Molecular and Crystal Structure of Purine Derivatives as Reflected in Solid-State Nuclear Magnetic Resonance Parameters

Name of the Project: Molecular and Crystal Structure of Purine Derivatives as Reflected in Solid-State Nuclear Magnetic Resonance Parameters

Purine derivatives represent an integral part of all living organisms. Purine-based nucleobases adenine and guanine are involved in numerous metabolic actions, purine receptors are found in all organs of the human body. A variety of purine-based compounds appear as clinical candidates for drug development. The wide range of biological functions displayed by purines, bearing different types and combinations of substituents at one or more of the seven reactive centres that make up the bicyclic structure, have recently been reviewed, both for naturally occurring and for synthesised compounds.[1,2]

Small change in molecular structure of a purine derivative can significantly influence its biological activity. Different type of substituent in the same position at the purine ring often leads to completely different biological effect. Different position of the same substituent also largely influences interactions with biological targets and hence modifies the biological function. Furthermore, position of proton in the structure is equally important as position of a bulky substituent.

In addition, one chemical substance can form different crystals. This phenomenon known as polymorphism is of importance in many industries including pharmaceutical companies. Polymorphs differ, in principle, in all their properties, so that producing the correct form for a particular application is fundamental.

Changes in molecular structure as well as differences in crystal packing are reflected in solid-state nuclear magnetic resonance (SS NMR) parameters. The relevant data come mainly from two experimental sources, namely dipolar coupling constants and chemical shifts - for the latter involving isotropic values as well as full chemical shift tensors (CSTs).

The start of high-resolution SS NMR can be dated to 1976, when the power of combining techniques of magic-angle spinning (MAS), proton-to-carbon cross polarisation (CP) and high-power proton decoupling was shown.[3] In the two decades after that, the use of this (CP/MAS) suite of techniques was dominated by applications to synthetic polymers on the one hand and zeolite-related materials on the other. During the last decade SS NMR techniques have been rapidly developing and amount of work on organic solids has increased.

Variety of complementary techniques are used to understand the relationship between the solid-state structure and the observed NMR parameters. It is common to utilize SS NMR alongside structure determination from diffraction measurements. Furthermore, recent advances in NMR chemical shift calculations[4] have significantly increased their power in prediction of solid-state structure from an examination of the observed chemical shifts.

Our project is designed to describe relations between SS NMR parameters and solid-state structure of purine derivatives. The aim is to find out, how changes in molecular structure (substitution, tautomerism) influence crystal structure (intermolecular interactions including hydrogen bonding, formation of solvates and polymorphs) of purine-based compounds, and how the structural changes are reflected in isotropic chemical shifts and CST principal components as the relevant SS NMR parameters.

We already started working in this field by analyzing two purine derivatives, hypoxanthine and 6-mercaptopurine.[5] Obtained results confirmed significance of the study and laid foundation to the project proposed here.

Due to cooperation with Dr. Michal Hocek from Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, variously substituted purine derivatives are available for the project. Most of them have already been analyzed by solution-state NMR spectroscopy, which can simplify analysis of their structure in solid state. Additional samples can be purchased from commercial sources. SS NMR isotropic chemical shifts and CST principal components of the compounds will be determined on a high-resolution Bruker AVANCE-500 spectrometer equipped with a MAS unit and a Bruker 4 mm CP/MAS probe available in our NMR laboratory. Since powder samples will be used for the measurements, orientations of the CST principal components in the molecular framework cannot be determined. Therefore, quantum-chemical calculations of the chemical shifts will be performed to obtain this data. In advance, calculations can be used to explore structural origin of the observed chemical shifts. Gaussian 03 program[6] available in our laboratory will be employed for this purpose. Crystal structures of the compounds will be obtained from Cambridge Structural Database (CSD). If the data are not available, the powder samples will be recrystallized to yield single crystals appropriate for single-crystal X-ray diffraction analysis. Identity of the powder samples with the structures obtained either from CSD or from the single-crystal X-ray diffraction will be confirmed by using powder X-ray diffraction analysis. When the analysis reveals presence of more crystal forms for the same compound (solvates or polymorphs), SS NMR and the diffraction methods will be used in complementary fashion towards their structural characterization. The single-crystal X-ray diffraction analysis will be done in cooperation with Dr. Jaromr Marek from Laboratory of Functional Genomics and Proteomics, Masaryk University, and for the powder X-ray diffraction analysis, cooperation with University of Jyvskyl, Finland (Dr. Manu Lahtinen, Department of Chemistry) was recently established.

Structural variety of available compounds enables examination of wide range of effects, which have not yet been described. Experienced from our first study of hypoxanthine and 6-mercaptopurine, we believe that reasonable time for running this project is 3 years. Results of the project are assumed to be published in international journals with impact factor higher than 2.

Prerequisite for successful completion of the project is participation of two co-workers.

Since SS NMR parameters of purine derivatives represent topic of Ph.D. study of Kateřina Maliňkov, she will be the main person responsible for SS NMR measurements, analysis of the results obtained by this method, interpretation of results obtained from SS NMR as well as from diffraction measurements in terms of molecular and crystal structure, and preparation of publications. Last year she spent 10 months working with Prof. Kay Saalwchter in Solid-State NMR Group at Martin-Luther-University Halle-Wittenberg, Halle (Saale), Germany, on a project devoted to study of CP transfer.[7] Experience gained during this work will be used in the application of CP/MAS methods in the project proposed here. Kateřina Maliňkov will defend her PhD thesis in 2010 and since 2011 she can be almost fully employed in this project.

Lucie Novosadov is well experienced in quantum-chemical calculations of NMR parameters and that is the part of the project she will be responsible for as a part-time employee.

The role of applicant in the project will be mainly regulatory. He will plan and manage the project, analyze the results, coordinate work of all researchers involved in the project, and solve problems, which may appear. As the most experienced scientist in the field of structural chemistry of purine derivatives he will also assist the interpretation of experimentally and theoretically obtained data in terms of molecular and crystal structure and supervise the preparation of publications.

References

H. Rosemeyer, Chem. Biodivers. 2004, 1, 361.

M. Legraverend, D. S. Grierson, Bioorg. Med. Chem. 2006, 14, 3987.

J. Schaefer, E. O. Stejskal, J. Am. Chem. Soc., 1976, 98, 1031.

J. C. Facelli, Concepts Magn. Reson. Part A, 2004, 20A

K. Maliňkov, L. Novosadov, M. Lahtinen, E. Kolehmainen, J. Brus, R. Marek, J. Phys. Chem. A, 2010, 114, 1985.

GAUSSIAN 03, Revision D.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, and J. A. Pople, Gaussian, Inc., Wallingford CT, 2004.

M. F. Cobo, K. Maliňkov, D. Reichert, K. Saalwchter, E. R. deAzevedo, Phys. Chem. Chem. Phys., 2009, 11, 7036.