A Further Investigation on Collagen-Cr(III) Interaction at Molecular Level

Abstract

As we know, the reaction of chrome tanning primarily occurs at carboxyl groups of collagen, tacitly assumed at aspartic and glutamic side chains. However, the differences of the reactivity and reaction extent at these sites, as well as the effect of environmental conditions on the reaction are still unclear at molecular level. This paper presents a theoretical analysis of the reactivity of collagen carboxyl groups and Cr(III) species based on electrophilicity index (ù), and the driving force for tanning reaction by using density functional theory (DFT) with Lanl2dz for chrome atom and 6-31g(d) for non-metal atoms. Glutamatic acid (Glu) and aspartatic acid (Asp) were used as the models of collagen side chains containing carboxyl group. Based on visual minteq calculation, the main Cr(III) species in chrome tanning liquor were found to be Cr(SO4)(H2O)4 +, Cr(H2O)6 3+, Cr2(OH)2(H2O)8 4+, andCr3(OH)4(H2O)10 5+ under the pH range of tanning process, and the content of multi-nuclear Cr(III) species increases with the rise of pH. The reactivity of collagen carboxyl groups is in the sequence of ionized Asp (Asp-) > ionized Glu (Glu-) > unionized Glu > unionized Asp, while the reactivity of the main Cr(III) species is sequenced by Cr3(OH)4(H2O)10 5+ > Cr2(OH)2(H2O)84+ > Cr(H2O)6 3+ > Cr(SO4)(H2O)4 +. The data ofthermodynamic driving force for tanning reaction indicate that ionized carboxyl groups have stronger tendency to form more stable carboxyl-C (III) complexes compared with unionized carboxyl groups, and the order of the tendency is Glu- > Asp- > Asp > Glu. Obviously, for Glu- and Asp- or Glu and Asp, the thermodynamic driving force is exactly the opposite with reactivity. Therefore, as for collagen carboxyl groups, the predominant reaction bonding site is determined by both thermodynamic and kinetic conditions. As for Cr(III) species, the order of thermodynamic driving force is Cr3(OH)4(H2O)10 5+ > Cr2(OH)2(H2O)8 4+ > Cr(H2O)6 3+ > Cr(SO4) (H2O)4 + which is consistent with the order of reactivity.