Fig. 6: Amino acids can abolish cluster formation at very low concentration.

a Molecular structure of amino acids investigated. Molecular orientations depict averages observed in MD simulations at a silica interface. Average interfacial dipole moment vectors for proline and tyrosine (green arrows) imply normal components that are oriented downward, opposite to that of pure water at an interface (see Fig. 2a). b Digitised images of negatively charged SiO₂ particles suspended in various concentrations of net-neutral amino acids in water, shown here for serine and tyrosine (see Supplementary Fig. 24 for all amino acids studied). The interparticle attraction significantly weakens at \({c}_{{{{\rm{b}}}}}=0.5\) mM for serine and \(0.1\) mM for tyrosine and vanishes at higher concentrations. Scale bars \(20\) μm. c Inferred pair-interaction potentials \(U(x)\) for serine and tyrosine-containing solutions inferred from BD simulations (see Supplementary Table 28 for parameters). Error bars denote estimated uncertainties of ± 100 nm on particle diameter and ± 1.5 \({{{{\rm{k}}}}}_{{{{\rm{B}}}}}T\) in \(w\). d Normalised well depths, \(w/{w}_{\max }\), as a function of \({c}_{{{{\rm{b}}}}}\) with sigmoidal fits to the data (coloured lines). At \({{c}_{{{{\rm{b}}}}}=c}_{1/2}\), the clustering strength is half-maximal, i.e., \(w/{w}_{\max }=\) 0.5. e Plot of measured \({{\mathrm{ln}}}{c}_{1/2}\) vs. PMF minima values, \(u\), taken from MD simulations in ref. ⁴⁴ (symbols) with a linear fit (black dashed line). f Excess interfacial dipole moment densities, \({\mu }_{{{{\rm{av}}}}}(0)\), from MD simulations of \({c}_{{{{\rm{b}}}}}=1\) M aqueous solution of amino acid in contact with a neutral silica surface. Error bars depict uncertainty arising from simulation convergence (see MD simulation methods). \({\mu }_{{{{\rm{av}}}}}(0)\) values for the amino acid-containing electrolyte are substantially lower than the pure water value, \({\mu }_{{{{\rm{av}}}}}\left(0\right)\approx 0.3\) D nm⁻² (blue dashed horizontal line), with the magnitude decreasing with the increasing surface affinity of the amino acid reflected in \(\left|u\right|\). g Averaged amino acid density profiles in solution, \({\rho }_{{{{\rm{aa}}}}}\) (as a function of distance from the silica surface (\(z-{z}_{{{\mathrm{int}}}}\))), for the simulations described in (f), present qualitative trends of the interfacial amino acid concentration\(,{c}_{{{{\rm{s}}}}}\), which is highest for tryptophan and lowest for glycine.