Mechanism of Degradation of Rice Starch Amylopectin by Oryzenin Using ONIOM Quantum Calculations [DFT/B3LYP/6-31+G(D, P): AM1]

Understanding the molecular factors of rice degradation during its aging concerns our research team. This article emphasizes oryzenin-amylopectin. It aims to reveal the mechanism of amylopectin deterioration during rice aging. The research exploits the Natural Bond Analysis and ONION method at theory level DFT/B3LYP/6-31+G(d, p) and AM1. This methodological approach allows highlighting amylopectin transformation; oryzenin converts amylopectin into amyloidosis in continuous. This led to monosaccharides and disaccharides.


Introduction
Rice is the staple food of the Ivorian people. However, Côte d'Ivoire is struggling to fulfill food self-sufficiency in relation to this product. Post-harvest losses are the main cause. They're explained by the inefficiency of storage techniques or its alteration during storage. Understanding the molecular factors of this modification concerns our research team. This work follows up on two others on the subject. The first focuses on the roles of phospholipids in this process [1]. The second analyzes the oryzenin-amyloidosis interaction [2]. This article emphasizes oryzenin-amylopectin. It aims to elucidate the role of this latter in the degradation of rice.
Previous work on amyloidosis establishes that its degradation occurs due to its transformation into polysaccharides and monosaccharides [2]. On the contrary, authors [3] explain its alteration by the slow transformation of amylose into amylopectin. They conjecture that this transformation results in a compensatory increase in the second. This debate suggests the following research question: How does oryzenin interact with amylopectin? This work aims to understand the mechanism that degrades the components of starch; literature ignores it. Moreover, theoretical studies on amylopectinoryzenin remain unknown. The research targets to contribute to filling this gap. It assumes that the hydrogen bond (HB) between oryzenin and amylopectin decomposes this latter; its formation impacts the stability of many molecules in the solid state and in solution. It justifies those of molecular structures such as DNA, water [4] [5] and carboxylic acids [6] [7].
HB influences some physical constants such as the melting point temperature of chemical compounds. To answer the research question, this work utilizes the resources of theoretical chemistry. It deploys NBO (Natural Bond Orbital) analysis [8]; it uses charge transfer (CT) [9] [10] [11] to describe the formation of HB as in [2]. Additionally, it exploits the branching model to guide calculations of oryzenin-amylopectin interactions. It relies on an amylopectin subunit; this contains three (AMP3G) or four (AMP4G) glucose molecules linked by sugar bridges. Also, the research employs the resources of quantum chemistry.
This work applies the ONION method at precision [DFT/B3LYP/6-31+G(d, p): AM1] and AM1. Furthermore, amylopectin molecules schematize with three (AMP3G) and four (AMP4G) molecules of α-D glucose or synthons connected by osidic bridges. This methodology highlights the mechanism shaping the degradation of amylopectin under the action of oryzenin. This knowledge, associated with that of the amyloidosis degradation, helps to clarify the mechanism underlying the degradation of rice starch during its prolonged storage. This article consists of three parts.
The first details the results of the Natural Bond Orbital (NBO) analysis. The second focuses on the AMP3G-oryzenin interaction. The third examines the one between the latter and AMP4G. These last two parts present the geometrical, energetic, and spectroscopic parameters of these complexes. Also, it discusses the convergent or divergent aspects between the results obtained with amyloidosis. Before, the article summarizes the hardware and the materials and the method of research.

Materials and Method of Research
This work retains the strategy used to study the amylose-oryzenin interaction. The amylopectin subunits comprise three (AMP3G) or four (AMP4G) glucose molecules linked by osidic bridges. The arrangement of glucose molecules considers the branching of amylopectin. The amylopectin-oryzenin complex is divided into two parts. The ONIOM/[DFT/B3LYP/6-31+G(d, p): AM1] offers the possibility of calculating its parameters with different levels of precision [12] [13]. The active part concerns the interaction site between the hydrogen of oryzenin and the oxygen of amylopectin. This part undergoes a calculation with high precision DFT/(B3LYP/6-31+G(d, p)). On the contrary, the remaining part of the complex is treated with the low precision semi-empirical AM1 method. Moreover, the research uses Gaussian 09 software to optimize all the parameters of the complexes [14].
Vibration frequency calculations help validate their geometric parameters. From an ideal structure, a "single point" calculation at the same level of theory offers the opportunity to perform the NBO calculation [15]. As in the amylose-oryzenin complexes, the interactions considered are those established between osidic oxygen of amylopectin (by its subunits) and oryzenin-hydrogen. The numbering of the atoms is generated automatically by the GaussView06 software. The electrostatic potential [16] [17] [18] [19] at the level of the B3LYP/ 6-31+G(d, p) theory makes it possible to identify the HB donor or acceptor sites of oryzenin [2]. More, this article presents and discusses its results.

