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PubMed1991

SELEXION. Systematic evolution of ligands by exponential enrichment with integrated optimization by non-linear analysis.

Recently, novel technologies for isolation of nucleic acid molecules with specific biological activities have been reported. In each case, the enrichment process involves repeated rounds of selection from complex mixtures of nucleic acid sequences, followed by polymerase chain reaction (PCR) amplification of ligand sequences that function in the desired manner. Particular variations in experimental conditions can dramatically alter the outcome of these processes. In this study, we use mathematical analysis and computer simulation to predict which variations have the greatest impact and to develop strategies and guidelines for enhanced effectiveness. First, we perform reconstruction tests to demonstrate that a mathematical description based on equilibrium binding is sufficient to explain the high levels of enrichment attained in the laboratory after just a few rounds. Then, we show the expected enrichment for an extensive range of conditions; and, finally, we determine the optimum protein and nucleic acid concentrations to use for maximum enrichment, while also ensuring a high likelihood of recovering even the rare molecule that binds well. The strategies and guidelines for enhanced effectiveness are generally applicable to processes for systematic enrichment of DNA, RNA or peptide ligands and have been implemented in an interactive simulation program for integrated non-linear optimization of enrichment using any target of interest.

Biological Evolution↗
PubMed1994

Water diffusivity in porcine stratum corneum measured by a thermal gravimetric analysis technique.

Water is a natural constituent of the stratum corneum (sc) affecting its plasticity and modulating its barrier function. Diffusion coefficients (D) were calculated by measuring the desorption rates of water from porcine sc and dermis samples by a thermal gravimetric analysis (TGA) technique at isothermal conditions in the range 30-80 degrees C. Water-loaded samples were exposed to a flow of dry air, and the change of weight and of temperature were monitored with time. Distinct abrupt decreases in rate of desorption marked three different phases of water in the sc, designated as free, bulk, and bound water. Concomitant with the sharp change in evaporation rate, an increase in temperature was observed, in accordance with the absorption of heat accompanied with the water desorption process. Desorption curves were plotted against time and optimized. Values of D were estimated from the ratio of the evaporated water to the initial sc water content, as a function of the square root of time. The "initial slope" and the t1/2 (time for which Mt/Mo = 0.5, where Mo is the equilibrium amount of water absorbed in the membrane and Mt is the amount of water released by the membrane in a time t) methods gave similar results. The water D values of sc at 30 degrees C calculated by the two methods were 3.3 +/- 0.6 x 10(-10) and 2.7 +/- 0.8 x 10(-10) cm2/s, respectively. These values were about two orders of magnitude lower than the calculated D value for water in the dermis.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMedDate not supplied

Rapid assessment of ventilation by measurement of carbon dioxide elimination during high-frequency ventilation of kittens.

Monitoring of the effectiveness of ventilation is a significant problem during high-frequency ventilation (HFV). The time necessary to achieve equilibrium of the arterial tension of carbon dioxide (Paco2) following step changes in ventilation is appreciable, because of large body stores of CO2. Waiting for Paco2 to reach equilibrium is not only time-consuming but a potentially dangerous means of monitoring ventilator adjustments during HFV. Five kittens of mean +/- SD 1,082 +/- 383 gm weight were studied during HFV, both with normal lungs and lungs injured by saline lavage-induced surfactant depletion. The transcutaneous tension of carbon dioxide (Ptcco2) was monitored continuously to determine the time required to achieve equilibrium of Paco2 following a step change in ventilation. The rate of pulmonary CO2 elimination (VECO2) was measured immediately before and immediately after (less than 12 sec) step changes in ventilation and was used to predict the change in Paco2 achieved once equilibrium was reestablished. With normal lungs, equilibration time following step changes in ventilation was found to be approximately 20 minutes. After step decreases in ventilation of the injured lung, achieving equilibrium state took significantly longer, approximately 30 minutes. The Paco2 predicted was significantly related to the change in Paco2 achieved at equilibrium for both normal and injured lung studies. We concluded that direct monitoring of VECO2 during HFV may be a useful clinical monitoring technique, allowing rapid and accurate assessment of the efficiency of ventilation following step changes in ventilation and potentially assisting in optimizing ventilator settings.

Animals↗
PubMed1980

Purification and some properties of hydroxypyruvate isomerase of Bacillus fastidiosus.

