Biomedical subjects

Monica C Concha

Publications and source records attributed to Monica C Concha.

7 recordsLinked to original sources

PubMed2006

An overview of halogen bonding.

Halogen bonding (XB) is a type of noncovalent interaction between a halogen atom X in one molecule and a negative site in another. X can be chlorine, bromine or iodine. The strength of the interaction increases in the order Cl<Br<I. After a brief review of experimental evidence relating to halogen bonding, we present an explanation for its occurrence in terms of a region of positive electrostatic potential that is present on the outermost portions of some covalently-bonded halogen atoms. The existence and magnitude of this positive region, which we call the sigma-hole, depends upon the relative electron-attracting powers of X and the remainder of its molecule, as well as the degree of sp hybridization of the s unshared electrons of X. The high electronegativity of fluorine and its tendency to undergo significant sp hybridization account for its failure to halogen bond. Some computed XB interaction energies are presented and discussed. Mention is also made of the importance of halogen bonding in biological systems and processes, and in crystal engineering.

Halogens↗

PubMed2006

Molecular surface electrostatic potentials and anesthetic activity.

General anesthetics apparently act through weak, noncovalent and reversible interactions with certain sites in appropriate brain proteins. As a means of gaining insight into the factors underlying anesthetic potency, we have analyzed the computed electrostatic potentials V (S)(r) on the surfaces of 20 molecules with activities that vary between zero and high. Our results are fully consistent with, and help to interpret, what has been observed experimentally. We find that an intermediate level of internal charge separation is required; this is measured by Pi, the average absolute deviation of V (S)(r), and the approximate window is 7 < Pi < 13 kcal mol(-1). This fits in well with the fact that anesthetics need to be lipid soluble, but also to have some degree of hydrophilicity. We further show that polyhalogenated alkanes and ethers, which include the most powerful known anesthetics, have strong positive potentials, V (S,max), associated with their hydrogens, chlorines and bromines (but not fluorines). These positive sites may impede the functioning of key brain proteins, for example by disrupting their normal hydrogen-bond patterns. It has indeed been recognized for some time that the most active polyhalogenated alkanes and ethers contain hydrogens usually in combination with chlorines and/or bromines.

Anesthetics↗

PubMed2006

Analysis of the reaction force for a gas phase S(N)2 process: CH3Cl + H2O --> CH3OH + HCl.

The "reaction force" F(R(c)) is the negative derivative of a system's potential energy V(R(c)) along the intrinsic reaction coordinate of a process. If V(R(c)) goes through a maximum, as is commonly the case, then F(R(c)) has a characteristic profile: a negative minimum followed by zero at the transition state and then a positive maximum. These features reflect four phases of the reaction: an initial one of reactant preparation, followed by two of transition to products, and then relaxation of the latter. In this study, we have analyzed, in these terms, a gas-phase S(N)2 substitution, selected to be CH3Cl + H2O --> CH3OH + HCl. We examine, at the B3LYP/6-31G level, the geometries, energetics, and molecular surface electrostatic potentials, local ionization energies, and internal charge separation.

Journal Article↗

PubMed2005

An unusual feature of end-substituted model carbon (6,0) nanotubes.

We have examined the effects of substituents on the computed electrostatic potentials V(S)(r) and average local ionization energies I(S)(r) on the surfaces of model carbon nanotubes of the types (5,5), (6,1) and (6,0). For the (5,5) and the (6,1), the effects upon both V(S)(r) and I(S)(r) of substituting a hydroxyl group at one end are primarily localized to that part of the system. For the (6,0) tube, however, a remarkable change is observed over its entire length, with V(S)(r) showing a marked gradation from strongly positive at the substituted end to strongly negative at the other; I(S)(r) correspondingly goes from higher to lower values. Replacing OH by another resonance- donor, NH2, produces similar results in the (6,0) system, while the resonance withdrawing NO2 does the opposite, but in equally striking fashion. We explain these observations by noting that the arrangement of the C-C bonds in the (6,0) tube facilitates charge delocalization over the full length and entire surface of the tube. Substituting NH2 and NO2 at opposite ends of the (6,0) tube greatly strengthens the gradations in both V(S)(r) and I(S)(r). The first hyperpolarizability of this system was found to be nine times that of para-nitroaniline, suggesting possible nonlinear optical applications. [figure: see text]. HF/STO-5G electrostatic potential on outer surface of open (6,0) C72H10NH2NO2. The nitro group is at the right end of the tube, the amino group at the left. In eV: purple is less than 14, blue is between 14 and 15, green is between 15 and 16.5, yellow is between 16.5 and 17.5, and red is more than 17.5.

