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By Richard J. Pankhurst

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S u d h a et al (1983) h a v e studied a series of Aib-containing peptides by C D a n d n m r . T h e n u m b e r of p e p t i d e N H groups involved in intramolecu­ lar h y d r o g e n b o n d i n g h a v e b e e n estimated by t e m p e r a t u r e d e p e n d e n c e of 2 the H chemical shift in d e u t e r a t e d chloroform and dimethylsulfoxide. T h e C D s p e c t r a of t h e s e p e p t i d e s in trifluoroethanol and m e t h a n o l w e r e m e a s u r e d . All t h e s p e c t r a s h o w e d t w o negative b a n d s , o n e n e a r 200 n m and the o t h e r n e a r 205 n m .

Idealized band shapes for ab­ sorption (a), CD (b), and ORD (c). An isolated absorption band is represented as a Gaussian function of wavelength. The band is centered at k0, has a half-width of A, and amplitudes of € ma x , A e m a, x and [ M ] m xa in absorption, CD, and ORD, respectively. In (b) and (c), the solid curves represent a positive CD band and the corresponding positive Cotton effect, re­ spectively, while the dashed curves corre­ spond to a negative CD band and a negative Cotton effect.

T h e extent of twisting varies from o n e protein s t r u c t u r e to a n o t h e r , but the sense of the twist is always the s a m e . In addition t o t h e s e u n i q u e factors, the C D of ft sheets m a y d e p e n d o n t h e solvent and side chain and on the length and width of the sheet. Theoretical calculations (Pysh, 1966, 1970a; W o o d y , 1969; Madison and Schellman, 1972; Applequist, 1982) h a v e generally predicted qualita­ tively similar C D s p e c t r a for parallel and antiparallel /3 sheets.

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