N-Carboxyimidazolidone (1) and O-carboxyimidazolidone (2) were theoretically investigated as a model of carboxybiotin. The planar structure of 1 including CO2 on the molecular plane is more stable than the perpendicular one by 5.0 kcal/mol (MP4SDQ/6-31+G*). The CO2 binding energy is 26 kcal/mol for 1 and 15 kcal/mol for 2 (MP4SDQ/6-31+G*), where a standard (energy 0) is the sum of CO2 and deprotonated imidazolidone (abbreviated as imidazolidone- hereafter) taking their equilibrium structures. When solvated species such as imidazolidone(-)3H(2)O and CO2-3H(2)O systems are taken as standard, the CO2 binding energy is 27 kcal/mol for land -3 kcal/mol for 2 (SCRF-HF/6-31+G*). These results are in accord with the experimental result that only N-carboxybiotin was isolated in the biotin-dependent enzymic reaction. Population analysis indicates that the charge transfer from imidazolidone(-) to CO2 is of particular importance in carboxyimidazolidone and this charge transfer is greater in 1 than in 2. Because of this charge transfer interaction, the CO2 part in 1 and 2 is considerably activated for electrophile like transition metal eta(1)-C coordinated CO2 complexes. Various simple model compounds of carboxybiotin are compared with 1 and 2, and their reliability is discussed in detail.