The circular dichroism (CD) spectrum of fd bacteriophage has a deep minimum at 222 nm characteristic of highly α-helical protein, but there is a shoulder at 208 nm rather than a minimum, with a 222/208-nm amplitude ratio near 1.5 rather than near 1. Oxidation of fd phage with the tryptophan reagent N-bromosuccinimide (NBS) changes the ratio. In this report, the NBS titration of fd is followed by CD and three other spectroscopies, the results of which yield an explanation of the unusual CD spectrum. Absorbance, fluorescence, and Raman data show the oxidation to have two phases, the first of which involves the destruction of tryptophan and the second, tryptophan and tyrosine. Raman spectra reveal the invariance of an environmentally-sensitive tyrosine Fermi resonance doublet during the First oxidative phase. Raman spectra also show that little or no change of α-helicity occurs in the first or second oxidation phase, although very slight changes in the helix parameters might be occurring. Concurrent with the destruction of tryptophan during the first phase is the appearance in CD difference spectra ([Ɵ]NBS-treated fd - [Ɵ]native fd) of positive maxima at 208-210 nm and negative maxima at 224 nm, with crossovers at 217 nm. Enormous difference ellipticities, per oxidized subunit of 50 amino acids, of +490 000 ± 80 000 deg cm2dmol-1at 208 nm and -520 000 ± 110 000 deg cm2dmol-1at 224 nm have been derived from the data. With no apparent change in α-helicity, with no loss of other chromophores, and with the constant nature of the tyrosine environments, the entire spectral change has been assigned to tryptophan. Direct stacking of tryptophan-26 (W26) from one subunit upon phenylalanine-45 (F45) from another subunit, with coupling of the electronic transitions of the two chromophores, is proposed as an explanation of the assigned spectrum.
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