Figure 10.

Correlation of the measured fluorescence yield with the yield calculated by eqs. (24) – (27) with ξ = 2.5×10¹³ cm⁻¹s⁻¹, σ = 1.5×10⁴ cm⁻¹, δ = 100 cm⁻¹, k_f = 6.11×10⁷s⁻¹ and k_nr = 1.858×10⁸s⁻¹. Circles, hairpin peptides; squares, Trp-cage 16b peptides; triangles, Trp-cage 10b peptides. The fluorescence yields were measured at 285 K for the Trp-cage peptides and the Phe10His and Trp1Phe/Phe10Trp hairpin peptides, and at 295 K for the other hairpins. The observed values for hairpin peptides containing either Glu, Asp, His or Tyr with the side chain in its protonated form, or containing His or Tyr with the side chain deprotonated, were obtained from the asymptotes of the sigmoidal fits to the data in Figure 2. Data for 26 peptides are shown. (The hairpins with Cys, ornithine, Lys, or ionized Glu or Asp at position 8 were not simulated because the present treatment cannot handle ionized carboxyl groups, S atoms, or quenching by proton transfer. The Phe10Leu hairpin, the Asp9Ala Trp-cage 16b peptide, and the Arg16Orn, Arg16Nva and Asp9Glu/Arg16Lys Trp-cage 10b peptides were omitted because of poor folding or lack of fluorescence data at 285 K.) The calculations for hairpin peptides containing ionized Tyr considered direct electron transfer from Trp to backbone amides, electron transfer from the phenolate ring to the excited indole ring of the Trp, and resonance energy transfer from Trp (π–π*2) to Tyr (π–π*1). Charge transfer from the Tyr to backbone amides thus does not contribute to the calculated quenching, although the resulting CT states could serve as sinks for removal of π–π*1 (see Figure 7). The straight line is a least-squares fit to all the data (R² = 0.849, Pearson correlation coefficient = 0.922, y-intercept = -0.0026 ± 0.0124, slope = 1.053 ± 0.0906).