第六届青年地学论坛

1062 / 2019-06-26 20:55:45

Predicting equilibrium intramolecular isotope distribution within a large organic molecule by Cutoff calculation

2、地球化学 > 专题13：高维度稳定同位素：原理、方法及应用

A predicted equilibrium intramolecular isotope distribution (Intra-ID) serves as a fixed reference for measured position-specific (PS) isotope composition variation in a large organic molecule. Equilibrium Intra-ID can be estimated from calculated reduced partition function ratios (RPFR or 13β factor in the case of carbon isotope system), which are largely absent at this time. For relative small molecules, the PS β factors can be directly calculated. However, current computational resources do not permit the computation of PS β factors by considering an entire large organic molecule. Isotope effect is local in that the vibrational frequencies of an atom are only affected by its proximal surroundings. Therefore, the cluster-model, which are widely used to represent minerals or solutions in β factor calculations, should be applicable to large organic molecules. Here we calculated a series of small organic molecules to test the influences of molecular size and functional groups on the β value estimation of a target position. We found that the standard deviation for methyl carbon 13β value is smaller than 0.2‰ if we consider or ignore a functional group that is three bonds away from the target. Coenzyme A (CoA) was used as an example to test the accuracy of the cluster-model on large organic molecules. The results showed that the PS 13β values of CoA calculated by the cluster-model deviates from the calculation of the entire molecule by 0.3‰. In addition, the cluster-model took 0.2% of the computation time that was required to calculate the entire molecule. We conclude that the cluster- model can provide PS 13β values with high efficiency and sufficient accuracy for large organic molecules.