Understanding Microscale Thermophoresis: Contributions by simple building blocks of proteins M. Bjelčić1, D. Niether1, J.K.G. Dhont1,2, S. Wiegand1,3 1ICS-3 Soft Condensed Matter, Forschungszentrum Jülich GmbH, D-52428 Jülich, Germany 2Department of Physics, Heinrich-Heine-Universität Düsseldorf, D-40225 Düsseldorf, Germany 3Chemistry Department – Physical Chemistry, University Cologne, D-50939 Cologne, Germany

Introduction [1] System

Microscale Thermophoresis (MST) gained a lot of interest as analytical approach to monitor protein-ligand binding reactions. Since this method Propane‐1,3‐diol HO needs less sample compared to calorimetric methods it is therefore very Glycerol 1‐propanole useful for pharmaceutical applications. ‐3.33 ‐1.98 ‐1.31 ‐1.26 ‐0.81 0.24 Recent experiments for various amides and sugars Ribitol showed a clear correlation of the temperature dependence of the Soret coefficient with the OH hydrophilicity, quantitatively described by the logarithm Butane‐1,4‐diol of the 1-octanol/water partition coefficient P. To check HO OH the empirical correlation between the temperature OH OH Ethane‐1,2‐diol sensitivity of Soret coefficient as function of logP we performed additional measurements of various alcohols. Results 12 1-propanole (T) of 1‐propanol can be described with Thermophoresis 8 the empirical equation: -1 4 K 1-propanole

Flux along a temperature gradient T: -3 ethane-1,2-diol 4 propane-1,3-diol butane-1,4-diol (T) of the other polyhydric alcohols (1) Thermal diffusion DT along a temperature gradient T / 10 glycerol T ribitol

(2) Fickian diffusion D along the induced concentration S 3 follows an empirical equation suggested by [3] gradient c 2 Piazza 1 1 10 20 30 40 50 60 With increasing mass and number of Steady state (=0) defines the 1 ∆ temperature / oC hydroxy groups ∆(T) increases. ≡ Soret coefficient ST: 1 ∆ 2.1 1-propanole ethane-1,2-diol 1.8 propane-1,3-diol With increasing temperature diffusion IR-TDFRS Setup [2] -1 butane-1,4-diol s

2 1.5 glycerol increases, which is expected because the ribitol viscosity of solution decreases.

cm 1.2

-5 Diffusion coefficient decreases with 0.9 increasing chain length and number of / 10

D 0.6 hydroxy groups.

0.3

10 20 30 40 50 60 temperature / oC 1.0 1.5 8 20°C 1-Propanol o A A 30 C Around a mass fraction of 0.35 the 7 Grossman @25°C diffusion coefficient shows a minimum.

1.0 a.u. / / a.u. 0.5 -1 s 6 Only at very low concentrations single 2 0.5 molecules diffusion is observed.

intensity 5 intensity cm [4]

-6 Grossmann and Ebert explained the 0.0 0.0 4 0 1 2 3 4 5 diffusion minimum by the formation of 100 101 102 103 104 105 10 10 10 10 10 10

/ 10 3 time / s time / s clusters with eight 1‐propanol molecules D 2 and 40 water molecules in the hydration / / ς 1 th 1 / , 1 layer of the cluster. het 1 temperature / , 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 mass fraction Conclusion References

[1] D. Niether, H. Kriegs, J. K. G. Dhont, and S. 8 • Hydrophilic alcohols with Wiegand. Peptide model systems: Correlation 6 negative logP follow the between thermophilicity and hydrophilicity. J.

-1 empirical correlation. Chem. Phys. 2018, 149, 044506 cyclodextrins K 4 [2] S. Wiegand, H. Ning, and H. Kriegs. Thermal

-3 sugars • Measurements of 1‐propanol amides Diffusion Forced Rayleigh Scattering Setup 2 with a positive logP are difficult aspirin Optimized for Aqueous Mixtures. J. Phys. Chem. B / 10 1-propanole due to formation of aggregates. T 0 Ethane-1,2-diol 2007, 111, 14169-14174 S Propane-1,3-diol [3] SS. Iacopin and R. Piazza. Thermophoresis in  Hydrophilic -2 Butane-1,4-diol octanol glycerol protein solutions. Europhys. Lett., 63, 2003, 247– water compound: logP < 0 ribitol 253 -4 Exponential fit [4] G. H. Grossmann and K. H. Ebert. Formation of -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 clusters in 1-propanol-water-mixtures. Berichte Der

[solute] Bunsen-Gesellschaft-Physical Chemistry Chemical octanol logP logP  log( unionized ) [solute]water Physics, 85, 1981, 1026-1029.