Results and Discussion
This section presents the results related to the interactions between the two polysaccharides of the study and oryzenin. It specifies those linking to the NBO analysis. Table 1 and Table 2 present the stabilization energies E (2) , the differences Table 1. Stabilization energy of second-order E (2) perturbations in the AMP3G-oryzenin complex.

Interactions
Transitions  between the energies of NBO electron donors (i) and NBO electron acceptor (j). They also show the elements F(i, j) of the Fock matrix and the CT. The model of the latter, constructed by Reed and al [11], makes it possible to describe the interactions in the AMP3G-oryzenin and AMP4G-oryzenin complexes.

NBO Analysis of the Amylopectin (AMP3G/AMP4G)-Oryzenin Complexes
The NBO analysis presents the results for which the stabilization energies are greater than 1. with E (2) = 10.59 kcal·mol −1 and CT = 20.98 me The CT of 20.98 suggests to me that the strong HB in AMP3G-oryzenin comes from the oryzenin-H 30 …O 99 interaction. For those with the N 9 H 10 of oryzenin, no CT is observed with chlorine. Its position prohibits it within the complexes. The most important transfers are those concerning O 49 and N 9-H 10 bonds. They're − For the interaction N 9 -H 10 …O 49 , → with E (2) = 7.24 kcal·mol −1 and CT = 11.40 me. HB is strong with the oryzenin-H30 probe associated with O 99 in the AMP3G and in AMP4G complexes. This interaction results from the lone chlorine pairs of oryzenin. It initiates the transformation of amylopectin at its branching level. The research exploits this result to construct the ideal complexes. It uses it to model the interactions between oryzenin and AMP4G or AMP3G. In previous research [2], the electrostatic potential of molecular interaction sites indicated that the oryzenin H 30 and H 10 constitute the highest potential V S,max . This work appropriates this result to study AMP3G.

Interaction AMP3G-Oryzenine
These atoms illustrate the hydrogen donor potential of the oryzenin. The geometrical, energetic, spectroscopic parameters and the NBO analysis relate to the interactions involving this hydrogen. Figure 1 shows the geometries of the optimized complexes. A red ball represents an oxygen atom. A big white corresponds to a carbon. A small one schematizes hydrogen.   [24]. On the other hand, the α and β associated with the N 9 H 10 …O 49 interaction deviate from the ideal values. These geometrical parameters establish that oryzenin-H 30 …O 99 is the strongest HB. Table 4 shows the variations of the enthalpy, the entropy and the free enthalpy of reaction linked to the interactions between AMP3G and oryzenin. Those relating to the enthalpies of reaction are all negative. They fluctuate between −2.730 kcal·mol −1 and −20.175 kcal·mol −1 . These values suggest that all reactions are exothermic. Those associated with the oryzenin-H 30 …O 99 interaction represent the weakest.
This interaction corresponds to the most stable. Its free enthalpy of reaction (∆rG = −5.30 kcal·mol −1 ) indicates that its formation is spontaneous. The other interactions (N 9 H 10 …O 49 and N 9 H 10 …O 99 ) are exothermic but not spontaneous (∆rG > 0). Figure 2 shows free enthalpies of reaction changes associated with AM3G-oryzenin and AMP3G-oryzenin complexes. It illustrates that the others (N 9 H 10 …O 49 and N 9 H 10 …O 99 ) aren't spontaneous (ArG > 0).
These results suggest the degradation modality of amylopectin; this process begins with the establishment of HB oryzenin-H 30 …O 99 . It breaks the osidic bridges of the ramification. It leads to the formation of amyloidosis. Its production linked to this process and its natural disappearance [2] occurs concomitantly. They explain the slow decrease in its rate presence in the starch during the aging of the rice. Furthermore, spectroscopic parameters precise this finding.   The rupture of its osidic bridge conducts to the formation of this starch latter component. This result refutes the conclusions of [3]. Its authors conjecture that the storage of rice favours the fall in the proportion of amylose in the starch; they affirm that an increase in that of amylopectin in the same proportion simultaneously. Thus, degradation of rice starch changes the amylose-amylopectin ratio.
This result corroborates the observations of Cao and al [25] regarding the variation over time of this latter. The first degradation of amylopectin also supports the conclusions drawn from the analysis of geometric, energetic, and spectroscopy. Moreover, this research concerns the AMP4G-oryzenin complex.

