Figure 5.

T-Jump IR responses reflect sequence-dependent conformational heterogeneity. The final temperature for each measurement is AT-all, 320 K; GC-end, 319 K; GC-core, 327 K; and GC-mix, 326 K. (a) Mid-IR t-HDVE difference spectra for GC-core at time delays from 5 ns to 560 μs. Normalized time traces for each sequence are shown at (b) 1600 cm^–1 and (c) 1660 cm^–1. The signal at 1600 cm^–1 corresponds to changes in A and T ring vibrations while the signal at 1660 cm^–1 contains contributions from G and T carbonyl vibrations. Each time trace is fit to the sum of a stretched exponential with two exponentials (solid lines): S(t) = A exp(−(t/τ_fast)^β_fast) + B exp(−t/τ_slow) + C exp(−t/τ_cool). The stretched exponential describes the process from 5 ns to 1 μs, and the two exponentials describe the signal increase from 1 to 320 μs and signal decay from rehybridization induced by thermal relaxation back to the initial temperature. The stretch factor β_fast for the fits at 1600 and 1660 cm^–1 are reported directly on the plots in panels b and c.