Carbon↗

PubMed2004

Comparative analysis of surface electrostatic potentials of carbon, boron/nitrogen and carbon/boron/nitrogen model nanotubes.

We have extended an earlier study, in which we characterized in detail the electrostatic potentials on the inner and outer surfaces of a group of carbon and B(x)N(x) model nanotubes, to include several additional ones with smaller diameters plus a new category, C(2x)B(x)N(x). The statistical features of the surface potentials are presented and analyzed for a total of 19 tubes as well as fullerene and a small model graphene. The potentials on the surfaces of the carbon systems are relatively weak and rather bland; they are much stronger and more variable for the B(x)N(x) and C(2x)B(x)N(x). A qualitative correlation with free energies of solvation indicates that the latter two categories should have considerably greater water solubilities. The inner surfaces are generally more positive than the corresponding outer ones, while both positive and negative potentials are strengthened by increasing curvature. The outsides of B(x)N(x) tubes have characteristic patterns of alternating positive and negative regions, while the insides are strongly positive. In the closed C(2x)B(x)N(x) systems, half of the C-C bonds are double-bond-like and have negative potentials above them; the adjacent rows of boron and nitrogens show the usual B(x)N(x) pattern. When the C(2x)B(x)N(x) tubes are open, with hydrogens at the ends, the surface potentials are dominated by the B+-H- and N(-)-H+ linkages.

Boron↗

PubMed2004

Analysis of two intramolecular proton transfer processes in terms of the reaction force.

The negative derivative of the potential energy along an intrinsic reaction coordinate defines a force that has qualitatively a universal form for any process having an energy barrier: it passes through a negative minimum before the transition state, at which it is zero, followed by a positive maximum. We have analyzed two intramolecular proton transfer reactions in terms of several computed properties: internal charge separation, the electrostatic potentials of the atoms involved, their Fukui functions, and the local ionization energies. The variation of each of these properties along the intrinsic reaction coordinate shows a marked correlation with the characteristic features of the reaction force. We present a description of the proton transfer processes in terms of this force.

Journal Article↗

PubMed2002

Evaluation of the role of multiple hydrogen bonding in offering stability to negative ion adducts in electrospray mass spectrometry.

An experimental approach, electrospray mass spectrometry (ES-MS), and a theoretical approach employing computer modeling, have been used to characterize the interaction between small inorganic anions and neutral analyte molecules that form anionic adduct species in negative mode ES mass spectrometry. Certain anionic adducts of small saccharides (e.g., alpha-D-glucose, sucrose) have shown exceptional stability in ES mass spectra even when internal energies are raised at high "cone" voltages. Computer modeling studies reveal that multiple hydrogen bonding strengthens the interaction between these neutral molecules and the attaching anion. The equilibrium structures and stabilization energies of these anionic adducts have been evaluated by semi-empirical, ab initio, and density functional theory (DFT) methods. Chloride anion is found to be capable of forming "bridging" hydrogen bonds between monosaccharide rings of polysaccharides resulting in the stabilization of chloride adducts, thus reducing the tendency for the glycosidic bond to decompose. Moreover, the tendency for various hydroxyl hydrogens on saccharide molecules to dissociate in the form of HA (A-, anion) during decomposition of anionic adducts, thereby forming [M - H]-, has also been evaluated by computer modeling.

Journal Article↗