Interaction AMP4G-Oryzenin
This section presents and discusses the results related to the geometrical, ener-  Table 6 shows the geometric parameters (d, α, β) of the interactions characterizing HB in the AMP4G-oryzenin complex. Its data agree with those established by Desiraju and al [22].   Its distance d respects the criterion of Desiraju and al [22]. Its angle α and β remain close to their ideal values. These parameters lead to the conclusion that    The O 99 site is the most favourable for the formation of HB. This suggests that AMP4G mostly breaks down from there. Its deterioration begins with this osidic bridge in position α (1.6). The rupture of the latter promotes the formation of linear α-D-glucose chains; it leads to amyloidosis. These consist of tetra saccharides. They degrade according to the mechanism described by [2]. Then, this work elucidates the mechanism of starch degradation following the appearance of amyloidosis. Figure 4 shows the free enthalpy changes associated with interactions between O 29 H 30 or N 9 H 10 and the appropriate proton acceptor sites of AM4G and AMP4G. Its data reveal that all variations linked to AM4G and AMP4G are negative for the O 29 H 30 probe. The modifications related to the interactions between N9H10 and AM4G are also. The formation of HB following oryzenin-O 29 H 30 -AMP4G or  In other words, their thermodynamic similarity doesn't make it possible to detect the second stage of the AMP4G disintegration during its progressive transformation into AM4G. This limit justifies the choice of spectroscopy. This can explain degrada-tion order of the two saccharides in this phase deterioration; the local nature of the phenomenon pleads for this approach.
For the same oryzenin-H 30 probe, the two oxygen osidic bridges O(1) and O(2) possible for the oryzenin-H 30 …O 91 interaction, the difference in frequency between the free and associated forms is 257 cm −1 . Moreover, that between AM4G and AMP4G is −30 cm −1 for O(1) and +7 cm −1 around O(2) as the receptor site. The frequency variation is greater for amylopectin near O(1) compared to amyloidosis. It reflects a stronger attraction of oxygen from the osidic bridge oxygen O 91 towards H 30 . It breaks the latter more easily.
The degradation of amylopectin into amylose is carried out continuously when the first interact with O 91 . The same reasoning leads to similar conclusions with O 99 and O 49 . In the latter case, the transformation of AMP4G takes place around O(1) mainly; a smaller proportion comes from its O(2). Furthermore, this research extends to NBO analysis of the AMP4G-oryzenin and AMP3G-oryzenin complexes.

Conclusions
This work aims to clarify the chemical processes underlying the degradation of amylopectin with oryzenin. It exploits resources from quantum chemistry as a method. It uses NBO analysis and calculates the geometric, energetic, and spectroscopic parameters of the AMP3G-oryzenin and AMP4G-oryzenin complexes. suggests that the degradation of amylopectin is easier than that of amylose. So, these forces readily break the branching of amylopectin. Its degradation into amyloid becomes continuous. The latter deteriorates gradually according to the mechanism described by [2]; its products are monosaccharides and disaccharides during rice aging or storage. This finding contributes to the debate on rice starch degradation; it refutes the conclusions of [3].
According to [3], the storage of rice promotes a reduction in the proportion of amylose in the starch; it causes an equivalent increase in that of amylopectin. On the other hand, the observations of Cao et al. [25] confirm the findings of this research. These authors state that amylopectin is degraded before amyloidosis. Moreover, research reinforces the team's findings [2]. It specifies that the deterioration of rice starch begins with that of its amylopectin. Therefore, it explains the origin of disaccharides and monosaccharides during the aging of rice. Their presence is linked to two types of amyloidosis. Some of it comes from the continual breakdown of amylopectin. The other relates to amyloid, a constituent of starch. Moreover, the article highlights the main difference between amylopectin and amyloid regarding action of oryzenin.
The attraction of oxygen from the saccharide bridge by H 30 differs between amylopectin-oryzenin and amyloid-oryzenin. The O-H stretching frequency difference associated with it exceeds that of the second complex. Simply, the attraction of the oryzenin's H 30 for amylopectin's oxygen on the osidic bridge remains higher than that observed for amyloid. This result constitutes further evidence that amylopectin degrades before amyloid in rice starch.