Transient electric birefringence of human erythroid spectrin dimers and tetramers at ionic strengths 4 mM and 53 mM

In conventional electrooptic studies the sample ionic strength must f or technical reasons be kept below about 3 mM, which is only 2% of th e ionic strength at physiological conditions. Here we report on the first electrooptic study of human erythroid spectrin dimers and tetra mers at ionic strengths higher than 3 mM. All measurements in this st udy were carried out at both ionic strength 4 mM (2.5 mM HEPES + 1 mM NaCl) and 53 mM (2.5 mM HEPES + 50 mM NaCl). Spectrin tetramers wer e studied only at 4 deg whereas the dimers were studied at both 4 deg 37 deg. At 4 deg there is a striking quantitative similarity betwee n the transient electric birefringence (TEB) of spectrin dimers and t etramers. Also, the TEB of spectrin dimers at 37 deg was very similar to the results at 4 deg. The contour length and the molecular weight of spectrin dimers and tetramers are known. The dominating TEB relax ation time is in all cases only a fraction of what is predicted theor etically if the spectrin dimers and tetramers are assumed to be stiff and extended molecules. In sum the new TEB data constitute strong el ectrooptic evidence confirming that spectrin dimers and tetramers hav e a highly flexible structure, and demonstrate for the first time tha t a major part of the intra-chain dynamics of the spectrin is quite i nsensitive to an increase of the ionic strength from 4 to 53 mM. Use of the reversing electric field pulse technique for all conditions st udied yields TEB data suggesting that the orientation of both spectri n dimers and tetramers in an electric field is dominated by a permane nt rather than an induced electric dipole moment.