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Optical activity of single‐stranded polydeoxyadenylic and polyriboadenylic acids; dependence of adenine chromophore cotton effects on polymer conformation
Author(s) -
Bush C. Allen,
Scheraga Harold A.
Publication year - 1969
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.1969.360070310
Subject(s) - chemistry , chromophore , circular dichroism , exciton , absorption band , crystallography , wavelength , polymer , cotton effect , solvation , absorption (acoustics) , stereochemistry , molecule , photochemistry , optics , organic chemistry , physics , quantum mechanics
Circular dichroism (CD) curves are reported for poly dA, (pdA) 6 , (pdA) 2 , poly A, ApAp, ApA, AMP, dApA, pdApA, A‐2′‐ O ‐methyl pA, and A‐2′‐ O ‐methyl pAp. Analysis of these curves indicated the presence of single CD bands at 228–230 mμ and at 278–280 mμ in oligomers longer than dinucleotides. In the case of dinucleotides and mononucleotides (from the literature, in addition to those studied here), the 230 mμ CD of band appears but the 280 mμ CD band does not. We assign the 230 mμ band to a very weak π–π* transition at this wavelength. From theoretical considerations, we show that the 280 mμ band is not an exciton component of the strong π–π* transition at 260 mμ in adenine. We conclude that the 280 mμ CD band must be assigned to a distinct absorption, not previously reported, which we suggest arises from an n –π* transition. The fact that the n –π* CD band at 280 mμ is not seen in mononucleotides or dinucleotides is ascribed to solvation of the adenine ring by water, which shifts the band to shorter wavelengths. Therefore, only interior residues of oligomers have the 280 mμ band, and the optical activity of a polymer cannot be computed from that of a dinucleotide, by using a nearest‐neighbor approximation. The existence of this end effect hag been tested, by taking it into account in computing the rotational strengths of the 278 mμ n –π* transition for several oligomers; it is pointed out that a more sensitive test of this end effect would require CD data for the oligo dA series of 3 to 5 residues. We speculate about the structural and optical differences between poly dA and poly A, and point out the need for a theoretical treatment of n –π* Cotton effects in polynucleotides.