Charge Invariant Protein–Water Relaxation in GB1 via Ultrafast Tryptophan Fluorescence

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• 作者：Arianna Biesso ; Jianhua Xu ; Pedro L. Muí?o ; Patrik R. Callis ; Jay R. Knutson
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：February 19, 2014
• 年：2014
• 卷：136
• 期：7
• 页码：2739-2747
• 全文大小：416K
• ISSN：1520-5126

文摘

The protein鈥搘ater interface is a critical determinant of protein structure and function, yet the precise nature of dynamics in this complex system remains elusive. Tryptophan fluorescence has become the probe of choice for such dynamics on the picosecond time scale (especially via fluorescence 鈥渦pconversion鈥?. In the absence of ultrafast (鈥渜uasi-static鈥? quenching from nearby groups, the TDFSS (time-dependent fluorescence Stokes shift) for exposed Trp directly reports on dipolar relaxation near the interface (both water and polypeptide). The small protein GB1 contains a single Trp (W43) of this type, and its structure is refractory to pH above 3. Thus, it can be used to examine the dependence of dipolar relaxation upon charge reconfiguration with titration. Somewhat surprisingly, the dipolar dynamics in the 100 fs to 100 ps range were unchanged with pH, although nanosecond yield, rates, and access all changed. These results were rationalized with the help of molecular dynamics (including QM-MM) simulations that reveal a balancing, sometimes even countervailing influence of protein and water dipoles. Interestingly, these simulations also showed the dominant influence of water molecules which are associated with the protein interface for up to 30 ps yet free to rotate at approximately 鈥渂ulk鈥?water rates.