The 32 kDa hydrophilic and acidic enamelin, the most stable cleavage fragment of the enamel specific glycoprotein, is believed to play vital roles in controlling crystal nucleation or growth during enamel biomineralization. Circular dichroism and Fourier transform infrared spectra demonstrate that the secondary structure of the 32 kDa enamelin has a high content of alpha-helix (81.5%). Quantitative analysis on the circular dichroism data revealed that the 32 kDa enamelin undergoes conformational changes with a structural preference to beta-sheet with increasing concentration of calcium ions. We suggest that the increase of beta-sheet conformation in the presence of Ca2+ may allow preferable interaction of the 32 kDa enamelin with apatite crystal surfaces during enamel biomineralization. The calcium association constant (K-a = 1.55 (+/-0.13) x 10(3) M-1) of the 32 kDa enamelin calculated from the fitting curve of ellipticity at 222 nm indicated a relatively low affinity. Our current biophysical studies on the 32 kDa enamelin structure provide novel insights towards understanding the enamelin-mineral interaction and subsequently the functions of enamelin during enamel formation. (C) 2008 Elsevier Inc. All rights reserved.