Hydroxypyruvate isomerase of Bacillus fastidiosus is a novel enzyme (Braun, W. and Kaltwasser, H. (1979) Arch. Microbiol. 121, 129-134) which catalyzes the reversible conversion of tartronate semialdehyde into hydroxypyruvate. The enzyme was purified to homogeneity. The native molecule had a molecular weight of 265 000-280 000 and was composed of six subunits with a molecular weight of 45 000. The enzyme showed optimal activity at pH 6.6-7.4 and 57 degrees C. Hydroxypyruvate isomerase is stable on heating for 10 min at 67 degrees C. The enzyme appeared to be specific for tartronate semialdehyde and hydroxypyyruvate and no cofactors were involved in the reaction. The equilibrium constant K = [tartronate semialdehyde] divided by [hydroxypyruvate] was found to be 2.5 at pH 7.1, and 30 degrees C.

Aldose-Ketose Isomerases↗
PubMed1997

Retention profiles of some commercial pesticides, pyrethroid and acaricide residues and their application to tomato and parsley plants.

This work deals with the preconcentration of some water soluble pesticides, pyrethroids and acaricides by polyurethane foams. The retention profiles of the tested species were found quickly and reached equilibrium in a few min. Various parameters--e.g. pH, extraction media, shaking time, salt effect, temperature and sample volume--affecting the preconcentration of the tested species by the unloaded foams and tri-n-octylamine and tri-n-methylphosphate treated foams were optimized. The unloaded foams were employed in a column mode to study the quantitative retention and recovery of the tested species. The sorption efficiency and recovery of the compounds by the unloaded foam column were found to be up to 99.5% +/- 2.1. The height equivalent of a theoretical plate for the unloaded foam column was found to be in the range 1.9-2 +/- 0.2 mm. The sorption mechanisms of the tested compounds by the foams are discussed. Analysis of N, P, Na, K, Cu, Zn, Mn, Fe, humidity, wet and dry mass of tomato and parsley untreated and sprayed for different time intervals--i.e. 24, 72 and 120 h--with Chlorpyrifos, was carried out.

Benzilates↗
PubMed1996

Binding dynamics and electron transfer between plastocyanin and photosystem I.

The mechanism of the electron transfer from the soluble protein plastocyanin to the multiprotein complex of photosystem I from spinach has been studied in detail. The two kinetic components of P700+ reduction by plastocyanin after a laser flash, showing a constant half-life of 11 microseconds and a variable half-life of the second-order reaction, respectively, are used to monitor the electron transfer from bound and soluble plastocyanin. The effect of increasing concentration of reduced plastocyanin on both of these kinetic components and the competition by oxidized plastocyanin is used to estimate the individual dissociation constants of the complex between the proteins in each of its oxidized and reduced state. The dissociation constant of oxidized plastocyanin is about six times larger than that of 7 microM found for reduced plastocyanin and purified PSI. Consistent with this result the midpoint redox potential of plastocyanin bound to photosystem I either in equilibrium with soluble plastocyanin or after cross-linking to photosystem I is found to be 50-60 mV higher than that of soluble plastocyanin. It is concluded that the driving force of the intracomplex electron transfer is decreased in favor of an optimized turnover of photosystem I. Double-flash excitation shows that oxidized plastocyanin has to leave the complex after the electron transfer before a new reduced plastocyanin molecule can bind to photosystem I. This release of oxidized plastocyanin with a half-life of about 60 microseconds limits the turnover of photosystem I. All data are consistently described by a model including the formation of a complex at a single binding site of photosystem I. Differences in the rate and binding constants are discussed with respect to the structure and the electrostatic and hydrophobic interactions stabilizing the complex as well as their modification by the membrane environment in situ.

Algorithms↗
PubMed1996

Laser flash absorption spectroscopy study of flavodoxin reduction by photosystem I in Synechococcus sp. PCC 7002.

