Collision-induced dissociation Fourier Transform ion cyclotron resonance mass spectrometry (CID-FTICR MS) was developed to determine structural building blocks in heavy petroleum systems. Model compounds with both single core and multicore configurations were synthesized to study the fragmentation pattern and response factors in the CID reactions. Dealkylation is found to be the most prevalent reaction pathway in the CID. Single core molecules exhibit primarily molecular weight reduction with no change in the total unsaturation of the molecule (or Z-number as in chemical formula C(c)H(2c+Z)N(n)S(s)O(o)VNi). On the other hand, molecules containing more than one aromatic core will decompose into the constituting single cores and consequently exhibit both molecular weight reduction and change in Z-numbers. Biaryl linkage, C(1) linkage, and aromatic sulfide linkage cannot be broken down by CID with lab collision energy up to 50 eV while C(2)+ alkyl linkages can be easily broken. Naphthenic ring-openings were observed in CID, leading to formation of olefinic structures. Heavy petroleum systems, such as vacuum resid (VR) fractions, were characterized by the CID technology. Both single-core and multicore structures were found in VR. The latter is more prevalent in higher aromatic ring classes.

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synopsisThe enthalpy of hydrogen-bond formation between guanine (G) and cytusine (C) in o-dichlorobenzene and in chloroform at 25°C has been determined by direct calorimetric measurement. We derivatized 2'-deoxyguanosine and 2'-deoxycytidine at the 5'-and 3'-hydroxyls with triisopropylsilyl groups; these group increase the solubility of the nucleic acid bases in nonaqueous solvents. Such derivatization also prevents the r i b hydroxyls from forming hydrogen bonds. Consequently, hydrogen-bond formation in our system is primarily between the bases, and to a lesser extent, between base and solvent, and can be measured directly with calorimetry. To obtain the data on baae-pair formation, we first took into account the contributions from self-association of each base, and where possible, have determined the AH of self-association. From isoperibolic titration dorimetry, our measured AH of C2 formation in chloroform is -1.7 kcal/mol of C. Our measured AH of C : G base-pair formation in o-dichlorobenzene is -6.65 0.32 kcal/mol. Since o-dichlorobenzene does not form hydrogen bonds, the AH of C : G base-pair formation in this solvent represents the AH of the hydrogen-bonding interaction of C with G in a nonassociating solvent. In contrast, our measured AH of C : G basepair formation in chloroform is -5.77 f 0.20 kd/mol; thus, the absolute value of the enthalpy of hydrogen bonding in the C : G base pair is greater in o-dichlorobenzene than in chloroform. Since chloroform is a solvent known to form hydrogen bonds, the decrease in enthalpic contribution to C : G base pairing in chloroform is due to the formation of hydrogen bonds between the bases and the solvent. The AH of hydrogen bonding of G with C reported here differs from previous indirect estimates: Our measurements indicate the AH is 50% less in magnitude than the AH based on spectroscopic mensurements of the extent of interaction. We have also observed that the enthalpy of hydrogen bonding of C with G in chloroform is greater when G is in excess than when C is in excess. This increased heat is due to the formation of C : G, > complexes that we have observed using H-nmr. Although C:G2 structures have previously been observed in triplestranded polymeric nucleic acids, higher order structures have not been observed between C and G monomers in nonaqueous solvents until now. By using monomers as a model system to investigate hydrogen-bonding interactions in DNA and RNA, we have obtained the following results: A direct measurement of the AH of hydrogen bonding in the C : G complex in two nonaqueous solvents, and the first observation of C : G,, , complexes between monomers. These results reinforce the importance of hydrogen bonding in the stabilization of various nucleic acid secondary and tertiary structures.

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