(443b) Solubility Measurement and SAFT-? Mie Prediction of Peptides Based on Amino Acid Residues

Peptides are short chain biomolecules containing 50 or fewer amino acids, which are known to have important therapeutic properties and huge potential application in the treatment of chronic and metabolic diseases (e.g. cancer, obesity and diabetes).^1-4Peptide crystallisation, as a good alternative to chromatographic purification, can solve the shortcomings of traditional purification method, such as high cost, proteolytic degradation and physiochemical instability. Accurate solubility data is a crucial thermodynamic property during the crystallisation, as corresponds to the stable phase boundary in the phase diagram, it is needed to characterize supersaturation, calculate nucleation parameters in the prediction and design of crystallisation conditions. Currently only limited solubility data of peptides in organic solvent systems is available in the literature.

We present a study of the chain length and side chain effect on the thermodynamic properties of several peptides and homopeptides during crystallisation to establish a rational design of the conditions for glycine homopeptide crystallisation. The solubility of glycine homopeptides (glycine, diglycine, triglycine, tetraglycine, pentaglycine and hexaglycine), amino acids with different side chains (aspartic acid, phenylalanine, histidine and tyrosine) and their dipeptides (asp-phe, gly-asp, gly-phe, phe-phe, gly-gly, tyr-phe, gly-tyr, gly-his) in water from 278.15K to 313.15K were measured using the gravimetric method.

Compared with small molecules, the solubility measurement of longer chain peptides is challenging because of their flexible structure, especially in the case of the peptides composed of more than five amino acid residues. During the crystallisation process, the adding of the salts and precipitants which stabilise the nature and the structure of the peptides also makes the measurement of solubility become more complex. In this context the development of tools to deliver accurate predictions of solubility of peptides from limited experimental data is also of critical interest.

The SAFT- γ Mie group contribution equation of state in which a heteronuclear model is implemented, with molecules split into their chemical moieties (CH₃, CH₂, NH₂, COOH, etc...) to calculate the thermodynamic properties and phase equilibrium (including solid-liquid solubility)⁵ of fluids predictively. We model the peptides and amino acids of interest divided into the relevant groups. Known group-group interaction parameters are adopted from the literature or developed in this work using literature data as well as data measured in this study. The NH₂ -COOH parameters are determined based on the thermodynamic properties of glycine, alanine and valine. These parameters can be used to predict the properties of other amino acids (alanine, valine, aspartic acid, glutamic acid, isoleucine, leucine and lysine) and peptides.

In summary, the effect of chain length and side chain on the solubility of peptides is studied and the group-group interaction matrix of the SAFT-γ Mie approach is extended for the prediction of the solubility of amino acids and peptides. This research establishes a thermodynamic foundation for the further design of peptides crystallisation process and explores the application of SAFT-γ Mie to biomolecular thermodynamic properties, providing a flatform to explore the relationship between their physicochemical properties and biostructures.

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