The photoreduction of flavodoxin by trimeric photosystem I, both from the cyanobacterium Synechococcus sp. PCC 7002, was investigated by flash absorption spectroscopy. After addition of flavodoxin in darkness, single flash experiments show that the transient signals change between individual flashes. This behavior is assigned to a progressive accumulation of flavodoxin semiquinone, which is relatively stable under most experimental conditions. Different conditions were devised in order to study the reduction of the oxidized and semiquinone forms of flavodoxin separately. Both processes were identified by their differential spectra measured between 460 and 630 nm. Detailed kinetic characteristics of flavodoxin reduction were obtained at pH 8.0 in the presence of salts. The kinetics of reduction of oxidized flavodoxin displays a single-exponential component. The rate of this component increases with the flavodoxin concentration up to an asymptotic value of about 600 s-1. The semiquinone form of flavodoxin being protonated, this rate corresponds to a rate-limiting reaction which could be either an electron transfer reaction or a protonation reaction. In contrast, the reduction of flavodoxin semiquinone is biphasic. A fast first-order phase with t 1/2 approximately 10 microseconds is interpreted as an electron transfer process within a preformed complex. A dissociation constant of 2.64 microM is calculated for this complex by assuming a simple binding equilibrium between photosystem I and flavodoxin semiquinone. The slower phase observed for semiquinone reduction is concentration dependent, and a second-order rate constant of 1.7 x 10(8) M-1 s-1 is calculated. For both one-electron reduction steps, different optimal salt concentrations are observed indicating slightly different interactions between photosystem I and flavodoxin in its oxidized and semiquinone states.

Cyanobacteria↗
PubMed2000

Leaf photosynthetic characteristics of beech (Fagus sylvatica) saplings during three years of exposure to elevated CO(2) concentration.

Beech (Fagus sylvatica L.) seedlings were cultivated from seeds sown in pots or directly in the ground in outdoor chambers that were transparent to solar radiation, and provided either ambient air or CO(2)-enriched air (ambient + 350 &mgr;mol mol(-1)). The rooting volume was high in all experiments. In the short-term experiment, potted plants were assigned to a factorial CO(2) x nutrient treatment (optimal nutrient supply and severe nutrient shortage) for 1 year. In the long-term experiment, plants were grown directly in the ground and received an optimal supply of water and nutrients in both CO(2) treatments for 3 years. Nutrient stress caused carboxylation capacity (V(m)) to decrease in the potted seedlings exposed to CO(2)-enriched air during their first growing season. In the long-term experiment with optimal nutrient supply, CO(2)-enriched air did not affect V(m), but caused an upward acclimation of maximum electron transport rate (J(m)). Consequently, there was a 14% increase in the J(m)/V(m) ratio, indicating nitrogen reallocation to maintain an equilibrium between RuBP consumption and RuBP regeneration. Both V(m) and J(m) decreased during the growing season in both CO(2) treatments. Although upward acclimation of J(m) was no longer apparent at the end of the third growing season, plants in CO(2)-enriched air maintained a higher J(m)/V(m) ratio than plants in ambient air, indicating that photosynthetic acclimation always occurred. Second flush leaves appeared during each growing season. When expressed on the basis of foliar nitrogen concentration, their photosynthetic characteristics (V(m) and J(m)) were enhanced compared with other leaves. Because the number of second flush leaves was also increased in the elevated CO(2) treatment, this response should be taken into account when modeling the effects of elevated CO(2) concentration on canopy photosynthesis. Stomatal conductance decreased in response to atmospheric CO(2) enrichment; however, the stomatal response to irradiance followed a single relationship based on two stomatal conductance models.

Journal Article↗
PubMed1986

The dependence on internal pH of Ca2+-fluxes across sarcoplasmic reticulum vesicular membranes.

The interdependence of the competition between Ca2+ and hydrogen ions for the internally located low-affinity Ca2+ binding sites of sarcoplasmic reticulum vesicles and the pH-dependent splitting rate of phosphoenzyme was investigated. Sarcoplasmic reticulum vesicles were preincubated at a selected pH and passive Ca2+ loading, active Ca2+ uptake at the same pH as well as active Ca2+ uptake at a distinct pH (pH-jump method) were observed. In addition, Cai-Cao exchange in the absence and presence of ADP and ATP-ADP exchange were measured. The overall ATP splitting rate was assayed with leaky vesicles in the presence of varied Ca2+ concentration and four different pH. All experiments were carried out at Ca2+ concentrations sufficient to saturate the externally located activating high-affinity binding sites at all pH and in the absence of affecting concentrations of monovalent cations. Active Ca2+ transport (particularly evident applying the pH-jump method) is facilitated at low intravesicular pH, reflecting the favoured Ca2+ release to the intravesicular space, in contrast to the reverse pH-dependence of passive Ca2+ accumulation and the initial rate of Cai-Cao exchange, both favoured by elevated internal Ca2+ binding capacity. The rates of ATP splitting, the continuing slow rate of Cai-Cao exchange, and the ATP-ADP exchange are optimal at an intermediate proton concentration, reflecting the influence of protons on partial reaction steps occurring later in the reaction cycle and the accelerated exchange of Ca2+ at the internal low-affinity sites as well as the establishment of a new pseudo equilibrium between the possible reaction intermediates. The pool of rapidly exchangeable Ca2+ is enlarged whereas the rate of slow exchange is unaltered or diminished (pH 7.8) by ADP.

