Browsing by Author "Silva, Carlos M."
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- Application of Fourier transform infrared spectroscopy and orthogonal projections to latent structures/partial least squares regression for estimation of procyanidins average degree of polymerisationPublication . Passos, Cláudia P.; Cardoso, Susana M.; Barros, António S.; Silva, Carlos M.; Coimbra, Manuel A.Fourier transform infrared (FTIR) spectroscopy has being emphasised as a widespread technique in the quick assess of food components. In this work, procyanidins were extracted with methanol and acetone/water from the seeds of white and red grape varieties. A fractionation by graded methanol/chloroform precipitations allowed to obtain 26 samples that were characterised using thiolysis as pre-treatment followed by HPLC-UV and MS detection. The average degree of polymerisation (DPn) of the procyanidins in the samples ranged from 2 to 11 flavan-3-ol residues. FTIR spectroscopy within the wavenumbers region of 1800-700 cm(-1) allowed to build a partial least squares (PLS1) regression model with 8 latent variables (LVs) for the estimation of the DPn, giving a RMSECV of 11.7%, with a R(2) of 0.91 and a RMSEP of 2.58. The application of orthogonal projection to latent structures (O-PLS1) clarifies the interpretation of the regression model vectors. Moreover, the O-PLS procedure has removed 88% of non-correlated variations with the DPn, allowing to relate the increase of the absorbance peaks at 1203 and 1099 cm(-1) with the increase of the DPn due to the higher proportion of substitutions in the aromatic ring of the polymerised procyanidin molecules.
- Solubility of amino acids by group-contribution methodPublication . Macedo, Eugénia A.; Pinho, Simão; Silva, Carlos M.Crystallization and precipitation are widely used for the separation of proteins. Therefore, the study of the solubility of amino acids, peptides, and proteins is of fundamental importance for the design of separation units. The correlation and prediction of the solubilities of amino acids are the basis for further progress in this area. However, the successful representation of the solubilities depends on the ability to correlate and predict the activity coefficients of the amino acids in solution. In this way, several models have been proposed in the last few years: Nass (1988) has assumed the activity coefficients of amino acids to be a product of two terms due to chemical reaction equilibrium and physical interactions using for this the Wilson equation. Although the results are satisfactory, they are limited to a few amino acids; Chen et al. (1989) combined electroestatic and physical interactions (NRTL model) without considering the chemical equilibrium, which they recognize Lo be very important for same ranges of pH; Gupta and Heidemann (1990) have considered only short range interactions using the UNIFAC model, but the results were very poor. Orella and Kirwan (1991) tried to correlate solubility data of amino acids in mixtures with alcohols using an excess solubility approach based on three different models: Margules, Wilson and NRTL. The best approach is the one related Lo Wilson equation, with deviations smaller than 15%. The major disadvantage of the referred models comes from the fact that they can’t be used for prediction purposes and therefore are of limited application. In this work a generalized thermodynamic model has been developed based on the UNIFAC method; new groups have been defined according to the group-contribution concept, and chemical equilibrium is taken into account simultaneously with the physical equilibrium.
- Solubility of amino acids: a group-contribution model involving phase and chemical equilibriaPublication . Pinho, Simão; Silva, Carlos M.; Macedo, Eugénia A.A new model is proposed to represent the solubility behavior of 14 amino acids and 5 small peptides in water. The UNIFAC model is combined with a Debye-Huckel term to describe the activity coefficients of the species present in the biomolecule/water system. New groups have been defined according to the group-contribution concept, and chemical equilibrium is taken into account simultaneously with the physical equilibrium. To estimate the new interaction parameters, molal activity coefficient data from the literature were used. These parameters, in addition to solubility data, were the basis for the correlation of the solubility product of the amino acids. Using this approach, satisfactory results were obtained in the representation and prediction of the solubilities of amino acids in aqueous solutions at different conditions of temperature and pH.