Using SEC H
PLC and fluorescence anisotro
py, absor
ption s
pectra were assigned to the s
pecificoligomeric structures found with
phycocyanin. The absor
ption s
pectra were used to quantify the
po
pulationof each oligomeric form of the
protein as a function of both urea concentration and tem
perature.Phycocyanin hexamers dissociate to trimers with equilibrium constants of 10
-6 to 10
-5. Phycocyanintrimers dissociate to monomers with equilibrium constants of 10
-15 to 10
-12. Both dissociation constantsincrease linearly with increasing urea concentration, and
G![](/images/entities/deg.gif)
values calculated from the equilibriumconstants fit best with an ex
ponential function. Our findings a
ppear in contrast with the commonly usedlinear extra
polation model,
Gurea![](/images/entities/deg.gif)
=
Gwater![](/images/entities/deg.gif)
+
A[denaturant], in which a linear relationshi
p exists betweenthe free energy of
protein unfolding or loss of quaternary structure and the denaturant concentration. Ourdata examines a smaller range of denaturant concentration than generally used, which might
partiallyex
plain the inconsistency.