RESEARCH large and complicated side chains (similar to benzyl glutamate), are al most hopeless, says Tiuoco. Thus cal culations of optical rotation cannot be used yet to assign the sense of the helix because of the influence of side chain groups. This problem might be over come, Tinoco suggests, through more careful analysis of how the optical rotation depends on the wave length of light ^ Helixes A p p e a r Short. T h e helical chain configuration in proteins is sta bilized by hydrogen bonds between C—Ο and Ν—Η groups in the mole cules. But length of helical regions in some protein molecules must b e limited by cross-linking and other steric effects. So the question arises as to how short a helix can b e and still be stable. For a specific model protein, poly glutamic acid ( P G A ) , Jon Applequist, now at the University of California, and Paul Doty at Harvard came up with a criti cal size of six amino acid units. Applequist and Doty partially race mized PGA and measured the amount of optical rotation, which in turn gives a measure of the amount of either the right or left hand type helix present. T h e technique is simple; first, poly-Lglutamic acid is heated at a high pH. T h e product contains random sequences of D and L amino acid units along the polymer chain. At low p H , PGA is helical, but the D and L· units each tend to form helixes of opposite handedness. Racemized PGA containing both types of units is helical only where a single type occurs in a sequence long enough to form a helix of the handedness pre ferred by that type, according to Apple quist. Thus the optical rotation of partially racemized PGA depends on the amount of helix of either handedness present. Therefore, says Applequist, rotation measures t h e fraction of amino acid units which are in sequences greater t-u *-u~ —i*-i^^l t i i c i i i LIIC; L11 v i t a l
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