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Biomedical subjects

S Scheiner

Publications and source records attributed to S Scheiner.

At least 19 recordsLinked to original sources
PubMed2001

Strength of the Calpha H..O hydrogen bond of amino acid residues.

Although the peptide C(alpha)H group has historically not been thought to form hydrogen bonds within proteins, ab initio quantum calculations show it to be a potent proton donor. Its binding energy to a water molecule lies in the range between 1.9 and 2.5 kcal/mol for nonpolar and polar amino acids; the hydrogen bond (H-bond) involving the charged lysine residue is even stronger than a conventional OH..O interaction. The preferred H-bond lengths are quite uniform, about 3.32 A. Formation of each interaction results in a downfield shift of the bridging hydrogen's chemical shift and a blue shift in the C(alpha)H stretching frequency, potential diagnostics of the presence of such an H-bond within a protein.

Amino Acids↗
PubMed2000

Calculation of isotope effects from first principles.

Various means of calculating the effect of changing the mass of a given atom upon a chemical process are reviewed. Of particular interest is the deuterium isotope effect comparing the normal protium nucleus with its heavier deuterium congener. The replacement of the bridging protium in a neutral hydrogen bond such as the water dimer by a deuterium strengthens the interaction by a small amount via effects upon the vibrational energy. In an ionic H-bond such as the protonated water dimer, on the other hand, the reverse trend is observed in that replacement of the bridging protium by dimer weakens the interaction. In addition to the stability of a given complex, the rate at which a proton transfers from one group to another is likewise affected by deuterium substitution, viz. kinetic isotope effects (KIEs). The KIE is enlarged as the temperature drops, particularly so if the calculation of KIE includes proton tunneling. The KIE is also sensitive to any angular distortions or stretches present in the H-bond of interest. KIEs can be computed either by the standard transition state theory which is derived via only two points on the potential energy surface, or by more complete formalisms which take account of larger swaths of the surface. While more time intensive, the latter can also be applied to provide insights important in interpretation of experimental data.

Deuterium↗
PubMed2000

[Standards, Options and Recommendations (SOR) for the use of appetite stimulants in oncology. Work group. Federation of the French Cancer Centres (FNCLCC)].

CONTEXT: The "Standards, Options and Recommendations" (SOR) project, started in 1993, is a collaboration between the Federation of the French Cancer Centres (FNCLCC), the 20 French Cancer Centres and specialists from French Public Universities, General Hospitals and Private Clinics. The main objective is the development of clinical practice guidelines to improve the quality of health care and outcome for cancer patients. The methodology is based on literature review and critical appraisal by a multidisciplinary group of experts, with feedback from specialists in cancer care delivery. OBJECTIVES: To develop clinical practice guidelines according to the definitions of the Standards, Options and Recommendations project for the use of appetite stimulants in oncology. METHODS: Data were identified by searching Medline and the personal reference lists of members of the expert groups. Once the guidelines were defined, the document was submitted for review to 55 independent reviewers, and to the medical committees of the 20 French Cancer Centres. RESULTS: The main recommendations for the use of appetite stimulants in oncology are: 1) Corticosteroids and the synthetic progestogens (megestrol acetate and medroxyprogesterone acetate) are appetite stimulants. 2) They can be useful in managing anorexia and weight loss in cancer patients, especially in the palliative setting, despite the potential side-effects of these agents. 3) The most effective way of using these drugs is not known. Inclusion in clinical trials is recommended. 4) Cyproheptadine, metoclopramide, nandrolone and pentoxif line should not be used outside prospective clinical trials. 5) Hydrazine sulfate should not be used.

Adrenal Cortex Hormones↗
PubMed1993

The effect of terodiline on hyperreflexia (in vivo) and the in vitro response of isolated strips of rabbit bladder to field stimulation, bethanechol and KCL.

Hyperreflexia, a condition characterized by contractions of the urinary bladder, is not mediated by a micturition reflex. The contractions can be of neurogenic origin through spinal or supraspinal reflexes or of myogenic origin, independent of neuronal mediation. There is a clear relationship between hyperreflexia and symptoms such as urgency, frequency, nocturia and urinary incontinence. Therapies to reduce the presence of uninhibited bladder contractions include oral drug therapy, instillation drug therapy and parasympathetic nerve ablation. The current study characterizes the ability of terodiline to inhibit an experimental form of hyperreflexia and compares the efficacy and potency of terodiline and other agents on hyperreflexia and evoked contractions. The results can be summarized as follows: (1) terodiline inhibits the amplitude of the hyperreflexia at lower concentrations than it inhibits the frequency of hyperreflexia; (2) terodiline had no statistically significant effect on mean blood pressure at any concentration utilized; (3) terodiline had approximately the same potency for inhibition of 2 and 32 Hz stimulation for both the bladder body and base; (4) terodiline inhibited the maximum contractile response to bethanechol and also shifted the curve to the right, demonstrating that terodiline is a mixed inhibitor, and (5) terodiline was a noncompetitive inhibitor of KCl.