Adenosine Diphosphate↗
PubMed1996

Pharmacokinetics of camptothecins administered orally.

The equilibrium between lactone and salt forms of camptothecin (CPT) and its derivatives including 9-nitrocamptothecin (9-NC) depends on pH, binding to albumin and other factors. Their antitumor activity is associated with the lactone form. Our goal was the development of dosing regimens optimal for chemotherapeutic activity of the drug. The effect of p.o., i.v. and i.m. administration on the tumor uptake of [3H]-CPT or [3H]-9-NC in tumor-bearing nude mice was studied by whole-body autoradiography. In all cases, [3H]-CPT or [3H]-9-NC accumulated mainly in the gastrointestinal tract. Comparatively lower levels of drug were detected in liver, kidney, tumor, and other sites. Consistently high tumor/blood ratios following oral administration of drug suggest this route as the most effective way of treatment. Within 4 h of i.s. administration of 2 mg/kg CPT or 1 mg/kg 9-NC to mice, lactone forms were 57-81% and 47-95% of total plasma drug levels, respectively. However in plasma of humans treated p.o. with varying doses of CPT or 9-NC, lactone forms were only a minor component of total drug levels. It is concluded that ratios of lactone/total drug are much higher in mice than in humans, which influence the therapeutic efficacies these drugs in the two species.

Administration, Oral↗
PubMed1994

Influence of some selected ions on system water activity and on ethanol vapour pressure and its inhibitory action on Saccharomyces cerevisiae.

The individual and interactive effects of some ionic species on the ethanol vapour pressure in equilibrium with the system, the water activity, and the fermentative performance of Saccharomyces cerevisiae were investigated. The concentrations of the ions Ca2+, Mg2+, and NH4+ were modulated according to a central composite design, within a range of values that can be individually considered optimal for S. cerevisiae. Although the temperature and ethanol and glucose concentrations were kept constant, the initial water activity and ethanol vapour pressure of the systems were remarkably affected by the interactions between the three ions studied. The data concerning the fermentative activity emphasize the role of the physical state of ethanol. In fact, when the ethanol concentration was kept constant, the highest fermentative performances of yeast were obtained when the ethanol vapour pressure, depending particularly on the Ca2+ and Mg2+ interaction, was the lowest in the matrix of values considered.

Ethanol↗
PubMed2002

Inhibition of the p38 MAP kinase pathway destabilizes smooth muscle length during physiological loading.

We tested the hypothesis that mechanical plasticity of airway smooth muscle may be mediated in part by the p38 mitogen-activated protein (MAP) kinase pathway. Bovine tracheal smooth muscle (TSM) strips were mounted in a muscle bath and set to their optimal length, where the active force was maximal (F(o)). Each strip was then contracted isotonically (at 0.32 F(o)) with ACh (maintained at 10(-4) M) and allowed to shorten for 180 min, by which time shortening was completed and the static equilibrium length was established. To simulate the action of breathing, we then superimposed on this steady distending force a sinusoidal force fluctuation with zero mean, at a frequency of 0.2 Hz, and measured incremental changes in muscle length. We found that TSM strips incubated in 10 microM SB-203580-HCl, an inhibitor of the p38 MAP kinase pathway, demonstrated a greater degree of fluctuation-driven lengthening than did control strips, and upon removal of the force fluctuations they remained at a greater length. We also found that the force fluctuations themselves activated the p38 MAP kinase pathway. These findings are consistent with the hypothesis that inhibition of the p38 MAP kinase pathway destabilizes muscle length during physiological loading.

Animals↗
PubMed2006

Rubitecan.

