Animation: pH-Dependent
Electrophoresis and Microstate Composition of Glycine
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Glycine (NH3+-CH2-COO-)
has 2 ionizable
groups:
The
a-amino
group (NH3+-) ionizes (i.e., loses a proton) with pK (NH2)
= 9.8.
The
a-carboxyl
group (-COOH) ionizes (i.e., loses a proton) with a pK (COOH) = 2.4.
The pH where an equilibrium solution
of glycine has zero average charge is defined as the isoeletric pH, or
pI; pI (glycine) = 6.1 = 1/2 [pK (NH2) + pK (COOH)].
Glycine molecules potentially exist in
1 of 4 different ionization states(microstates): (NH3+-CH2-COO-); (NH2-CH2-COOH);
(NH2-CH2-COO-); or (NH3+-CH2-COOH)
The equilibrium percentages or ratios
of microstates vary greatly with pH and are calculated below.
BEFORE running each animation
MAKE A PREDICTION for
the migration direction or the microstate composition of glycine at the
corresponding equilibrium pH value.
Self Assessment: Test your
predictions regarding the electrophoretic behavior and microstate
distributions of glycine at different pH values.
Macrostate Dynamics:
Spreading is cause by random diffusion (Brownian
motion) with a pH-dependent distribution of the bulk mass over time.
Microstate Dynamics:
Individual molecule movements
over time are caused by the electric field exerting its effects on charges
created by pH-dependent ionization reactions.
Click to change pH conditions.
Click to start animation.
Observe the
electrophoretic behavior of glycine at different pH values
selected with the buttons at the top.
Use the rewind, play, and
fast-forward buttons to navigate within animations.
Play an animation straight
through by clicking the play button.
Step through the animation
frame-by-frame by clicking the forward or reverse buttons.
