Two classes of β‐pleated sheet conformation in poly(L‐tyrosine): Evidence from solvent perturbation difference spectroscopy

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Abstract
We had earlier observed that the detailed nature of the β‐structure adopted by poly(L‐tyrosine) depends on molecular weight [Auer, H. E. & McKnight, R. P. (1978) Biochemistry 17, 2798–2805]. It was proposed that shorter molecules form an open, single‐layered sheet (class I) and that longer chains fold over to generate at least two layers (class II). In this work, solvent perturbation difference spectroscopy of the chromophoric side chain was applied to gain additional support for the model. Two sample‐reference pairs of media, sodium acetate vs sodium chloride and urea vs buffer, were studied. Great care was taken to achieve identical conditions in the sample and reference cells. The factors of importance in executing the experiments are the polypeptide concentration, the degree of side‐chain ionization, achievement of the same class of β‐conformation, and the absence of significant aggregation. Difference spectra obtained with both solvent pairs have similar features. Low‐molecular‐weight samples manifest difference bands arising from both ionized and neutral tyrosine side chains, while high‐molecular‐weight samples yield spectra with contributions primarily from ionized residues. The interpretation of these results is that the class I particle has both ionized and neutral residues exposed to the solvent medium, whereas the class II particle is folded, with neutral residues restricted to the interior of the fold and primarily ionized residues on the outer surfaces exposed to the solvent. Our results, therefore, corroborate the original model.