Structural and spectroscopic study of a pectin isolated from citrus peel by using FTIR and FT-Raman spectra and DFT calculations

MARÍA V. FIORI BIMBI; SILVIA ANTONIA BRANDÁN; MARÍA V. FIORI BIMBI; SILVIA ANTONIA BRANDÁN; LAURA C. BICHARA; HUGO VACA; LAURA C. BICHARA; HUGO VACA; PATRICIA E. ALVAREZ; CLAUDIO GERVASI; PATRICIA E. ALVAREZ; CLAUDIO GERVASI

INFRARED PHYSICS & TECHNOLOGY

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2016 vol. 76 p. 315 - 327

1350-4495

In this work, pectin isolated from citrus peel with a degree of esterification of 76% was characterized byFourier Transform Infrared (FTIR) and Fourier Transform Raman (FT-Raman) spectroscopies. Structuralstudies were carried out taking into account their partial degree of esterification and considering thepolygalacturonic acid chain as formed by two different subunits, one with both COOH and COOACH3groups (Ac) and the other one as constituted by two subunits with two COOACH3 groups (Es). Their structuralproperties, harmonic frequencies, force fields and force constants in gas and aqueous solutionphases were calculated by using the hybrid B3LYP/6-31G⁄ method. Then, their complete vibrational analyseswere performed by using the IR and Raman spectra accomplished with the scaled quantum mechanical(SQM) methodology. Reactivities and behaviors in both media were predicted for Ac and Es by usingnatural bond orbital (NBO), atoms in molecules (AIM), and frontier orbitals calculations. We report forfirst time the complete assignments of those two different units of polygalacturonic acid chain whichare the 132 normal vibration modes of Ac and the 141 normal vibration modes of Es, combining the normalinternal coordinates with the SQM methodology. In addition, three subunits were also studied.Reasonable correlations between the experimental and theoretical spectra were obtained. Thus, this workwould allow the quick identification of pectin by using infrared and Raman spectroscopies and also providesnew insight into the interactions that exist between subunits of a large pectin chain.