Hydrophobic core and surface charges of human beta-2-microglobulin probed by CD measurements

The roles of hydrophobic bonding and charge electrostatics in the stabilization of human beta-2-microglobulin have been probed by variations in solution conditions and monitored by circular dichroism in the near and far UV regions. Sodium perchlorate initially gives a decrease in intensity of the positive 234 nm peak in the near UV followed by a shift of this peak to negative ellipticity at high perchlorate concentration. These 234 nm changes indicate a new environment for a tyrosyl chromophore(s). A conformational rearrangement of the beta-sheet sandwich must occur since all six tyrosines of human beta-2-microglubulin are located in the two beta-sheets of this sandwich. A slight decrease in intensity for the 200 nm positive peak in the far UV indicates a less close packing of the beta-sheets at high perchlorate. In other experiments, enthalpy and entropy values have been calculated from thermal unfolding studies at 50 and 180 mm NaCl for pH values 6.0 and 8.0. Larger enthalpy values are obtained at higher NaCl concentration consistent with salt shielding of predominantly unfavorable charge interactions. These enthalpy differences are relatively large suggesting that charge electrostatics are energetically significant in stabilization of human beta-2-microglobulin.