Birefringence of Oriented Thin Filaments in the I-Bands of Crab Striated Muscle and Comparison with the Flow Birefringence of Reconstituted Thin Filaments

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Abstract
Birefringence of the I-band of glycerinated myofibrils of the walking legs of the crab, Plagusia dentipes was examined by rectified polarization optics. The coefficient of birefringence of the I-band was estimated to be 1.64 .times. 10-3, which attained about 60% of the birefringence of the H-zone. The number ratio of thin filaments to thick filaments in the A-band was 6 according to EM and the spacing between neighboring thick filaments was about 56 nm according to X-ray diffraction. Protein concentration in the I-band was 91 mg/ml. Birefringence of the I-band was of the same order as birefringence of the reconstituted thin filaments oriented by flow at the same protein concentration. The ratio of protein concentration in the I-band to that in the H-zone was determined by interferometry. The ratio of birefringence of the I-band to that of the H-zone per unit protein concentration was about 0.9, which was comparable to the ratio of birefringence of the thin filaments and thick filaments in solution. The imbibition experiment showed that 70% of the total birefringence of the I-band is from birefringence and the remaining 30% comes from intrinsic birefringence.