SynopsisFor the first time ORL) measurements in the near-infrared region from 0.7 to 2.(J / . L for well-known polypeptides, namely, poly( y-benzyl kglutamate), poly(kg1utamic acid), poly-~lysine. HCI, poly-S-carbobenzoxymethyl-Lcysteine, and Bombyx mori silk fibroiir, were carried out. I t was found that the value of the optical activity infrared term, which is proportional to the sum of rotational strengths of vibrational transitions, depends 0 1 1 polypeptide conformation. The optical activity infrared term value is equal to zero for the random-coil conformation, it is small but exceeds the measurement error for the a-helical state, and finally, for the p conformation it is an order of magnitude higher than for the a-helical state. The obtained results permit one to hope that on the basis of OIiD measurement. in the near-infrared region it will be possible to suggest a method of determining the p-form content in polypeptides and proteins It is well known that. t,he optical rotatory dispersion (ORD) of polypeptides arid proteins in t,he ultraviolet and visible regions depends greatly on their conformations, and that, the ORD method is widely applied in practice.l-j The theoretical interpretation of the optical act,ivit,y effect,s of electronic transitions in the pept.ide group is complicated in many res p e c t~.~,~ I n this connect,ion the st.udy of optical activity is of interest, for t'he region of vibrational transit,ions also, as the vibrational interactions of side groups with each other as well as with a polypeptide backbone must be much less than those involving the electronic transit.ions. Thus, it follows that t8he interpretation of t.he corresponding optical activit.y must also be simpler.Recently a t'heoret#ical evaluat,ion of t,he optical activity of vibrational t,ransitions appeared, and estimations of rotational strengt'hs n-ere given.8 AIoreover, an instrument for measurement of rotat'ory dispersion in the middle infrared region has been Strong optical activity can he expected in pept'ide compounds and proteins due t,o the asymmet'ric structure of molecular chains, as was found for electronic t,ransitions. According to our estimates the rotabiorial strengths of the main amide 1nstit.ute of Biological Physics, Academy of Sciences of the