Hydration dynamic

Li TP, Hassanali AAP, Kao YT, Zhong DP, Singer SJ (2007) Hydration dynamics and time scales of coupled water-protein fluctuations. J Am Chem Soc 129(11) 3376-3382... 

Keszei, E., Murphrey, T. H. and Rossky, P. J. Electron hydration dynamics simulation results compared to pump and probe experiments, J.Phys.Chem., 99 (1995), 22-28... 

This bimodal dynamics of hydration water is intriguing. A model based on dynamic equilibrium between quasi-bound and free water molecules on the surface of biomolecules (or self-assembly) predicts that the orientational relaxation at a macromolecular surface should indeed be biexponential, with a fast time component (few ps) nearly equal to that of the free water while the long time component is equal to the inverse of the rate of bound to free transition [4], In order to gain an in depth understanding of hydration dynamics, we have carried out detailed atomistic molecular dynamics (MD) simulation studies of water dynamics at the surface of an anionic micelle of cesium perfluorooctanoate (CsPFO), a cationic micelle of cetyl trimethy-lainmonium bromide (CTAB), and also at the surface of a small protein (enterotoxin) using classical, non-polarizable force fields. In particular we have studied the hydrogen bond lifetime dynamics, rotational and dielectric relaxation, translational diffusion and vibrational dynamics of the surface water molecules. In this article we discuss the water dynamics at the surface of CsPFO and of enterotoxin. 

The hydration dynamics were also studied by Karim et al. [58], These authors used the TIP4P model of water in their molecular dynamics simulations. The observed hydration dynamics was nonexponential with average time constant in the range of 0.4-0.7ps. In this case simulated relaxation of the first solvation shell was also faster than that of the other shells. 

Hydration dynamics, Portable NMR, PEDRI, Overhauser MRI, RELIC. 

HYDRATION DYNAMICS AND COUPLED WATER-PROTEIN FLUCTUATIONS PROBED BY INTRINSIC TRYPTOPHAN... 

Femtosecond spectroscopy has an ideal temporal resolution for the study of ultrafast water motions from femtosecond to picosecond time scales [33-36]. Femtosecond solvation dynamics is sensitive to both time and length scales and can be a good probe for protein hydration dynamics [16, 37-50]. Recent femtosecond studies by an extrinsic labeling of a protein with a dye molecule showed certain ultrafast water motions [37-42]. This kind of labeling usually relies on hydrophobic interactions, and the probe is typically located in the hydrophobic crevice. The resulting dynamics mostly reflects bound water behavior. The recent success of incorporating a synthetic fluorescent amino acid into the protein showed another way to probe protein electrostatic interactions [43, 48]. 

We recently developed a systematic method that uses the intrinsic tryptophan residue (Trp or W) as a local optical probe [49, 50]. Using site-directed mutagenesis, tryptophan can be mutated into different positions one at a time to scan protein surfaces. With femtosecond temporal and single-residue spatial resolution, the fluorescence Stokes shift of the local excited Trp can be followed in real time, and thus, the location, dynamics, and functional roles of protein-water interactions can be studied directly. With MD simulations, the solvation by water and protein (residues) is differentiated carefully to determine the hydration dynamics. Here, we focus our own work and review our recent systematic studies on hydration dynamics and protein-water fluctuations in a series of biological systems using the powerful intrinsic tryptophan as a local optical probe, and thus reveal the dynamic role of hydrating water molecules around proteins, which is a longstanding unresolved problem and a topic central to protein science. 

With site-directed mutation and femtosecond-resolved fluorescence methods, we have used tryptophan as an excellent local molecular reporter for studies of a series of ultrafast protein dynamics, which include intraprotein electron transfer [64-68] and energy transfer [61, 69], as well as protein hydration dynamics [70-74]. As an optical probe, all these ultrafast measurements require no potential quenching of excited-state tryptophan by neighboring protein residues or peptide bonds on the picosecond time scale. However, it is known that tryptophan fluorescence is readily quenched by various amino acid residues [75] and peptide bonds [76-78]. Intraprotein electron transfer from excited indole moiety to nearby electrophilic residue(s) was proposed to be the quenching... 

