Ca. Haynes et al., STRUCTURAL AND ELECTROSTATIC PROPERTIES OF GLOBULAR-PROTEINS AT A POLYSTYRENE WATER INTERFACE, Journal of colloid and interface science, 164(2), 1994, pp. 394-409
The structural and electrostatic properties of two similar-sized globu
lar proteins, hen egg-white lysozyme and bovine milk alpha-lactalbumin
, adsorbed to microspheres of a negatively charged polystyrene latex,
are determined from adsorption isotherms, plateau adsorption values, a
nd isothermal titration microcalorimetry data. Particular attention is
given to the application of proton-titration techniques for which a n
ew robust protocol is presented which accurately predicts the contribu
tions of the aqueous solution (i.e., the blank) to the overall titrati
on behavior of the protein-containing sample. Plateau values for the a
dsorbed proteins show a complex dependence on pH with a maximum occurr
ing at the isoelectric point of the protein/sorbent complex. Adsorbed-
state titration curves differ markedly from the corresponding dissolve
d-state curves; the data suggest that alpha-lactalbumin undergoes subs
tantial changes in secondary and tertiary structure upon adsorption. I
n the adsorbed state, alpha-lactalbumin does not exhibit the low-pH na
tive-to-apo-state transition which promotes release of the Ca2+ ion fr
om the ion-binding pocket. Average pK(a) data for the two proteins sug
gest that a fraction of the charged carboxyl groups are in close proxi
mity to the sorbent surface and the adsorbed protein is largely denatu
red. The data also indicate that protein adsorption is driven by a syn
ergistic combination of sorbent dehydration, protein structural change
s, and charge redistribution. (c) 1994 Academic Press, Inc.