Substrate structure and amino acid/K+ symport in brush-border membrane vesicles from larval Manduca sexta midgut

The effects of amino acid sidechain length, substituent position and c chirality on amino acid/K+ symport have been examined in rapid filtration experiments on brush-border membrane vesicles prepared from larval Manduca sexta midgut. Cis-inhibition and trans-stimulation protocols were used to examine the effects of amino acid analogs on the uptake of alanine, phenylalanine, leucine and lysine, which are cotransported with K+ by a zwitterionic symporter at the high pH characteristic of the midgut in vivo. The symporter was found to translocate both L- and D-stereoisomers of alanine, leucine and lysine, but only the L-form of phenylalanine. Alterations to substrate structure that leave the charge distribution unchanged do not affect symport. Thus, moving the methyl group from C-3 to C-5 in the sequence isoleucine, leucine and norleucine has no effect on their ability to inhibit leucine symport. Increasing sidechain length among alanine homologs has little effect on their ability to inhibit alanine uptake, but increasing the sidechain length of lysine homologs from 1 to 3 methylene groups enhances cis-inhibition and trans-stimulation of lysine symport. The substantial difference in molecular charge distribution among aminobutanoic acid isomers has a large impact on alanine symport with only alpha- (or 2-) aminobutanoic acid functioning as an alanine analog. Only those changes in substrate structure that are coupled to the molecular charge distribution seem to affect symport. The tolerance of the symporter may reflect a balance mandated by the conflicting demands of selectivity and throughput.

Anomalous glutamate/alkali cation symport in larval Manduca sexta midgut

Rapid filtration assays were used to characterize glutamate/cation uptake in brush-border membrane vesicles from the larval midgut of the lepidopteran Manduca sexta. At pH 10.5, which is close to the physiological pH in the midgut of M. sexta, an inwardly directed K+ gradient stimulated glutamate uptake, suggesting that glutamate was symported. Gradients of Na+ or Li+ were less effective. Neither Rb+ nor Cs+ stimulated glutamate uptake. Anion-specificity was less pronounced: the accumulation maximum was only slightly higher with thiocyanate (SCN-) than with Cl-, although initial uptake was noticeably faster with thiocyanate. A distinct set of amino acids that would cis-inhibit or trans-elicit glutamate uptake was not found. Even L-glutamate itself did not elicit accumulations of labeled glutamate. Taken together, these results suggest that a glutamate-specific symporter may not be present. Moreover, because glutamate symport was found to be electroneutral in vitro whereas amino acid uptake is electrophoretic in vivo, we infer that symport with K+ may not be an important mechanism of glutamate translocation by M. sexta midgut.

Protein sequence and structure relationship ARMA spectral analysis: application to membrane proteins

If it is assumed that the primary sequence determines the three-dimensional folded structure of a protein, then the regular folding patterns, such as alpha-helix, beta-sheet, and other ordered patterns in the three-dimensional structure must correspond to the periodic distribution of the physical properties of the amino acids along the primary sequence. An AutoRegressive Moving Average (ARMA) model method of spectral analysis is applied to analyze protein sequences represented by the hydrophobicity of their amino acids. The results for several membrane proteins of known structures indicate that the periodic distribution of hydrophobicity of the primary sequence is closely related to the regular folding patterns in a protein's three-dimensional structure. We also applied the method to the transmembrane regions of acetylcholine receptor alpha subunit and Shaker potassium channel for which no atomic resolution structure is available. This work is an extension of our analysis of globular proteins by a similar method.

Potential differences influence amino acid/Na+ symport rates in larval Manduca sexta midgut brush-border membrane vesicles

The time-dependent fluorescence intensity of an intravesicular potential-sensitive dye was used to probe the real-time kinetics of potential difference (PD)-dependent amino acid/Na+ symport at pH9 into brush-border membrane vesicles obtained from larval Manduca sexta midgut. Neutral amino acids (alanine, proline) are symported at higher rates as the vesicles are hyperpolarized. The symport rates of acidic (glutamate) and basic (arginine) amino acids are almost PD-independent. The half-saturation constant of alanine is PD-independent between -108 and -78 mV, although the maximal symport velocity increases by half as the voltage is increased. Amino acid throughput is evidently enhanced as the relatively high transmembrane PDs (> 150 mV, lumen positive) measured in vivo are approached. The half-saturation concentrations of Na+ were in the range 15-40 mmol l-1 for most of the amino acids examined and increased with voltage for alanine. The Vmax observed as a function of cation or amino acid concentration increased as the vesicle was hyperpolarized in the case of leucine and alanine. The data support the hypothesis that carrier and substrates are at equilibrium inasmuch as substrate translocation seems to be the rate-determining step of symport.

