The 2D IR lineshape evolution for the three systems under investigation were analyzed using center line slope (CLS) analysis (Figure and). The CLS is obtained by identifying the detection frequencies (vertical axis) with the ridge of maximum intensity along the excitation axis (horizontal axis) and then fitting a linear slope through these points (Section S4) . The CLS decays fit well to a monoexponential function (Figure ), serving as an experimental measure of the frequency-frequency correlation function (FFCF). More specifically, FFCF reflects the decorrelation between excitation and detection frequencies with waiting time, capturing the environmental fluctuations along the backbone decay almost twice as slow as that of the other two proteins. These results suggest that the long PEG chain significantly slows the local dynamics, while the short PEG chain has minor influence on dynamics.