Contributions of solvent-solvent hydrogen bonding and van der Waals interactions to the attraction between methane molecules in water.

The contribution of solvent-solvent hydrogen bonding and van der Waals interactions to the attraction between methane molecules in water was investigated by comparing the potential of mean force (PMF) between two methane molecules in TIP4P water to those in a series of related liquids in which the solvent-solvent interactions were progressively turned off while keeping the solvent-solute interactions unchanged. The magnitude of the attraction between methanes was not significantly changed when the hydrogen bonding interaction between solvent molecules was eliminated and the solvent was maintained in the liquid state by increasing either the pressure or the magnitude of the solvent-solvent van der Waals interaction. However, when solvent-solvent excluded volume interactions were eliminated, the methane molecules interacted no more strongly than in the gas phase. The results are consistent with the idea that the primary contribution of hydrogen bonding to the hydrophobic interaction is to keep water molecules in a liquid state; at constant density, packing interactions rather than hydrogen bonding appear to be critical as suggested by scaled particle theories of solvation. The overall shape of the PMF was, however, changed in the absence of hydrogen bonding, pointing to an influence of hydrogen bonding on the detailed form of the interactions between nonpolar solutes in water. The effects of correlations between the configurations sampled during the Monte Carlo procedure used in the free energy calculations on the estimation of errors was also characterized.

Nosocomial legionellosis traced to a contaminated ice machine.

OBJECTIVE: To investigate a case of nosocomial legionellosis, identify pathways of transmission, and effect control of the environmental source. DESIGN: Case investigation and environmental culture surveillance. SETTING: A 720-bed university teaching hospital. CASE PATIENT: A ventilator-dependent 66-year-old male developed nosocomial pneumonia due to Legionella pneumophila serogroup 6 after 3 months in an intensive-care unit (ICU). The patient had no intake of potable water except for ice chips from an ice machine in the ICU. RESULTS: Cultures revealed L pneumophila serogroup 6 in the ice (4.3 colony-forming units/mL) and ice machine cold water (too numerous to count). Cultures from adjacent hot and cold taps, plus taps located near the patient, all were negative; ice machines and cold water on seven other patient units also were negative. Only sterile water had been used for tube feedings, mouth care, suctioning, and ventilator humidification. Hospital hot water previously had been colonized with L pneumophila serogroup 6, but all surveillance water cultures had been negative since chlorination of the hot-water system began the previous year; cold-water cultures had never before grown Legionella. The ice machine was disinfected with a 2-hour flush of 2.625% sodium hypochlorite. The supply line to the ice machine was replaced, and the cold-water pipe from the floor below was treated with 83 ppm sodium hypochlorite for 48 hours. All follow-up surveillance cultures of the ice machine remained negative through mid-1996. No additional cases of nosocomial legionellosis occurred. CONCLUSIONS: Ice machines may be reservoirs of L pneumophila in hospitals. Both ice and water dispensed from these machines may be contaminated, and nosocomial transmission may occur. Successful long-term decontamination and control can be accomplished with shock chlorination.

A desolvation barrier to hydrophobic cluster formation may contribute to the rate-limiting step in protein folding.

To gain insight into the free energy changes accompanying protein hydrophobic core formation, we have used computer simulations to study the formation of small clusters of nonpolar solutes in water. A barrier to association is observed at the largest solute separation that does not allow substantial solvent penetration. The barrier reflects an effective increase in the size of the cavity occupied by the expanded but water-excluding cluster relative to both the close-packed cluster and the fully solvated separated solutes; a similar effect may contribute to the barrier to protein folding/unfolding. Importantly for the simulation of protein folding without explicit solvent, we find that the interactions between nonpolar solutes of varying size and number can be approximated by a linear function of the molecular surface, but not the solvent-accessible surface of the solutes. Comparison of the free energy of cluster formation to that of dimer formation suggests that the assumption of pair additivity implicit in current protein database derived potentials may be in error.