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MBB222-lecture3 - Ionic Interactions the result of attractions between positively charged cations and negatively charged anions e.g NaCl Na ClNaCl

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Ionic Interactions: – the result of attractions between positively charged cations and negatively charged anions g., NaCl a + Cl - e.g., NaCl Na + Cl – in aqueous solutions, simple ions of biological significance (Na + , K + , Ca ++ , Mg ++ , Cl - ) do not exist as free, isolated entities, but rather in hydrated forms: surrounded by a stable shell of water molecules held in place by ionic interactions between the central ion d the oppositely charged end of the and the oppositely charged end of the water dipole
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• this also explains why most ionic compounds dissolve readily in water: the energy released when ions bind water olecules ( ergy of hydration is greater than the lattice molecules ( energy of hydration ) is greater than the lattice energy that stabilizes the crystal structure of the ions in the form of a salt •t h i s water of hydration shell must be lost (at least in part) for direct ion interaction with cellular proteins nic interactions are sensitive to salt concentrations in ionic interactions are sensitive to salt concentrations in their environment – increasing the concentration of salts like NaCl in a solution of biomolecules can weaken and/or disrupt the ionic interactions holding the biomolecules together
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Van der Waals Interactions: •a w e a k, nonspecific attractive force created when any two atoms approach each other closely (due to the transient, random unequal distribution of electrons around any atom) • closeness is critical here; yet, if the atoms get too close together, the negative charges of their electrons will repel ht h each other • because of the weakness of these forces, multiple Van der Waals interactions and/or Van der Waals interactions in conjunction with other noncovalent interactions must occur to significantly effect intermolecular contacts
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(a) permanent dipoles polar molecules interact weakly, with negative poles ( δ - ) attracted to gp ( ) positive poles ( δ + ) (b) dipole-induced dipole interactions tdi l i d a permanent dipole can induce a dipole in a neighbouring group by distorting its electron distribution (c) London dispersion forces at any moment, nonpolar groups can polarize electrons in a neighbouring nonpolar group due to rapid fluctuation of their electrons attraction extremely weak, transient, fall off rapidly with distance
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dipole-induced dipoles • benzene has neither a net charge nor a permanent dipole • a + charge will induce a redistribution of electrons within the benzene ring London dispersion forces • planar molecules like benzene tend to stack because fluctuations in the electron clouds of the rings interact with each other, producing a dispersion force
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• Van der Waals interactions represent the sum of these dipole interactions along with any Van der Waals repulsion at may exist that may exist
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Hydrophobic Effects: • nonpolar molecules do not » contain charged groups » possess a dipole moment
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This note was uploaded on 12/13/2011 for the course MBB 201 taught by Professor -- during the Spring '11 term at Simon Fraser.

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MBB222-lecture3 - Ionic Interactions the result of attractions between positively charged cations and negatively charged anions e.g NaCl Na ClNaCl

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