Biochemistry 675, Lecture 8 Electrostatic Interactions Previous Classes Hydrophobic Effect Hydrogen Bonds Today: Electrostatic Interactions Reading: Handout:P.R. Bergethon & E.R. Simons, Biophysical Chemistry, Springer-Verlag Portions of Ch. 11 & 12
Charged residues are found on the surface of proteins
The surfaces of biological molecules can be highly charged The EMBO Journal (2006) 25, 163 173, doi:10.1038/sj.emboj.7600918 Molecular basis of RNA recognition by the human alternative splicing factor Fox-1 Sigrid D Auweter, Rudi Fasan, Luc Reymond, Jason G Underwood, Douglas L Black, Stefan Pitsch and Frédéric H-T Allain Overview of the solution structure of the RBD of Fox-1 in complex with UGCAUGU
The surfaces of biological molecules can be highly charged camp-dependent protein Kinase (1YDR) bound to peptidic H7 protein kinase inhibitor http://swissmodel.expasy.org/spdbv/text/epot.htm
Electrostatic interactions are long range and fall off in strength gradually with distance E 1/r n
Electrostatic interactions are strong Coulomb s law Interaction energy between an Na + and Cl - separated by 3Å is-110 kcal/mol (in vacuum) Interaction energies of 2 opposite charges in a protein 10 Å apart:-16 kcal/mol (if dielectric is 2) But!Competing interactions with water: desolvation of a unit charge costs about 60 kcal/mol
Interaction of Charges with water; The Born Model
Born Model: A model for understanding Ion-water interactions-a thermodynamic cycle w discharge + w transfer + w charging + (-ΔG i-s )=0 There are no interactions in the transfer of the uncharged sphere into water so w transfer =0 and ΔG i-s = w discharge + w charging
ΔG i-s = w discharge + w charging w discharge : Can be determined from the work of charging a sphere in a vacuum; work of transferring infinitesimal bits of charge to the sphere until it has the correct charge w ch arg ing = (z i e o )2 8πε o r i Therefore: w disch arg e = (z i e o )2 8πε o r i For the work of charging the sphere in solvent we have to consider solvent. The dielectric constant must influence charging in solvent. remember: F q 1 q 2 4πε o εr 2 Work of charging the sphere in the solvent: w ch arg ing = (z i e o )2 8πε o εr i
ΔG i s = w disch arg e + w ch arg ing ( ΔG i s = z e i o) 2 ( + z e i o) 2 8πε o r i 8πε o εr i ( ΔG i s = z e i o) 2 (1 1 8πε o r i ε ) MolarBasis ΔG i s = N A Enthalpy ( z i e o ) 2 (1 1 8πε o r i ε ) ΔG i s = ΔH i s TΔS i s ΔH i s = N A z i e o ( ) 2 8πε o r i (1 1 ε T ε 2 ε T )
Compare the theoretical prediction to the experimentally measured enthalpies of solvation (Fig 11.1-5)
Add the structural features of water: 1. Water is a dipole 2. Three types of water can be considered Bulk water An immobilized layer An intermediate layer
Thermodynamic cycle For the modified Born Model
Good agreement between the calculated and Measured enthalpies of ion-solvent interactions
Consider the significance of ion-solvent interactions for everyday procedures used in the lab Example: Salting out
What about the other ions?
Ion:ion interactions What is the free energy of ion:ion interactions ΔG i-i? Consider an ion in a solution with many other ions: ΔG i i = n i µ i i n i is the number of ions that the reference ion interacts with and µ i-i is the chemical potential for each ion that interacts with the reference. The chemical potential of a single ionic species will be related to the work of charging up a mole of ions of interest while in the proximity of other ions. w ch arg ing = (z i e o )2 8πε o εr i In terms of chemical potential: Δµ i i = N A w ch arg ing Δµ i i = N A z i e o 2 ψ ψ is the electrostatic potential of the ion
In order to determine the chemical potential change for the interaction between the total ionic assembly and the ion of interest we must know the electrostatic field at each individual ion that is a result of the other ions in solution. This requires information about the spatial distribution of the ions in the solution relative to the reference ion.
Erich Armand Arthur Joseph Hückel (in Engl. often spelled Huckel or Hueckel) b. August 9, 1896, Berlin, Germany d. 1980, Marburg, Germany A German physicist and physical chemist. He is known for two major contributions: (a) The Debye-Hückel theory of electrolytic solutions, (b) The Hückel method of approximate molecular orbital (MO) calculations on p-electron systems.
Petrus (Peter) Josephus Wilhelmus Debye was born March 24, 1884, at Maastricht, the Netherlands. Debye won Nobel Prize in Chemistry, 1936, "for his contributions to our knowledge of molecular structure through his investigations on dipole moments and on the diffraction of X-rays and electrons in gases". Debye died on November 2, 1966, and was buried on Pleasant Grove Cemetery, Cayuga Heights, Tompkins County, New York, U.S.A.
Debye-Hückel Model 1923 Treatment: A reference ion residing in a dielectric continuum. Dielectric constant is 80 Reference ion is charged-therefore, in its immediate vicinity there will be neutralization by the continuum of charge that surrounds it. At equilibrium the charge on the central ion will be exactly counterbalanced by the counter charge atmosphere: Principle of Electroneutrality: z i e o X i = 0