Dance Of Water With Proteins: Disco Becomes A Minuet

This has enabled them to demonstrate for the first time that proteins influence the movements of the surrounding water network over a broad area. Some 1000 water molecules are “brought into line” by one protein: If their movement without protein more closely resembles a bunch of unchoreographed disco dancers, then in the vicinity of a protein it looks more like they are dancing a minuet.

Forgotten Dance Partner

The focus of prior investigations of the protein fold was exclusively directed on the movements of the protein framework and the side chains. “But now we are assuming that the rapid movements of the water, in particular its coupling with the protein movement, play an important role and therefore have an important function in the protein fold”, explains Prof. Havenith-Newen. Fundamental questions that remain as yet unanswered: How far does the influence of the proteins reach? Do the rapid water movements change when two proteins move closer?

Absorption of Terahertz Radiation Allows Conclusions to be Drawn

The development of high-performance laser sources in the terahertz (THz) range opens up entirely new possibilities for research: Dependent upon its state, the water indeed absorbs the terahertz radiation in its characteristic manner, allowing conclusions to be drawn. One example: While at 370° Kelvin (97°C) only 0.7 % of the radiation (at a frequency of approximately 1.5 THz) penetrates a 100 micrometer thick layer of water, this already increases to 40 % at 270 ° Kelvin (-3°C). It is therefore far more transparent for terahertz radiation than water. The reason is the minute, rapid vibrations in which the water molecule networks constantly find themselves. They last less than one picosecond (one millionth of one millionth of a second) and are determined by an effort on the part of the water molecules to move away from one another and rotate against one another. At another frequency, frozen water absorbs the radiation as liquid water, subsequently making every measurement in THz frequency characteristic for the state of the water.

Proteins Bring Order to Water

The researchers now took advantage of the circumstances that the vibrations of water networks do not change only as a result of the temperature, but also in response to the proximity of proteins. “One can picture it something like a protein bringing the water molecules in its vicinity into a type of ordered movement”, states Martina Havenith-Newen. “The movement of the uninfluenced water is similar to how people dance in a disco; there is a loose connection to the next partner that breaks off after a while. Water in the vicinity of protein dances something more like a minuet. The movement is more coordinated and the bond with the closest partner is maintained for longer.” The result is that water in the vicinity of proteins permits less permeation of THz radiation. This phenomenon makes it possible to directly observe the effects of proteins on water. The researchers come to conclusions on the state of the water from the amount of the absorbed radiation.

Far-Reaching Influence

“On the basis of our measurements, we can demonstrate for the first time that proteins affect the rapid movements of the water network over a broad area”, declares the chemical engineer. Around 1000 water molecules are influenced by one protein in their network movements. One such far-reaching effect, measurable up to a distance of 15 to 20 Angstroms (1 Å = 1/10th of a nanometre), has been predicted in simulations but has not yet been experimentally observed. The process demonstrated with the help of the new measurements that the influence reaches far beyond the area in which static changes in the structure, e.g., local density changes, could be observed (~ 3 Å). “In the long run, what remains to be clarified is the role that the water’s terahertz dance plays with the protein for its biological function”, asserts Prof. Havenith-Newen.

Reference: Simon Ebbinghaus, Seung Joong Kim, Matthias Heyden, Xin Yu, Udo Heugen, Martin Gruebele, David M. Leitner, and Martina Havenith: An extended dynamical solvation shell around proteins. In: Proceedings of the National Academy of Science PNAS 2007; Early Edition, 17.-21.12.2007