Petroleum tricyclic terpanes: predicted physicochemical behavior from molecular mechanics calculations

Petroleum contains a diastereomeric doublet for each of the C-25 to C-29 tr icyclic terpanes due to stereoisomerization at C-22, where the elution orde r of the 22S and 22R epimers is unknown. Geometry-optimized molecular mecha nics models for each pair of epimers show similar calculated total energies , indicating similar thermal stability. Similar stability explains the near ly equivalent size of the 22S and 22R chromatographic peaks for each double t in nonbiodegraded petroleum. Molecular mechanics MM + and COMPASS force-f ield calculations indicate an abrupt conformational change between the C-28 and C-29 tricyclic terpanes, corresponding to a discontinuity on plots of molecular mass versus log of gas chromatographic retention time. The second -eluting peak in each C-26 to C-29 doublet is more readily biodegraded, wit h (Alberdi, M., Moldowan, J.M., Peters, K.E., Dahl, J.E., 2000. Stereoselec tive biodegradation of tricyclic terpanes in heavy oils from Bolivar Coasta l Fields, Venezuela. Submitted to Organic Geochemistry) or without microbia l demethylation to form 17-nor-tricyclic terpanes. Factors controlling the chromatographic elution order of epimers are not fully understood. However, elution order can be inferred if one assumes that epimers with greater cal culated surface areas are more susceptible to microbial attack, as for the extended hopanes where C-22 epimer elution order is known. Surface areas of 22R epimers exceed 22S for C-25 to C-29 tricyclic terpanes, suggesting tha t the 22R epimers elute after 22S. Proof of elution order will require co-i njection of authentic standards. Four epimers are possible for each of the C-30 and C-31 tricyclic terpanes. Molecular mechanics and high-resolution c hromatography suggest that all four peaks occur in petroleum, but only two are normally observed due to co-elution. Complete resolution of these epime rs will require improved chromatographic methods. (C) 2000 Elsevier Science Ltd. All rights reserved.