New clues to how proteins dissolve and crystallize

Hofmeister himself discovered that sodium salts-out egg white protein more efficiently than potassium, as does calcium. It's a difference with profound biological significance. "You don't want to precipitate salts inside the cells!" Saykally remarks. "That's part of why living organisms spend a lot of energy pumping calcium and sodium out of cells and pumping potassium in."

Computer simulations and quantum calculations of how sodium and potassium bind to proteins were performed by Luboš Vrbka and his colleagues in the Pavel Jungwirth research group, working in the Czech Republic in 2006. Their work indicated that the large difference between the binding efficiency of the two cations (which are otherwise similar in many ways) were consonant with the Law of Matching Water Affinities. In essence, Vrbka's simulations and calculations supported the Law's theoretical predictions.

Still needed was what Saykally calls "a new class of experimental support, stronger than previous experiments." His team, working with colleagues at beamline 8.0.1 of the Advanced Light Source, had developed just such an approach. Incorporating liquid microjet technology into the high-vacuum environment of a synchrotron x-ray experiment has allowed the group to perform near-edge x-ray absorption fine structure (NEXAFS) measurements on liquid samples that would otherwise be difficult or impractical to measure with synchrotron radiation.

Janel Uejio, a graduate student of Saykally's, recalls how she first became involved in the selective-binding investigation. She was working overnight on a different project at the 8.0.1 beamline when the phone rang at three o'clock in the morning.

"Rich had just read Vrbka's paper and had a brainstorm," says Uejio. "He wanted me to use liquid microjet technology to measure the selective binding of sodium and potassium to formate and acetate, two simple carboxylate groups characteristic of proteins. At that hour, all I had on hand were acetic acid and sodium chloride and potassium chloride" -- essentially, vinegar and table salt -- "but even so, the preliminary results were promising."

With liquid microjet technology, precisely mixed chemicals flow rapidly through a fused silica capillary shaped to a fine tip, a nozzle with an opening only a few micrometers (millionths of a meter) in diameter. The resulting liquid jet travels through a few centimeters of vacuum inside the beam chamber and is intersected by the synchrotron's x-ray beam, then collected by a cold skimmer and condensed out, to prevent any liquid molecules from contaminating the pristine vacuum of the synchrotron.

The great advantages of the system, says Saykally, are that in a vacuum the soft x-ray beam encounters only the liquid target -- there's no interference from air or windows or the like -- and that the rapid passage of the sample through the beam minimizes x-ray damage, which otherwise can be severe.