The perception and experience of health personnel about the integration of mental health in general health services

The National Mental Health Program of India focuses on the integration of mental health in general health services. Using a structured questionnaire, 100 health personnel (40 Medical Officers and 60 Health Assistants) in the District Mental Health Program were interviewed regarding their perception and knowledge about the integration of mental health in general health services. Most personnel were found to be satisfied with their new role in carrying out mental health services and only a few felt that it was an extra burden. Suggestions were made by them about the free supply of drugs, short and long term training and active supervision and support by the higher authorities.

Interaction of liposome-associated all-trans-retinoic acid with squamous carcinoma cells

Because of their antiproliferative and differentiation-inducing properties, retinoids have been used clinically as therapeutic and chemopreventive agents against squamous-cell carcinomas (SCC). As is the case for many therapeutic agents, however, the administration of retinoids is associated with toxic effects. Because encapsulation of certain drugs in lipid vesicles (liposomes) has been shown to result in reduced toxic effects, we studied the in vitro interaction of liposome-encapsulated all-trans-retinoic acid (L-ATRA) with a SCC line (MDA 886Ln) and its multicellular tumor spheroid (MTS) model. Various L-ATRA formulations were tested for incorporation of retinoic acid, toxic effects against human red blood cells, uptake and retention by tumor cells, and antiproliferative effects against SCC. Of the different formulations tested, L-ATRA containing diphosphatidyl palmitoylcholine (DPPC) and stearylamine (SA; 9:1, w/w) showed optimal drug incorporation, high stability, and minimal toxicity toward red blood cells and was highly efficacious in delivering ATRA and, thus, in inhibiting the growth of MDA 886Ln and its MTS model. DPPC: SA L-ATRA inhibited the expression of the enzyme keratinocyte transglutaminase in epidermal cells as effectively as did the free drug. These results suggest that liposomes can serve as an effective carrier system for the delivery of retinoids to SCC.

Biochemical and molecular properties of lithium-sensitive myo-inositol monophosphatase

Myo-inositol monophosphatase is a pivotal enzyme of the inositol second messenger system which is specifically inhibited by therapeutic levels of lithium salts, implicating inhibition of this enzyme as a potential site of its action in bipolar disease. This enzyme has a native molecular weight of 59,000, and has traditionally been found in the cytosolic fraction, although a membrane-bound form has also been identified. Possessing two identical subunits, this enzyme hydrolyzes those monophosphates which are equatorially located within the inositol ring, and several nucleoside monophosphates phosphorylated at the 2-position. Each subunit of the native enzyme contains an active site with unusually large caverns as revealed by crystallographic studies, which may explain the accommodation of these structurally unrelated substrates. We have suggested that the uncompetitive inhibition of this phosphatase by lithium ions may prevent the formation of an enzyme-bound non-isomeric (meso) intermediate, Mg(2+)-inositol 1,3 or 4,6 cyclic monophosphate when this enzyme hydrolyzes its respective isomeric substrates.

Lung transplantation without the use of antilymphocyte antibody preparations

Survival, infection, and rejection rates and functional data are reported in 35 lung recipients treated with triple-drug immunosuppression without antilymphocytic antibody therapy. Early mortality (less than 60 days) was 6%. Thirty recipients (86%) are alive, with a mean follow-up period of 16 months (range, 1 to 36 months). Actuarial survival was 91% at 1 year and 83% at 2 years. Thirty-seven infections occurred in 24 patients. Actuarial freedom from infection was 61% at 3 months and 27% at 1 year. Only one patient died of infection (aspergillosis). Thirty-six episodes of acute rejection were treated in 23 patients. Time to first rejection was 14 +/- 7.8 days. Actuarial freedom from acute rejection was 36% at 2 months. Significant functional improvement was evident in all operative survivors. Pulmonary function and exercise performance data in patients without bronchiolitis obliterans remain stable 1 year after transplantation. We conclude that the use of triple-drug immunosuppression without antilymphocytic antibody preparations in lung transplantation provides effective immunosuppression with a low risk of serious infectious complications.

