Consider the \"electrostatic model\" of the a-helix hydrogen bond, where the cha
ID: 495661 • Letter: C
Question
Consider the "electrostatic model" of the a-helix hydrogen bond, where the charge-charge interactions contributing to the H-bond are between the O-H and O-N atomic pairs (assume C is low in electronegativity such that its partial charge is negligible). The geometry is shown below where a = 100 pm, b = 66 pm, and c = 124 pm, and the partial charges of N, H, and O are -0.36 e, +0.18 e, and -0.38 e, respectively. How much energy (in kJ/mol) is needed to break this H-bond (a) in a vacuum, (b) inside the hydrophobic core of a protein where the dielectric coefficient is similar to that of the inside of a biomembrane (spailon_hydrophobic = 2.0), and (c) in water with epsilon_water = 78.5?Explanation / Answer
The electric potential energy U of a system of two point charges is defined as: (1/(4*pi*epsilon))*(q1*q2)/r. Where, q1 and q2 are the charges on individual points/atoms, r is the distance between them. pi is (22/7) and epsilon is the electric permittivity of the medium. To break the H-bond energy difference between the current system and that when the bond is broken needs to be calculated. The difference will essentially be the electrostatic energy owing to electrostatic interaction between H and O and that between N and O as with breaking of the same the interaction energy will be zero in both cases (think r goes to infinity) Accordingly;
summation of (q1*q2/r) = (-0010364 + 0008241)X (electron charge^2) = -5.4476 X 10^(-30) C^2/m
a) epsilon = 8.854X10^(-12) C^2/(J.m)
Energy Required = [5.4476 X 10^(-30)]/(8.854X10^(-12)) = 6.15X10^(-19) J/molecule = 370.6 kJ/mol
b) epsilon = 2*8.854X10^(-12) C^2/(J.m) implies
Energy Required = 370.6/2 kJ/mol = 185.3 kJ/mol
c) epsilon = 78.5*8.854X10^(-12) C^2/(J.m) implies
Energy Required = 370.6/78.5 kJ/mol = 4.72 kJ/mol
Related Questions
drjack9650@gmail.com
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.