A CD determination of the ?-helix contents of the coat proteins of four filamentous bacteriophages: fd, IKe, Pf1, and Pf3

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Abstract
The CD spectra of four filamentous bacteriophages—fd, IKe, Pf1, and Pf3— were analyzed to determine the α-helix contents of their major coat proteins. Measured spectra included the 192-nm band so that analyses could be carried out over the full wavelength range of the reference spectra for protein secondary structures available (a) from globular proteins [J. T. Yang, C.-S. C. Wu, and H. M. Martinez (1986) Methods in Enzymology 130, 208–269] and (b) from poly(L-lysine) [N. Greenfield and G. D. Fasman (1960) Biochemistry 8, 4108–4116]. Extended analyses were also performed with the addition of the spectrum of a model β-turn to the Greenfield and Fasman reference set, with the spectrum of a short α-helix in the Yang et al. reference set, and with an estimate of the spectrum of Trp added to both reference sets. The reference set based on the simple poly(L-lysine) polypetide, plus a spectrum of a model β-turn or of Trp, gave reasonably good fits to the measured spectra for all four phages and yielded the largest percentages of α-helix. The class I phages—fd and IKe—had large percentages of α-helix of 98 ± 2 and 97 ± 5%, respectively, while the two class II phages—Pf1 and Pf3— had similar but smaller α-helix contents of 83 ± 6 and 84 ± 2, respectively. While these α-helix contents were within the ranges previously reported from CD spectra of these phages in solution, they were more precise, and they indicated that the coat proteins of the intact phages have CD spectra that are probably modeled better by the reference spectra of polypeptides than by those of globular proteins.