myo-inositol monophosphatase from rat testes: purification and properties

myo-Inositol monophosphatase (EC 3.1.3.25) has been purified to homogeneity from the high-speed supernatant of rat testes and its properties were investigated. By means of ammonium sulfate precipitation, followed by heating, anion exchange, and gel filtration high-pressure liquid chromatographic techniques, polylysine agarose and phenyl-Sepharose column chromatographic methods, this phosphatase was purified 2563-fold to a specific activity of 7972 mU/mg protein. It showed an apparent native molecular weight of 58,000 as determined by gel filtration chromatography and was composed of two identical subunits of molecular weight of 29,000 as determined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. Among several divalent cations tested for activation of the enzyme, Mg2+ was most effective and optimally active at pH 7.8. The Km values for D- and L-myo-inositol 1-phosphate (which were equal) and 2'-AMP were 0.12 +/- 0.02 and 0.17 +/- 0.03 mM, respectively. Lithium ions inhibited this phosphatase specifically and kinetic studies demonstrated uncompetitive inhibition. Preparations of polyclonal antibodies against the homogeneous enzyme in rabbits cross-reacted with the partially purified enzyme preparations from liver, kidney, heart, and brain show immunological identity. Western blot analysis after SDS-polyacrylamide gel electrophoresis confirmed a major band corresponding to a subunit molecular weight of 29,000. A sensitive enzyme staining method was also developed to localize the site of myo-inositol monophosphatase activity on polyacrylamide gels which helped to differentiate this phosphatase from nonspecific contaminating phosphatases. To explain the unusual stereospecificity of this enzyme on its isomeric substrates, a working model was suggested involving the production of a myo-inositol 1,3-cyclic phosphate intermediate during the course of its reaction.

Cation-dependent leucine, alanine, and phenylalanine uptake at pH 10 in brush-border membrane vesicles from larval Manduca sexta midgut

Using the rapid filtration technique, cation gradient driven leucine, alanine and phenylalanine uptake by brush-border membrane vesicles (BBMV) from the highly studied model insect, Manduca sexta, is characterized at the physiological pH of 10. The vesicles are sealed and nonspecific binding is small. Almost identical initial time courses of leucine uptake are obtained whether the vesicles are osmotically balanced initially or at equilibrium. The maximum accumulation values are also similar and the equilibrium values are identical with either treatment. Equilibrium is reached by 60 min. Amino acid accumulation is cation gradient dependent and is abolished by 18 microM valinomycin. Uptake of all three amino acids occurs over a broad pH range with maximum rates at approximately pH 10 and lower rates at pH 7.5. The cation selectivity of phenylalanine and alanine uptake changes with pH; the sequence is K+ > Na+ > Cs+ >> Rb+ = Li+ at pH 10.0, whereas K+ = Na+ at pH 8.0; the selectivity of leucine uptake is K+ = Na+ > Cs+ >> Rb+ = Li+ at pH 10. Maximum K+ driven accumulation of all three amino acids decreases with anions in the order: SCN- > NO3- > Cl- = CO(3)2- = So(4)2- = HPO(4)2- > gluconate-.Vmax values are similar for all three amino acids. There are large differences in initial uptake rates (leucine > phenylalanine = alanine), and maximum accumulation values (leucine > phenylalanine > alanine).

Synthesis and immunological properties of bombesin analogs

Bombesin (Bn, pGlu-Gln-Arg-Leu-Gly-Asn-Gln-Trp-Ala-Val-Gly-His-Leu-Met-NH2) is one of the most potent peptides, possessing a variety of physiological and pharmacological functions. We find from CD spectroscopy that the eight C-terminal residues of bombesin [Bn(7-14)NH2] have an ordered structure, and replacement of His-12 with Pro of Bn(7-14)NH2 changes the conformation from ordered to a more unordered form. Antibodies to Bn(7-14)NH2 cross-react to Bn and gastrin releasing peptide (GRP) in a dose-dependent manner. Antibodies to the Pro-analog do not recognize Bn or GRP. Substitution of the C-terminal amide by isopropylamide [Bn(7-14)NHC3H7(i)] makes its antibodies more specific to Bn than to GRP. It appears that this region of the peptide is an important antigenic determinant, which makes these antibodies differentiate between BN and GRP.