Melittin, which is an amphipathic peptide from honeybee venom, consists of 26 amino acid residues and adopts different conformations from a random coil, to an a-helix, and to a self-assembled tetramer under certain aqueous environments see Fig. 9. We have carried out our systematic studies of the hydration dynamics in these three conformations using a single intrinsic tryptophan ( W19) as a molecular probe. The folded a-helix melittin was formed with lipid interactions to mimic physiological membrane-bound conditions. The self-assembled tetramer was prepared under high-salt concentration (NaCl = 2 M). The tryptophan emission of three structures under three different aqueous environments is 348.5 nm, 341 nm, and 333.5 nm, which represents different exposures of aqueous solution from complete in random-coil, to locating at the lipid surface of a nanochannel (50 A in diameter) in a-helix and to partially buried in tetramer. Figure 10 shows... 

Biomolecular recognition is mediated by water motions, and the dynamics of associated water directly determine local structural fluctuation of interacting partners [4, 9, 91]. The time scales of these interactions reflect their flexibility and adaptability. For water at protein surfaces, the studies of melittin and other proteins [45, 46] show water motions on tens of picoseconds. For trapped water in protein crevices or cavities, the dynamics becomes much slower and could extend to nanoseconds [40, 71, 92], These rigid water molecules are often hydrogen bonded to interior residues and become part of the structural integrity of many enzymes [92]. Here, we study local water motions in various environments, from a buried crevice to an exposed surface using site-selected tryptophan but with different protein conformations, to understand the correlation between hydration dynamics and conformational transitions and then relate them to biological function. 

The response range of the local environment to the excited Trp-probe is mainly within 10 A because the dipole-dipole interaction at 10 A to that at —3.5 A of the first solvent shell drops to 4.3%. This interaction distance is also confirmed by recent calculations [151]. Thus, the hydration dynamics we obtained from each Trp-probe reflects water motion in the approximately three neighboring solvent shells. About seven layers of water molecules exist in the 50-A channel, and we observed three discrete dynamic structures. We estimated about four layers of bulk-like free water near the channel center, about two layers of quasi-bound water networks in the middle, and one layer of well-ordered rigid water at the lipid interface. Because of lipid fluctuation, water can penetrate into the lipid headgroups, and one more trapped water layer is probably buried in the headgroups. As a result, about two bound-water layers exist around the lipid interface. The obtained distribution of distinct water structures is also consistent with —15 A of hydration layers observed by X-ray diffraction studies from White and colleagues [152, 153], These discrete water stmctures in the nanochannel are schematically shown in Figure 21, and these water molecules are all in dynamical equilibrium. 

The emission of Trp 19 in melittin shifts to the red side peaking at 341 nm (Fig. 18), and the probe location slightly moves away from the lipid interface toward the channel center. Consistently, we observed a larger fraction of the ultrafast solvation component (35%) and a smaller contribution of slow ordered-water motion (38%). Melittin consists of 26 amino acid residues (Fig. 9), and the first 20 residues are predominantly hydrophobic, whereas the other 6 near the carboxyl terminus are hydrophilic under physiological conditions. This amphipathic property makes melittin easily bound to membranes, and extensive studies from both experiments [156-161] and MD simulations [162-166] have shown the formation of an 7-helix at the lipid interface. Self-assembly of 7-helical melittin monomers is believed to be important in its lytic activity of membranes [167-169]. Our observed hydration dynamics are consistent with previous studies, which support the view that melittin forms an 7-helix and inserts into the lipid bilayers and leaves the hydrophilic C-terminus protruding into the water channel. The orientational relaxation shows a completely restricted motion of Trp 19, and the anisotropy is constant in 1.5 ns (Fig. 20b), which is consistent with Trp 19 located close to the interface around the headgroups and rigid well-ordered water molecules. 

When tryptophan is excited to the La state, the local equilibrium is shifted and the system is in a nonequilibrium state. Our observed hydration dynamics, with two distinct time scales, reflect the temporal evolution of two types of motions from the initial nonequilibrium configuration to new equilibrated state... 

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