S has been positively correlated with the magnitude of carbon isotope fractionation (represented by ΔC), with larger values of S corresponding with a larger values of ΔC. It has been theorized that because increasing S means the transition state is more like the product, the OC---C-2 bond will be shorter, resulting in a higher overall potential energy & vibrational energy. This creates a higher energy transition state, which makes it even harder for CO (lower in the potential energy well than CO) to overcome the required activation energy. The RuBisCOs used by varying photosynthetic organisms vary slightly in their enzyme structure, and this enzyme structure results in varying transition states. This diversity in enzyme structure is reflected in the resulting ΔC values measured from different photosynthetic organisms. However, overlap exists between the ΔC values of different groups because the carbon isotope values measured are generally of the entire organism, and not just its RuBisCO enzyme. Many other factors, including growth rate and the isotopic composition of the starting substrate, can affect the carbon isotope values of whole organism and cause the spread seen in C isotope measurements.