Animals↗
PubMed1991

Ab initio investigation of interactions between models of membrane-active compounds and polar groups of membranes: complexes involving amine, ether, amide, phosphate, and carboxylate.

Ab initio (MINI-1) molecular orbital calculations were performed on model systems to investigate the hydrogen bonds and proton transfer between antiarrhythmics and polar groups of the cell membrane. Methylamine cation, dimethyl ether, and N-methylacetamide served as models of associative sites for the antiarrhythmics mexiletine and tocainide. Formate and phosphate anions, the methylamine cation, and formamide were chosen as models for the membrane polar groups. Protonated methylamine forms a very strong complex with the formate and phosphate anions. However, the formate COO- group is a better proton acceptor than the phosphate PO4- group. The effect of specific hydration on the proton potential functions was investigated in the HCOO- ... +HNH2CH3 and H2PO4- ... +HNH2CH3 systems. The proton potential functions, calculated at the equilibrium distances RO ... N, with a single minimum were found. The ab initio calculations at the longer RO ... N = 0.275 nm distance indicate double-minimum potentials. The increasing hydration stabilizes a second minimum corresponding to the charged O- ... +HN structures. The complexes involving the amide and ether groups of tocainide and mexiletine and the protonated primary amine group of the membrane are considerably weaker. The weakest hydrogen bonds are formed by the amine group of the drug (in its neutral and ionized state) with the peptide group.

Amides↗
PubMed1991

Effect of intermolecular orientation upon proton transfer within a polarizable medium.

Ab initio calculations are used to investigate the proton transfer process in bacteriorhodopsin. HN = CH2 serves as a small prototype of the Schiff base while HCOO- models its carboxylate-containing counterion and HO- the hydroxyl group of water of tyrosine, leading to the HCOO-..H+..NHCH2 and HO-..H+..NHCH2 complexes. In isolation, both complexes prefer a neutral pair configuration wherein the central proton is associated with the anion. However, the Schiff base may be protonated in the former complex, producing the HCOO-..+HNHCH2 ion pair, when there is a high degree of dielectric coupling with an external polarizable medium. Within a range of intermediate level coupling, the equilibrium position of the proton (on either the carboxylate or Schiff base) can be switched by suitable changes in the intermolecular angle. pK shift resulting from a 60 degrees reorientation are calculated to be some 5-12 pK U within the coupling range where proton transfers are possible. The energy barrier to proton transfer reinforces the ability of changes in angle and dielectric coupling to induce a proton transfer.

Bacteriorhodopsins↗
PubMed1988

Ab initio investigation of interactions between models of local anesthetics and receptor: complexes involving amine, phosphate, amide, Na+, K+, Ca2+, and Cl-.

Ab initio molecular orbital methods are used to study the interactions between models of local anesthetic molecules and the putative receptors within the nerve membrane: phospholipids and lipoproteins. The tertiary amine terminus of local anesthetics was modeled by ionized and un-ionized trimethylamine, while phosphate monoanion and formamide emulated the appropriate portions of the receptor. The protonated amine forms a very strong complex with the phosphate anion in which the charge is transferred to the phosphate. While somewhat weaker than this, the complex involving the amine (in either its ionized or un-ionized state) and peptide is considerably stronger than interpeptide H bonds, suggesting the anesthetic can disrupt the normal H-bond patterns in a protein. On the other hand, such interactions must compete with the rather tight binding of the anesthetic with the Na+, K+, Ca2+ and Cl- ions present in vivo.

Amides↗
PubMed1985

Modification of pK values caused by change in H-bond geometry.

