The optical rotatory properties of 7-globulins have been extensively investigated by Jirgensons et al.1 and characterized by the finding that thebe globulins have abnormally low dispersion constants A, and that their bo values are near zero. This finding indicates the absence of helical conformation of one sense. However, the facts that denaturation causes some increase in A, and bo and a marked increase in [a10 suggest that a disorganization occurs upon denaturation different from a helix-coil transition.
The fact that the bo value is zero can be explained by assuming an internally compensated structure; that is, either exactly equal amounts of right-and left-handed helices or of quantities of any other two periodic structures which contribute equally and oppositely to bo. However, the relative constancy of b~ on denaturation2 almost completely rules out compensation by right-and left-handed helices, since a real difference in the stability of right-and left-handed helices composed of &residues should exist.
Another periodic structure which has a bn value opposite to that of a right-handed helix is the 8-structure.3.4 Thus, Schellmad had raised the possibility that both a-helix and the 8-structure are present in ?-globulins: however, neither Winkler and Doty2 nor Imahori and MomoiG could find the @-band by infrared spectroscopy.
An alternative explanation is a periodic structure which does not contribute to the bo value. Such a structure has been described by us in poly-O-acetyl-~-serine~ and poly- O-acetyl-~-threonine.g Since 7-globulins are unusually rich in serine and threonine, i t seems desirable to study poly-bserine or poly-L-threonine as a model polypeptide whose conformational preferences may be related to that of ?-globulin. Indeed, their precursors, poly-O-acetyl-1~erine7 8 and poly-0-acetyl-rAhreonine,S have already rereived some attention. I n the form of oriented films they were found to exist in a cross+ rather than the parallel-0 structure, and this cross-p structure was shown to persist in solution. Now, two cross-@ structures are possible: an intramolecular form with transverse folding into hairpinlike bends, and an interchain structure made of extended rhains which are associated as in a unitary needlelike particle. These have been distinguidhed in model experiments. The specific rotation of poly-O-acetyl-I>-serine a t various wavelengths is completely independent of concentration while short peptides whirh have been shown to be in the interchain cross-8 structure display a strong concentration dependenre.8 However, the rotatory dispersion constants for the poly-O-acetyl-I>-serine were found to be very similar to those of -,-globulin.8 This leads us to suggest that the folded structure in 7-globulin is intrachain cross-0.
Although no indication of the presence of the &conformation is found by infrared