Helix-forming tendencies of amino acids depend on their sequence contexts: tripeptides AFG and FAG show incipient beta-bulge formation in their crystal structures

Many of the theoretical methods used for predicting the occurrence of alpha-helices in peptides are based on the helical preferences of amino acid monomer residues. In order to check whether the helix-forming tendencies are based on helical preferences of monomers only or also on their sequence contexts, we synthesized permuted sequences of the tripeptides GAF, GAV, and GAL that formed crystalline helices with near alpha-helical conformation. The tripeptides AFG and FAG formed good crystals. The x-ray crystallographic studies of AFG and FAG showed that though they contain the same amino acids as GAF but in different sequences, they do not assume a helical conformation in the solid state. On the other hand, AFG and FAG, which contain the same amino acids but in a different sequence, exhibit nearly the same backbone torsion angles corresponding to an incipient formation of a beta-bulge, and exhibit nearly identical unit cells and crystal structures. Based on these results, it appears that the helix-forming tendencies of amino acids depend on the sequence context in which it occurs in a polypeptide. The synthetic peptides AFG (L-Ala-L-Phe-Gly) and FAG (L-Phe-L-Ala-Gly), C14H19N3O4, crystallize in the orthorhombic space group P2(1)2(1)2(1), with a = 5.232(1), b = 14.622(2), c = 19.157(3) A, Dx = 1.329 g cm-3, Z = 4, R = 0.041 for 549 reflections for AFG, and with a = 5.488(2), b = 14.189(1), c = 18.562(1) A, Dx = 1.348 g cm-3, Z = 4, R = 0.038 for 919 reflections for FAG. Unlike the other tripeptides GAF, GGV, GAL, and GAI, the crystals of AFG and FAG do not contain water molecule, and the molecules of AFG aor FAG do not show the helical conformation. The torsion angles at the backbone of the peptide are psi 1 = 144.5(5) degrees; phi 2, psi 2 = -98.1(6) degrees, -65.2(6) degrees; phi 3, psi 13, psi 31 = 154.1(6) degrees, -173.6(6) degrees, 6.9(8) degrees for AFG; and psi 1 = 162.6(3) degrees; phi 2, psi 2 = -96.7(4) degrees, -46.3(4) degrees; phi 3, psi 13, psi 31 = 150.1(3) degrees, -168.7(3) degrees, 12.2(5) degrees for FAG. The conformation angles (phi, psi) for residues 2 and 3 for both AFG and FAG show incipient formation of an beta-bulge.

Acute renal failure following binge drinking and nonsteroidal antiinflammatory drugs

Two college students who developed reversible acute deterioration in renal function following binge drinking of beer and the use of nonsteroidal antiinflammatory drugs (NSAIDs) are reported. Both patients presented with back and flank pain with muscle tenderness, but showed no evidence of overt rhabdomyolysis. The first case had marked renal failure, with a peak serum creatinine reaching 575 mumol/L (6.5 mg/dL), and acute tubular necrosis was documented by renal biopsy. The second case had only modest elevation in serum creatinine, and renal function rapidly improved on rehydration. The contribution of the potential muscle damage associated with alcohol ingestion to the changes in renal function in these two cases is not clear. However, the major mechanism for the acute renal failure was thought to be related to inhibition of renal prostaglandin synthesis in the face of compromised renal hemodynamics secondary to alcohol-induced volume depletion.

