc and the carbonyl oxygen of the Thr residues. Due to this bond, the orientation of the sugar residue of 1 appears to be fairly restricted relative to its peptide backbone. Despite the lack of the clear evidence for such intramolecular hydrogen bond in H2O, 1H coupling constant data suggested the structural similarity of 1 in DMSO and H2O, indicating the presence of the intramolecular hydrogen bond even in H2O, which may play an important role in determining the orientation of the sugar moiety with respect to the peptide backbone in glycoprotein.     

Glutamic acid-dihydrogen phosphate hydrogen-bonded networks: their proton polarizability as a function of cations present. Infrared investigations.

Glutamic acid [(L-glu)n] + dihydrogen phosphate systems are studied by infrared (IR) spectroscopy dried and hydrated at 75% relative humidity, as a function of both the phosphate-glutamic acid residue (Pi/glu) ratio and the type of cations present. It is shown that the glutamic acid groups form hydrogen-bonded chains with the phosphates. In these chains the positive charge fluctuates, and they show very large proton polarizability which increases in the series Li+,Na+,K+ systems. These chains are cross-linked via phosphate-phosphate hydrogen bonds, in which the proton is almost localized at one Pi. The comparison of the (L-glu)n + dihydrogen phosphate systems with the results obtained earlier in the case of (L-glu)n + hydrogen phosphate systems shows that the behavior of (L-glu)n + Pi systems strongly depends on the pH. Only with decreasing pH the conducting chains are formed. Finally, a hypothesis is discussed with regard to the charge conduction in the F0 subunit of the H+-ATPase in mitochondria.     

Proton transfer in and polarizability of hydrogen bonds coupled with conformational changes in proteins. II. IR investigation of polyhistidine with various carboxylic acids

Polyhistidine-carboxylic acid systems are studied by ir spectroscopy. It is shown that OH ?N ? O^?…H⁺N bonds formed between carboxylic groups and histidine residues are easily polarizable proton-transfer hydrogen bonds when the pK_a of the protonated histidine residues is about 2.8 units larger than that of the carboxylic groups. From these results it bis concluded that OH ?N ? O^? ?H⁺N bonds between glutamic or aspartic acid histidine residues in proteins may be easily polarizable proton-transfer bonds. Furthermore, it is demonstrated that water molecules shift the proton-transfer equilibria in these hydrogen bonds in favor of the polar structure, i.e., due to water or polar environments OH ?N ? O^? ?H⁺N bonds with smaller ΔpK_a values become easily polarizable proton-transfer hydrogen bonds. A consideration of the amide bands of polyhistidine shows that it can be present in five different conformations. It is shown that these conformational changes are strongly related to the degree of proton transfer. Hence, the degree of proton transfer, the degree of hydration, and conformation are not independent of each other, but are strongly coupled. Further proof for the interdependence of proton transfer and conformational changes are hysteresis effects, which are observed with studies of polyhistidine dependent on carboxylic acid, adsorption and desorption. OH ?N ? O^? ?H⁺N bonds between aspartic and glutamic acid and histidine residues are present in hemoglobin, in ribonucleases, and in proteases, whereby this type of bond is preferentially found in the active centers of these enzymes. It is pointed out that hydrogen bonds with such interaction properties should be of great significance for structure and especially functions of proteins in which they are present.     

Proton nuclear magnetic resonance studies on glutamine-binding protein from Escherichia coli. Formation of intermolecular and intramolecular hydrogen bonds upon ligand binding

Proton nuclear magnetic resonance studies have revealed several structural and dynamic properties of the glutamine-binding protein of Escherichia coli. When this protein binds L-glutamine, six low-field, exchangeable proton resonances appear in the region from +5.5 to +10 parts per million downfield from water (or +10.2 to +14.7 parts per million downfield from the methyl proton resonance of 2,2-dimethyl-2-silapentane-5-sulfonate). This suggests that the binding of L-glutamine induces specific conformational changes in the protein molecule, involving the formation of intermolecular and intramolecular hydrogen bonds between the glutamine-binding protein and L-glutamine, and within the protein molecule. The oxygen atom of the gamma-carbonyl group of L-glutamine is likely to be involved in the formation of an intermolecular hydrogen bond between the ligand and the binding protein. We have shown that at least one phenylalanine and one methyl-containing residue are spatially close to this intermolecular hydrogen-bonded proton. The intermolecular and intramolecular hydrogen-bonded protons of the ligand-protein complex undergo solvent exchange. The local conformations around these intermolecular and intramolecular hydrogen bonds are quite stable when subjected to pH and temperature variations. From these results, the utility of proton nuclear magnetic resonance spectroscopy for investigating such binding proteins has been shown, and a picture of the ligand-binding process can be drawn.     

Nuclear-magnetic-resonance study of aggregations and conformations of melanostatin and related peptides.

