Analysis of the near-ultraviolet circular dichroic spectra of staphylococcal enterotoxins A, B and C

The near-ultraviolet CD spectra from 260 to 300 nm of staphylococcal enterotoxins A, B and C were resolved empirically into Gaussian curves. Each spectrum contained the same six components with maximum ellipticities at virtually identical wavelengths. The rotatory strength of the bands of enterotoxin A, however, differed significantly from that of enterotoxins B and C. Each Gaussian curve was identified as corresponding to a prominent CD transition of an aromatic chromophore: two phenylalanine bands from its ¹L_b transition ($\text{0-0}\text{and}\text{0 + 930}\text{cm}{}^{\mathrm{-1}}$); four tyrosine bands from its ¹L_b transition (the 800 cm⁻¹ primary vibronic progression) with the weakest of these overlapping a phenylalanine band; and one ¹L_b tryptophan band. At alkaline pH, tyrosylate CD bands arose, centered at 247–249 nm and at 295–298 nm. The low intensity of the 295–298-nm bands indicated that most of the tyrosine CD in the neutral enterotoxins was contributed by buried residues. The tryptophan contribution to the CD of enterotoxin A was positive, while that of B and C was negative. Also indicative of a different milieu was the ready oxidation of the A toxin by $\text{N-}\text{bromosuccinimide}$ and the unavailability of the single tryptophan residue in the other two toxins to this reagent. The environment of the disulfide bond was markedly diverse in the three enterotoxins. Enterotoxin C was refractory to reduction by mercaptoethanol under conditions where enterotoxins B and A were readily reduced. The contribution of the disulfide of enterotoxin B to its CD spectrum was positive with an intensity of about 9000 deg · cm² · dmol⁻¹ and was centered near 273 nm. The difference spectrum between native and reduced enterotoxin A was much smaller and appeared to be conformational in origin.