The identification of a membrane-bound myo-inositol 1-phosphatase in rat brain, liver, and testes

A membrane-bound myo-inositol 1-phosphatase has been solubilized and partially purified from rat tissues. This particulate enzyme was detected in brain, liver and testis and certain physicochemical and enzymological properties were examined. Previously this major enzyme of the inositol signaling system was considered strictly cytosolic. The ratio of activity in the membrane form was approximately one-eighth of the activity found with the cytosolic fraction. The molecular weight of this phosphatase was found to be 59,000 by gel filtration chromatography and a subunit molecular weight of 29,000 by Western blot analysis, values comparable to the cytosolic form. This phosphatase cleaves both D- and L- myo-inositol 1-phosphates which originate from two different cellular pathways and is inhibited by lithium ions. Polyclonal antibodies were raised against homogeneous testicular cytosolic myo-inositol 1-phosphatase and cross-reacted with this membrane form as determined by western blot analysis showing immunological identity.

Helix-forming tendencies of amino acids depend on the restrictions of side-chain rotamer conformations: crystal structure of the tripeptide GAI in two crystalline forms

In our attempts to design crystalline alpha-helical peptides, we synthesized and crystallized GAI (C11H21N3O4) in two crystal forms, GAI1 and GAI2. Form 1 (GAI1) Gly-L-Ala-L-Ile (C11H21N3O4.3H2O) crystals are monoclinic, space group P2(1) with a = 8.171(2), b = 6.072(4), c = 16.443(4) A, beta = 101.24(2) degrees, V = 800 A3, Dc = 1.300 g cm-3 and Z = 2, R = 0.081 for 482 reflections. Form 2 (GAI2) Gly-L-Ala-L-Ile (C11H21N3O4.1/2H2O) is triclinic, space group P1 with a = 5.830(1), b = 8.832(2), c = 15.008(2) A, alpha = 102.88(1), beta = 101.16(2), gamma = 70.72(2) degrees, V = 705 A3, Z = 2, Dc = 1.264 g cm-3, R = 0.04 for 2582 reflections. GAI1 is isomorphous with GAV and forms a helix, whereas GAI2 does not. In GAI1, the tripeptide molecule is held in a near helical conformation by a water molecule that bridges the NH3+ and COO- groups, and acts as the fourth residue needed to complete the turn by forming two hydrogen bonds. Two other water molecules form intermolecular hydrogen bonds in stabilizing the helical structure so that the end result is a column of molecules that looks like an incipient alpha-helix. GAI2 imitates a cyclic peptide and traps a water molecule. The conformation angles chi 11 and chi 12 for the side chain are (-63.7 degrees, 171.1 degrees) for the helical GAI1, and (-65.1 degrees, 58.6 degrees) and (-65.0 degrees, 58.9 degrees) for the two independent nonhelical molecules in GAI2; in GAI1, both the C gamma atoms point away from the helix, whereas in GAI2 the C gamma atom with the g+ conformation points inward to the helix and causes sterical interaction with atoms in the adjacent peptide plane. From these results, it is clear that the helix-forming tendencies of amino acids correlate with the restrictions of side-chain rotamer conformations. Both the peptide units in GAI1 are trans and show significant deviation from planarity [omega 1 = -168(1) degrees; omega 2 = -171(1) degrees] whereas both the peptide units in both the molecules A and B in GAI2 do not show significant deviation from planarity [omega 1 = 179.3(3) degrees; omega 2 = -179.3(3) degrees for molecule A and omega 1 = 179.5(3) degrees; omega 2 = -179.4(3) degrees for molecule B], indicating that the peptide planes in these incipient alpha-helical peptides are considerably bent.

The effects of lithium isotopes on the myo-inositol 1-phosphatase reaction in rat brain, liver, and testes

Enzyme inhibition studies were performed with several lithium isotopes in order to more precisely define how lithium inhibits the enzyme myo-inositol 1-phosphatase. This lithium-induced inhibition is thought to be central to the therapeutic effects of lithium in the treatment of manic-depressive disorder. Naturally occurring lithium (NLi) exists as a combination of isotopes: 6Li and 7Li. Lethality studies were performed comparing 6LiCl, 7LiCl, and NLiCl, did not demonstrate a differential effect as previous studies had suggested. Enzyme inhibition studies were performed with these individual lithium isotopes, and compared to the effects of the naturally occurring combination (NLi) on the inhibition of myo-inositol 1-phosphatase using a partially purified enzyme preparation from rat brain, liver and testes. Identical inhibition was observed with all lithium isotopes and their combinations. In addition, both D- and L-myo-inositol 1-phosphates were used as enzyme substrates and found to be equivalent. These experiments, along with previous work demonstrating lithium acting as an uncompetitive inhibitor in the reaction, and the lack of lithium binding sites on the enzyme, suggests the hypothesis that lithium is possibly inhibiting this reaction by interfering with the formation of a transition cyclic intermediate, myo-inositol 1,3-cyclic phosphate, which may be formed from either the D- or L-substrates. This proposal is in contrast to previous suggestions regarding the inhibitory mechanism of action of lithium on the myo-inositol 1-phosphatase reaction.