The 1H and 13C nuclear magnetic resonance spectra of melanostatin (Pro-Leu-Gly-NH2) and related peptides (Pro-Leu-Gly, Z-Pro-Leu-Gly, Z-Pro-Leu-Gly-NH2 and Z-Pro-Leu-Gly-OCH3, where Z = benzyloxycarbonyl) were analysed in a variety of solvents. At physiological pH, the melanostatin molecule is N-protonated in aqueous solution. The concentration dependences of the chemical shifts of amide-proton and carbonyl-carbon resonances and of proton spin-lattice relaxation times were observed in relation to molecular aggregations. In dimethylsulfoxide solution, aggregations were observed for N-protonated melanostatin and Pro-Leu-Gly prepared with HCl and for the Na salt of Z-Pro-Leu-Gly but not for N-protonated melanostatin prepared with HClO4 or HNO3, unprotonated melanostatin, Z-Pro-Leu-Gly-NH2, or Z-Pro-Leu-Gly-OCH3. The leucine NH and glycine CO groups of N-protonated melanostatin are involved in the intermolecular hydrogen bonds of aggregates. The leucine NH group of N-protonated Pro-Leu-Gly also forms the intermolecular hydrogen bond. The solvent and temperature dependences of the chemical shifts of amide-proton and carbonyl-carbon resonances were measured to determine intramolecular hydrogen bonding. In dimethylsulfoxide solution, N-protonated melanostatin molecules in part take the beta-turn structure and the trans carboxamide NH proton and carbonyl oxygen of the proline residue form an intramolecular hydrogen bond.     

Infrared studies of fully hydrated unsaturated phosphatidylserine bilayers. Effect of Li+ and Ca2+

Infrared spectroscopy has been used to characterize the thermal-phase behavior of fully hydrated 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine (POPS) and 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPS) as well as their interaction with Li+ and Ca2+. The order-disorder transition of POPS-NH4+ is at 17 degrees C; in the presence of Li+ a POPS-Li+ complex is formed, and the transition temperature of this complex is 40 degrees C. DOPS-NH4+ has an order-disorder transition at -11 degrees C, and unlike POPS the addition of Li+ has no effect on the thermal behavior of DOPS-NH4+. This indicates that the binding of Li+ to DOPS is negligible or very weak. Li+ binds to the phosphate and carboxylate groups of POPS, and as a result these groups lose their water of hydration. Li+ binding induces a conformational change, probably in the glycerol backbone of POPS; however, the conformation of the two P-O ester bonds remains gauche-gauche as in POPS-NH4+. Both POPS and DOPS form crystalline complexes with Ca2+. As a result of Ca2+ binding to the phosphate, this group loses its water of hydration and there is a conformational change in the P-O ester bonds from gauche-gauche to antiplanar-antiplanar. In contrast to the POPS-Li+ complex, the carboxylate group remains hydrated in the Ca2+ complexes. Furthermore, in these PS-Ca2+ complexes a new hydrogen bond is formed between one of the ester C=O groups and probably water. Such a situation is not found in the NH4+ and Li+ salts of phosphatidylserine.     

Switching of turn conformation in an aspartate anion peptide fragment by NH . . . O- hydrogen bonds

Aspartic acid protease model peptides Z-Phe-Asp(COOH)-Thr-Gly-Ser-Ala-NHCy (1) and AdCO-Asp(COOH)-Val-Gly-NHBzl (3), and their aspartate anions (NEt4)[Z-Phe-Asp(COO-)-Thr-Gly-Ser-Ala-NHCy] (2) and (NEt4)[AdCO-Asp(COO-)-Val-Gly-NHBzl] (4), having an invariant primary sequence of the Asp-X(Thr,Ser)-Gly fragment, were synthesized and characterized by 1H-NMR, CD, and infrared (IR) spectroscopies. NMR structure analyses indicate that the Asp O(delta) atoms of the aspartate peptide 2 are intramolecularly hydrogen-bonded with Gly, Ser, Ala NH, and Ser OH, supporting the rigid beta-turn-like conformation in acetonitrile solution. The tripeptide in the aspartic acid 3 forms an inverse gamma-turn structure, which is converted to a beta-turn-like conformation because of the formation of the intramolecular NH . . . O- hydrogen bonds with the Asp O(delta) in 4. Such a conformational change is not detected between dipeptides AdCO-Asp(COOH)-Va-NHAd (5) and (NEt4)[AdCO-Asp(COO-)-Val-NHAd] (6). The pK(a) value of side-chain carboxylic acid (5.0) for 3 exhibits a lower shift (0.3 unit) from that of 5 in aqueous polyethyleneglycol lauryl ether micellar solution. NMR structure analyses for 3 in an aqueous micellar solution indicate that the preorganized turn structure, which readily forms the NH . . . O- hydrogen bonds, lowers the pK(a) value and that resulting hydrogen bonds stabilize the rigid conformation in the aspartate anion state. We found that the formation of the NH . . . O- hydrogen bonds involved in the hairpin turn is correlated with the protonation and deprotonation state of the Asp side chain in the conserved amino acid fragments.