The competition between various groups for a proton is studied by ab initio molecular orbital methods. It is found that reorientations of the two groups involved in a H-bond can reverse the equilibrium position of the proton shared between them. Specifically, the carbonyl and hydroxyl groups were modeled by H2CO and HOH. In the H-bond between these two groups, association of the proton with the carbonyl (H2COH...OH2)+ is favored over the hydroxyl (H2CO...HOH2)+ when the latter group is situated along a lone pair of the carbonyl oxygen. However, displacement of the water to the C = O axis between the two carbonyl lone pairs reverses the situation and (H2CO...HOH2)+ is more stable. A similar reversal of stability is observed in the H-bond involving a Schiff base (modeled by CH2NH) and amine (NH3). In one arrangement where the lone pairs of the two groups point toward one another, the proton prefers the Schiff base to the amine--i.e., (H2CHNH...NH3)+ is more stable than (H2CHN...HNH3)+. On the other hand, rotation of the lone pair of the amine away from the Schiff base nitrogen results in proton transfer across to the amine. These shifts in stability correspond to reversal of relative pK of the groups involved. A fundamental principle emerging from the calculations is that ion-dipole electrostatic interactions favor transfer of a proton to the group that is positioned as closely as possible to the negative end of the dipole moment vector of the other. The ideas developed here suggest a number of means by which conformational changes may be utilized to shift protons from residue to residue within a protein molecule such as an enzyme or bacteriorhodopsin.

Amines↗
PubMed1984

Angular dependence of the interaction energy between the N lone pair of amines and a proton: relevance to drug-receptor systems.

Ab initio molecular orbital calculations with a 4-31G basis set have been performed to study the angular dependence of the interaction energy between a lone electron pair of nitrogen and a proton. In this study ammonia and trimethylamine were used as models of biologically active amines. A proton was used as a model of an electrophilic site at the receptor. Results obtained confirm previous indications that the energy required to bend the proton from the lone pair direction decreases markedly as the two species are further separated from one another. Implications regarding the interactions of drugs and hormones at specific receptors are discussed.

Amines↗
PubMed1984

New insights in the clastic binding hypothesis for opiate-receptor interactions II: Proton-transfer mechanism.

Ab initio (4-31G) molecular orbital calculations were performed on model systems to investigate the proton-transfer version of the clastic binding hypothesis for opiate-receptor interactions. Ammonia was chosen as the model for the nitrogen-containing portion of the opiate molecule, while ammonia and water were chosen as models for the proton acceptor at the receptor. The equilibrium position of a proton situated between the two molecules is found to be determined primarily by the orientation of the proton-donor molecule with some influence also from the other molecule. Misalignments of the lone pairs can significantly alter equilibrium populations when the proton affinities of the two molecules are similar.

Chemical Phenomena↗
PubMed1981

Quantum chemical studies of proton transport through biomembranes.

A proposed mechanism of proton transport through biological membranes was investigated with the aid of ab initio molecular orbital methods. It requires the existence within a transmembrane protein of a hydrogen-bonded chain of residues. The full transport process was broken down into a number of simpler steps, each of which involves the transfer of a proton from one residue to the next along the chain. The hydroxyl-containing residues were modeled by water molecules. Linear hydrogen-bonded systems H+ (H2O)n, n = 2, 3, 4, 5 were studied using the 4-31G basis set. The dimer was shown to furnish an excellent model for study of proton transfer in larger systems. Deformations of the hydrogen bond lead to large increases in the energy barrier to proton transfer. Certain properties of the electron density distribution show a close correspondence with transfer energetics; consequently, they may have some predictive use. The transition state to double proton transfer in the trimer and pentamer involve partial transfer of only one proton. Implications of the above observations upon the proposed mechanism were discussed.

Biological Transport↗
PubMed1980

Influence of intramolecular hydrogen bonding on the electronic structure of oxymorphone.

Approximate ab initio molecular orbital methods are used to examine the structural and electronic properties of oxymorphone. The most stable conformation of the molecule is found to include an intramolecular hydrogen bond between the C-14 hydroxyl group and the nitrogen atom in agreement with available experimental data. The total molecular electron density is transformed to a set of localized molecular orbitals, one of which corresponds to the lone electron pair on nitrogen. The hydrogen bond is shown to produce substantial bending and stretching of the lone pair when compared to its shape when such hydrogen bonding is precluded.

Journal Article↗
PubMed1978

Energies of polypeptides: theoretical conformational study of polyglycine using quantum mechanical partitioning.

A method is developed for evaluating the total energy of polypeptides based on a combination of quantum mechanical and empirical potentials. Adjacent and nonadjacent peptide units are allowed to interact through these respective means. Our hybrid procedure is applied to a study of polyglycine and compared to the results obtained by the method of Scheraga and coworkers. We find the alpha helical conformation of a single strand of polyglycine to be most stable in vacuo. Other less-stable configurations include the 3(10) helix, the 2(7) ribbon structure, and the fully extended conformation.

Glycine↗