Spectroscopic studies on human lens crystallins

Human lens crystallins were studied by absorption, circular dichroism and fluorescence spectroscopy. The absorption spectra in the near-ultraviolet region show some differences in intensity, but spectral features are similar, except for the α-crystallin, which gives a fine structure due to phenylalanine between 250 and 270 nm. Tryptophan fluorescence and near-ultraviolet circular dichroism indicate that tryptophan residues are more exposed in α-crystallin than in either β- or γ-crystallin, and that the degree of exposure decreases in the order of α < β₁ > β₂ > β₃ > γ. The far ultraviolet CD suggests that these proteins exist mainly in a β-sheet conformation and that the amount does not vary much among them. The greater exposure of the tryptophan residues in the high-molecular-weight crystallins may reflect greater unfolding in their protein domains. Spectroscopic measurements are thus useful in predicting protein tertiary structure in the absence of the complete sequence and X-ray data. The fact that the high-molecular-weight proteins exist in a more unfolded state may render them more vulnerable to exogeneous insults, and these effects may be studied by spectroscopic measurements.