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Water molecules dance around a protein (in green).
Protein in water.
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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 � = 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 �). "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.
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