Circular dichroism in the 220–250-nm region of homopolypeptides of aliphatic amino acids in the random-coil conformation in trifluoroacetic acid
✍ Scribed by Gian Paolo Lorenzi; Giuseppe Greco; Roberto Mona
- Publisher
- Wiley (John Wiley & Sons)
- Year
- 1975
- Tongue
- English
- Weight
- 215 KB
- Volume
- 14
- Category
- Article
- ISSN
- 0006-3525
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✦ Synopsis
For deep penetration in the region of the peptide bond electronic transitions, the use of trifluoroacetic acid (TFA) as a solvent presents some technical problems' because its absorptivity becomes very large below 220 nm. However, even meastiremerits by normal techniques of the circiilar dichroism (CII) spectra of polypeptides in T F A appeared desirable in view of the coritroversy2-6 about the type of C U spectrum to be associated with random-coiled polypeptide chains. Considering the well-established random-coil enforcing nature of TFA, one would expect, CI) spectra of polypeptides in this solvent. to be unequivocally charact,eristic of the random-coil conformation. RIiich of the present discussion about the CI) profile of random-(wiled polypeptide chains concerns the presence or absence of a positive Cotton effect. near 218 nm, such as that exhibited by ionized poly-L-glrltamic acid7 and p o l y -~~l y s i n e ~~~ in water; hence C1) spectra in T F A not even extending below 220 nm would be informative. This commiinication presents CII as well as optical rotatory dispersion (ORD) spectra in the 220-2.70-rim region for poly-L-alanine, * poly-L-leiicine, poly-L-valine and poly-kisoleucine dissolved in TFA. These homopolypeptides with alkyl side chains were investigated first because polar interactions among the side chains, which might. exert. a regularizing effect on the conformation of the main chain, are absent in their case. Indeed, data presented in this communication confirm that they are in the random-coil conformat ion in TFA.
Poly-L-alanine (s 46) and poly-I,-leucine (I>P unspecified but probably larger than 50 on the basis of the GO-AIHz nmr spectrum) were purchased from Rliles-Yeda Ltd.
Poly-L-valine (I>P 20) and the samples of poly-L-isoleucine with DF's of 8.5, 13.5, and 20 were synthesized by the NCA method using n-butylaniine as the initiator. T h e UP of these polymers was determined by 100-;\IIlz nmr in T F A by measuring the ratio