Conformation and hydrogen bonding of N-formylpeptides: crystal and molecular structure of N-formyl-L-alanyl-L-aspartic acid

Crystals of N-formyl-L-alanyl-L-aspartic acid (C8H11N2O6) grown from aqueous methanol solution are orthorhombic, space group, P2(1)2(1)2(1) with cell parameters at 294K of a = 13.619(2), b = 8.567(2), c = 9.583(3)A, V = 1118.1A3, M.W. = 232.2, Z = 4, Dm = 1.38 g/cm3 and Dx = 1.378 g/cm3. The crystal structure was solved by the application of direct methods and refined to an R value of 0.075 for 1244 reflections with I greater than or equal to 3 sigma collected on a CAD-4 diffractometer. The structure contains two short intermolecular hydrogen bonds: (i) between the C-terminal carboxyl OH and the N-acyl oxygen (2.624(3)A), a characteristic feature found in many N-acyl peptides and (ii) between the aspartic carboxyl OH. and the peptide oxygen OP1 (2.623(3)A). The peptide is nonplanar (omega = 165.5(6) degrees). The molecule takes up a folded conformation in contrast to N-formyl peptides which form extended beta-sheets; the values of phi 1, psi 1, phi 2, psi 2(1), and psi 2(2) are, respectively -65.7(6), 152.0(5), -107.2(5), 30.9(5), and -150.3(6). The aspartic acid side chain conformation is g- with chi 1 = 73.1(5). The formyl group, as expected, is transplanar [OF-CF-N1-CA1 = -4.0(8) degrees]. The presence of the short O-H ... O hydrogen bond emerges as a structural feature common to this peptide and several other N-formyl peptides. There are no C-H ... O hydrogen bonds in this structure.

Structure and conformation of 5-bromo-2',3'-dideoxyuridine

C9H11BrN2O4, Mr = 291.11, monoclinic, P2(1), a = 11.307 (1), b = 5.954 (1), c = 15.829 (2) A, beta = 93.25 (1) degree, V = 1063.90 A3, Z = 4, Dx = 1.82 g cm-3, lambda(Cu K alpha) = 1.54184 A, mu = 53.58 cm-1, F(000) = 584, T = 295 K, R = 0.034 for 1927 observed reflections [I greater than 3 sigma(I)]. The crystal structure contains two independent molecules forming a dimer linked by a pair of N3--H...O2 hydrogen bonds; the crystal structure is stabilized by four additional hydrogen bonds. Two of these are internal C6--H6...O5' hydrogen bonds, one in molecule A and another in molecule B. These two molecules exhibit two different conformations; their sugar ring puckers are 2'-endo-3'-exo for molecule A and 3'-endo-2'-exo for molecule B. The Cl'--N1 distance, the chi CN torsion angle and the glycosidic conformation are 1.464 (8) A, -130.0 degrees and -anticlinal for molecule A and 1.506 (8) A, -168.9 degrees and -antiperiplanar for molecule B, respectively.