The only approved camptothecins for use in patients to date (topotecan and irinotecan) are delivered intravenously. Thus, an oral camptothecin analogue that would provide the convenience of oral delivery with the flexibility for a variety of prolonged treatment schedules would be advantageous. Rubitecan is an orally available camptothecin analogue that also has potential for delivery transdermally or by inhalation. Like all of the camptothecins, its antitumour activity is mediated through the inhibition of DNA topoisomerase I, which is involved in relaxing supercoiled DNA, which is important for the process of DNA replication and RNA transcription. Rubitecan exists in equilibrium as 9-nitro-camptothecin (9-NC) and 9-amino-camptothecin (9-AC), a metabolite that is thought to be active although it failed in clinical trials. Both 9-NC and 9-AC contain a lactone ring that is required for optimal activity with the carboxylic acid (open ring) forms being significantly less active or inactive. A more acidic environment favours the lactone ring structure, whereas neutral or basic conditions favour the conversion to the carboxylic acid form. In addition to issues of lactone ring stability at physiological pH (true for all of the camptothecin analogues), there is pharmacokinetic variability that has had to be dealt with during the development of rubitecan. Preclinically, rubitecan has shown activity against a broad spectrum of tumour types in in vitro and in vivo human tumour xenograft models. Frustratingly, the level of activity of an agent in preclinical models has not always translated into similar activity against human tumours in clinical trials. To date, with the exception of pancreatic and possibly ovarian cancer, rubitecan has had disappointing activity against a number of other solid tumours in relatively small Phase I/II trials; however, it has shown sufficient activity against pancreatic cancer, a malignancy that remains difficult to treat, to continue to be evaluated in clinical trials for this indication. Results of clinical trials in the next few years should determine whether rubitecan can find a role in cancer therapy.

Administration, Oral↗
PubMed2003

[Fast liquid chromatographic method to determine compounds binding to human serum albumin].

A fast gradient HPLC method (cycle time 15 min) has been developed to determine Human Serum Albumin (HSA) binding of discovery compounds using chemically bonded protein stationary phases. The 2- propanol concentration is increased from 0 to 25% in 3 minutes, thus allowing strongly bond compounds to elute within the 15 minutes. The HSA binding values were derived from the logarithmic values of the gradient retention times that were converted to logarithm of the equilibrium constants (logK HSA) using a calibration set of molecules. The logK HSA values for the calibration set of molecules have been derived from literature values of the % plasma protein binding. The method is fully automated and it has been used for lead optimization in more than 20 projects. The obtained HSA binding data on more than 4000 compounds were suitable to set up global and project specific quantitative structure binding relationships that helped compound design in the early drug discovery settings. The method has been validated using literature plasma protein binding data of 70 known drug molecules. It was shown that compound lipophilicity dominates the HSA binding, however compounds binding to a specific binding site show stronger HSA binding that can be expected from their lipophilicity values. The solvation equation approach has been used to characterize the non-specific lipophilic binding ability of HSA. Based on our approach we can identify compounds with specific and non-specific binding.

Automation↗
PubMed1991

Volition, deception, and the evolution of justice.

Criminal justice is inextricably associated with the attributive concept of volition. Although the voluntary-involuntary distinction is subjectively vivid, causal research shows its poles to be inseparable, i.e., the dichotomy is deceptive. Why a bulwark of civilization should be founded on paradox, may be clarified by examining the role of self-deception in man's evolutionary heritage. Natural selection for an optimal degree of self-deception probably occurred, both to facilitate deception of others and to foster human cooperation. This contributed to the evolution of psychiatric disorders, the voluntary-involuntary continuum, and large scale social systems. Society and its members reach an equilibrium within the truth-deception continuum, manifest in individuals by conscious versus unconscious and voluntary versus involuntary, and in society by tension between what actually occurs (realism) and its organizing ideals (idealism). Three legal models of criminal justice are understood in this context: The (1) utilitarian, most realistic, is essential to social survival but vulnerable to abuse; (2) rehabilitative, at an opposite idealistic pole, better supports the image of social beneficence that helps to bind society's members; (3) retributive, most heavily grounded in volition, puts greater emphasis on individual autonomy, and reciprocally modulates the other models. All are legitimized by evolutionary traditions that antedate homo sapiens, and none is sufficient in itself. Elements of all three models necessarily coexist within any existing society, their relative strength varying with its collective values, prosperity, and perceived safety.

Criminal Law↗
PubMed1975

Reversible autophosphorylation of a cyclic 3':5'-AMP-dependent protein kinase from bovine cardiac muscle.

