Time and time again studying ultrahigh dilutions (UHDs) provides new insights into biological and physical properties of complex systems. However, while such interactions can be easily detected via the effects they manifest onto other objects, studying UHDs directly remains a daunting challenge. Terahertz (THz) spectroscopy might provide just a way to do it.

THz spectroscopy is not that different from any other type of spectroscopy: electromagnetic waves pass through samples, some of it interacts with medium, and the resulting passing radiation is then picked up to study the interplay. Where THz differs massively is what kind of waves is used to observe the medium. Electromagnetic waves from visible spectra are barely able to interact with water and provide limited information about its structure. Microwave (MHz) radiation, while being less energetic, is almost entirely absorbed by water. This effect helps immensely with heating food, but is rather unhelpful when trying to study the interaction with the medium. THz radiation (radio waves), on the other hand, provides just enough interactivity between electromagnetic field and water molecules to be quite useful.

Not only that, but THz waves absorption in water is ununiform and heavily depends on the wavelength. Rotational and vibrational spectra of molecules have many distinct peaks in this range. This provides us with invaluable tool to study specific properties of water at the molecular level since many lines of rotational and vibrational spectra of water molecules lie specifically in those low energy zones (see an example of water vapour transmission in Fig. 1). By studying the position of the peaks, how they move and change intensity due to different conditions one can make reasonable predictions of molecular dynamics of the fluid system.

Fig. 1. This particular picture can be found on Wikipedia; however, it is created by using California’s Institute of Technology model (based on Pardo et al. 2001), that can be tweaked on their site. Feel free to play around with it. Source: http://www.submm.caltech.edu/cso/weather/

Recently published “Modeling of protein hydration dynamics is supported by THz spectroscopy of highly diluted solutions” by Kristina N. Woods from Ludwig-Maximilians-Universität, München, Germany is specifically devoted to the aforementioned method.

In the study Dr. Woods uses THz spectroscopy to study interactions between interferon gamma (IFN-γ), antibodies to IFN-γ and antibodies to IFN-γ receptor. These proteins were then used as basis for ultrahigh dilutions, that were subjected to THz spectroscopy. Spectral region of 100 to 250 cm^-1 were of particular interest to scientists, since this spectral region is responsible for local water molecules behaviour. Even in UHD solutions, which theoretically do not have proteins in any measurable amount, the spectra showed signs of contacts between proteins (Fig. 2). For example, a broad peak at 180 cm^-1 is responsible for networks of water molecules bonded by hydrogen interactions.

Fig. 2. Source: “Modeling of protein hydration dynamics is supported by THz spectroscopy of highly diluted solutions” by K. Woods.

However, the largest part of the work is not devoted to direct measurements, but rather to explanations of observable effects. To do this, Dr Woods has carried out an extensive modelling of molecular dynamics (Fig. 3). Those computer models show a few interesting details that help explain observable spectra:

• Ethanol added to water to produce HD samples has integral role in the observable changes. Less polar ethanol triggers reordering of molecular shell of proteins, that subsequently leads to protein surface deformations and mobility fluctuations of surface methyl groups.
• That, in turn, leads to side chain rearrangements, which are site dependent. Some parts of the proteins remain “solid”, while others change their spatial structures.
• Only certain proteins can undergo aforementioned changes, which indicates that certain build-up of internal structural energy should exist for the transformation to take place.
• Some of the observed changes in THz bands can be directly connected to chain deformations (or lack thereof) in proteins (for example, 80 cm-1 band in UHD of one of the antibodies reflects solvent induced torsional backbone fluctuations).
• Changes in spatial structures of proteins can lead to functional changes in UHD samples which might explain their novel binding associations and enhanced functional activity.

Fig. 3. Source: “Modeling of protein hydration dynamics is supported by THz spectroscopy of highly diluted solutions” by K. Woods.

The article showcases the potential of THz spectroscopy in exploring protein hydration dynamics in highly diluted solutions, emphasizing the critical role of technology in shaping the physical and chemical properties of these solutions. By applying molecular dynamics modeling, Dr. Woods' work has uncovered intriguing findings that pave the way for further investigations and advancements in the field.