A sequence preference for nucleation of alpha-helix--crystal structure of Gly-L-Ala-L-Val and Gly-L-Ala-L-Leu: some comments on the geometry of leucine zippers

The synthetic peptide Gly-L-Ala-L-Val (C10H19N3O4.3H2O; GAV) crystallizes in the monoclinic space group P21, with a = 8.052(2), b = 6.032(2), c = 15.779(7) A, beta = 98.520(1) degree, V = 757.8 A3, Dx = 1.312 g cm-3, and Z = 2. The peptide Gly-L-Ala-L-Leu (C11H21N3O4.3H2O; GAL) crystallizes in the orthorhombic space group P212121, with a = 6.024(1), b = 8.171(1), c = 32.791(1) A, V = 1614 A3, Dx = 1.289 g cm-3, and Z = 4. Their crystal structures were solved by direct methods using the program SHELXS-86, and refined to an R index of 0.05 for 1489 reflections for GAV and to an R index of 0.05 for 1563 reflections for GAL. The tripeptides exist as a zwitterion in the crystal and assume a near alpha-helical backbone conformation with the following torsion angles: psi 1 = -150.7 degrees; phi 2, psi 2 = -68.7 degrees, -38.1 degrees; phi 3, psi 32 = -74.8 degrees, -44.9 degrees, 135.9 degrees for GAV; psi 1 = -150.3 degrees; phi 2, psi 2 = -67.7 degrees, -38.9 degrees; phi 3, psi 31, psi 32 = -72.2 degrees, -45.3 degrees, 137.5 degrees for GAL. Both the peptide units in both of the tripeptides show significant deviation from planarity [omega 1 = -171.3(6) degrees and omega 2 = -172.0(6) degrees for GAV; omega 1 = -171.9(5) degrees and omega 2 = -173.2(6) degrees for GAL]. The side-chain conformational angles chi 21 and chi 22 are -61.7(5) degrees and 175.7(5) degrees, respectively, for valine, and the side-chain conformations chi 12 and chi 23's are -68.5(5) degrees and (-78.4(6) degrees, 159.10(5) degrees) respectively, for leucine. Each of the tripeptide molecule is held in a near helical conformation by a water molecule that bridges the NH3+ and COO- groups, and acts as the fourth residue needed to complete the turn by forming two hydrogen bonds. Two other water molecules form intermolecular hydrogen bonds in stabilizing the helical structure so that the end result is a column of molecules that looks like an alpha-helix.

Structure of N-methylnicotinamide

C7H8N2O, Mr = 136.2, monoclinic, P21/a, a = 7.055 (1), b = 9.849 (6), c = 10.066 (4) A, beta = 100.47 (2) degrees, V = 687.5 (5) A3, Z = 4, Dm = 1.32, Dx = 1.315 g cm-3, Cu K alpha, lambda = 1.5418 A, mu = 7.09 cm-1, F(000) = 288, T = 294 K, R = 0.048 for 1134 reflections [I greater than 3 sigma(I)]. The N-methylcarboxamide group is extended with the keto O(7) transoid to C(2) [C(2)--C(3)--C(7)--O(7) +/- 158.9 (3), C(3)--C(7)--N(7)--C(8) +/- 177.1 (3), C(2)--C(3)--C(7)--N(7) +/- 23.2 (3) and C(4)--C(3)--C(7)--N(7) +/- 158.2 (3) degrees]. The dihedral angle between the planes of the pyridine ring and the carboxamide plane is 22 degrees. The molecules are linked together by N--H...O hydrogen bonds involving the amino N(7) and the carbonyl O(7) atoms.

Design of crystalline helices of short oligopeptides as a possible model for nucleation of alpha-helix: role of water molecules in stabilizing helices

We have designed, synthesized, crystallized, and performed x-ray analysis of several hydrophobic tripeptides that show an extended near alpha-helical structure in the crystalline state. All of the tripeptides that show this remarkably stable helix crystallize with two or three water molecules; they all have glycine at the N terminus and have increasing hydrophobicity as one moves from the N to C terminus. Even though three residues in the oligomer are not sufficient to complete a turn, one of the water molecules acts as an added residue and links up adjacent tripeptide segments along the helix axis so that in the crystal, the helix appears effectively as one long continuous helix. Two of these tripeptides are stabilized by two water molecules that enable the peptides to complete a turn of the helix and extend the helical structure throughout the crystal by linking translationally related peptides by hydrogen bonds. In two other peptides, these roles are played by three rather than two water molecules. Though these tripeptides have different crystal symmetry, they all show the basic pattern of hydrated helix and packing, indicating the strong conformational preference for a stable structure even for these tripeptides. Such conformationally stable hydrated structures for short specific related sequences illustrate their possible importance in nucleating protein folding and in the role water molecules play in such events.