Purified cyclic adenosine 3':5'-monophosphate (cAMP)-dependent protein kinase of bovine cardiac muscles catalyzes the incorporation of 2 mol of 32P from [gamma-32P]ATP to seryl residues in its cAMP-binding protein. The reaction appears to be catalyzed by the protein kinase itself rather than by a protein kinase kinase and is enhanced by cAMP and by the addition of polyarginine. Phosphorylation of the purified enzyme facilitates its dissociation by cAMP (Erlichman, J., Rosenfeld, R., and Rosen, O.M. (1974) J. Biol. Chem. 249, 5000-5003) but does not affect cAMP binding. At equilibrium, 2 mol of cAMP are bound to both the phospho- and dephospho-enzymes. Phosphorylation of protein kinase is reversible. Upon addition of ADP and Mg2+, phosphate is transferred from the protein to ADP, and ATP is formed. The reverse reaction is optimal at pH 5.5 unlike the forward reaction which has a broad, more alkaline pH activity optimum. It is activated by polyarginine and dependent upon the addition of cAMP to a much greater degree than the forward reaction. The data suggest that the catalytic subunit of protein kinase catalyzes the forward and reverse reactions but do not exclude the possibility that the holoenzyme may also be active. Autophosphorylation by protein kinase and dephosphorylation by phosphrprotein phosphatases of by reverals of the autophosphorylation reaction may regulate the sensitivity of certain protein kinases to activation by cAMP in vivo.

Adenosine Diphosphate↗
PubMed1995

[Contribution of a mathematical model in the control of a parasitosis: the case of human African trypanosomiasis due to Trypanosoma brucei gambiense].

Trypanosoma brucei gambiense sleeping sickness transmitted by tsetse flies (Glossina spp.) is lethal if not treated adequately. The endemicity was generally well under control in the sixties. However, since the seventies the disease is returning in most of its old foci, with alarming endemic levels in several areas. Mathematical modelling provides a rational basis for finding the optimal strategies to control these recrudescences. We present a deterministic model of the basic transmission of trypanosomiasis between human and vector hosts in natural situations. The parameters were quantified on the basis of available evidence from the literature. The model predicts a stable equilibrium state with very high prevalences: approximately 95% of humans and 27% of flies being infected. The model further shows that the build-up of an epidemic is initially very slow, and it takes several months before the equilibrium state is reached. Consequently communities have enough time to avoid catastrophic situations by migrating to safer areas. If is therefore unlikely that such high equilibrium situations will occur in practice. The expression of the basic reproductive rate R0, the number of new infections during the lifetime of an infected subject with high values of R0 implies that efforts to diminish transmission to levels where the disease cannot maintain itself in the population, have to be substantial. The necessary reduction of fly numbers in order to enable eradication, has been calculated. In almost all situations a reduction of at least 90% is necessary, which is in accordance with the field experiences of vector control programmes. The present model can be considered as a starting point in the further development of a complete simulation model, which could be applied in supporting decision making in trypanosomiasis control.

Animals↗
PubMed1989

Salt or ion bridges in biological systems: a study employing quantum and molecular mechanics.

Equilibrium geometries and binding energies of model "salt" or "ion" bridge systems have been computed by ab initio quantum chemistry techniques (GAUSSIAN82) and by empirical force field techniques (AMBER2.0). Formate and dimethyl phosphate served as anions in the model compounds while interacting with several organic cations, including methyl ammonium, methyl guanidinium, and divalent metal ion (either Mg2+ or Ca2+) without and with an additional chloride; and a divalent metal ion (either Mg2+ or Ca2+), chloride, and four water molecules of hydration about the metal ion. The majority of the quantum chemical computations were performed using a split-valence basis set. For the model compounds studied we find that the ab initio optimized geometries are in remarkably good agreement with the molecular mechanics geometries. Several calculations were also performed using diffuse fractions. The formate anion binds these model cations more strongly than does dimethyl phosphate, while the organic cation methyl ammonium binds model anions more strongly than does methyl guanidinium. Finally, in model compounds including organic anions, Mg2+ or Ca2+ and four molecules of water, and a chloride anion, we find that the equilibrium structure of the magnesium complex involves a solvent separated ion pair (the magnesium ion is six coordinate), whereas the calcium ion complex remains seven coordinate. Molecular mechanics overestimates binding energies, but the estimates may be close enough to actual binding energies to give useful insight into the details of salt bridges in biological systems.

Chemical Phenomena↗