Prof. Robert Glaser, Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel 1

Special Topics in Advanced , a course

Concepts and Educational Goals presented in the course.

A. Introduction to the special topics course. points covered: 1. Models versus the 'real thing'. 2. Relationships between geometrical figures: isometry transformations, congruency , isometric and anisometric structures . 3. Symmetry : the equilibrium framework of the molecule, exchange of subunits without altering the identity or orientation of the molecule. 4. Eight symmetry operations: four of the 1st Kind and four of the 2nd Kind . 5. Reflection : the basic symmetry operation and the parity of its application. Even parity of reflections preserves the handedness of the permuted subunits, and an odd parity of reflections inverts the handedness of the permuted subunits. 6. Symmetry elements and symmetry operations : permutation of subunits versus invariant positions of points. 7. Group theory and symmetry operations: subunits permuted by symmetry operations fall into 'symmetry equivalent sets', elements of the set, binary operation of the group is ' multiplication ' or the ' successive application ' of two or more symmetry operations. 8. Point groups versus space groups . 9. Molecular dissymmetry and symmetry point groups: older term ' dissymmetric ' versus newer term ' chiral '. Chiral molecules having point groups containing (a) only the identity ( E) operation or (b) E plus one or more proper rotation axes ( Cn) are dissymmetric or chiral. Only chiral molecules having point groups containing the identity operation ( C1) as the sole symmetry operations are also ' asymmetric ' (totally lacking in symmetry). The Icosahedral symmetry viral capsid of proteins has I-symmetry (not Ih) due to the L-amino acids therein, and exhibits 60 symmetry operations all of the 1st Kind- it cannot be called ' asymmetric '. 10. While all asymmetric molecules are chiral, not all chiral molecules are asymmetric! 11. Metric or scalar properties versus pseudo-scalar properties, enantiomorphic shapes and enantiomeric molecules. 12. Molecular Structure: ensemble of particles vibrating with small amplitude around well-defined equilibrium positions in space. 13. Constitution and bonding parameters : bond lengths, bond angles, dihedral angles versus torsion angles. 14. Isomeric structures: isometric versus anisometric isomers, homomers (a congruent pair), , , and constitutionally heteromeric isomers . Differentiation in an achiral environment versus in a chiral environment. 15. Equivalence and non-equivalence of molecular subunits: topicity , internal versus external comparisons, homotopic, enantiotopic, diastereotopic, and heterotopic relationships. 16. and local site-Symmetry: chirotopicity versus achirotopicity, general versus special positions of symmetry in crystals or objects or molecules. 17. General positions of symmetry have only identity (E) symmetry. Chirotopic positions are either general positions or special positions of Cn-rotational symmetry, while achirotopic positions are always special positions of Sn-symmetry [remembering that when n = 1, S1 = σ, Prof. Robert Glaser, Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel 2

and when n = 2, S2 = i]. 18. Stereogenic elements : structural features giving rise to the generation of an isomer. 19. Stereogenicity is not related or correlated with local site-symmetry . 20. Van't Hoff's problem with the 'pseudo-asymmetric' carbon atom in appropriately constructed meso-structures. Rule versus Pseudo-Rule (or Rule versus Anti-Rule, e.g. the Octant and Anti- Octant Rules)- there is no rule. 21. Racemic Mixtures , Racemic Compounds , and Conglomerates of Chiral Crystals .

B. Chiral Discrimination, Helical Stereochemistry, & Chiral Apple Halves. points covered: 1. Cinquini and Cozzi chemical example of a ' La Coupe du Roi '. 2. 'La Coupe du Roi ' a polysection operation in solid geometry consisting of a desymmetrization of an achiral object into an ensemble of n-homochiral homomeric parts representing the repeat unit of a 2n/n -symmetry multiplex (a multistrand helix) of n-parallel chains. 3. Achiral objects loose all symmetry operations of 2nd kind when polysectioned by the coupe du roi cuts. Review: The eight symmetry operations. 3. Review: Symmetry Operations of the First Kind (preserving the handedness of the subunits that are exchanged): identity [ E], rotation [ Cn], translation, screw displacement [ n/m ]. 4. Review: Symmetry Operations of the Second Kind (inverting the handedness of the subunits that are exchanged): reflection [ σ], inversion [ i], rotatory-reflection [ Sn], glide reflection. 5. 'La Coupe du Roi ' is a desymmetrization. 6. 'La Coupe du Roi ' bisection affording a D2-symmetry ensemble of two homochiral halves, a trisection affording a D3-symmetry ensemble of three homochiral thirds, or a polysection affording a Dn-symmetry ensemble of n homochiral subunits. 7. Bisection of an object by ' La Coupe du Roi ' followed by translation of one of the halves affords a full turn of either a 41 right-handed single helix or a 43 left-handed single helix. 8. A ' La Coupe du Roi ' polysectioned object represents the unit-cell [ translational repeat unit] of a 2n n multiplex of n intertwined homochiral helices. 9. Uniqueness of the screw displacement operation due to its inherent . 10. Helicity is a stereogenic element . 11. Building of n1 right-handed ( P)-helices. 12. Building of nn–1 left-handed ( M)-helices. 13. The screw displacement operation can relate either individual molecules in a crystal lattice or subunits ligated together in a polymer chain. 14. A 2n n screw displacement is an inherently achiral arrangement unless made chiral by subunit ligation into n parallel chains. 15. Periodicity in crystals, the unit cell, and the asymmetric unit. 15. Tropicity (directionality). 16. A ' La Coupe du Roi ' bisection of a cube into two homochiral halves preserves three C2-axis but destroys its C3- and C4-axes. 17 Chiral discrimination . The two homochiral halves of the cube, readily intermesh to reform a cut cube, but two heterochiral halves of the cube can not. 18. Mislow and Anet chemical example of the ' inverse coupe du roi ' based upon chiral Prof. Robert Glaser, Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel 3

discrimination of two homochiral subunits versus two heterochiral subunits. 19. In a very intricate series of cuts, a cube can be bisected into two heterochiral halves in which a C3-axis is preserved [J. Jacques]. The resulting i-symmetry relationship between the two halves is representative of a common type of arrangement between enantiomeric molecules in many racemic compound crystals. 20.Chiral discrimination based on helical or screw sense of gas pressure regulator values. 21. Meso due to i-symmetry, and not σ-symmetry. 22. Intertwining of homochiral helices around a common axis. 23. Chiral discrimination . The extended intermeshing of heterochiral versus local intermeshing of homochiral helices. 24. Perkins chemical example of chiral discrimination enabling maximum π-π interactions via extended intermeshing of two heterochiral three-strand helices. 25. CRO gene regulating protein has a helical inprint engineered by Nature in its surface to enable intermeshing into the right-handed DNA helix. 26. Plectonemic coiling of chains in one full-turn repeat units of anti-parallel duplexes versus non- plectonemic coiling of chains in fractional-turn repeat units of parallel duplexes. 27. Diastereotopic versus homotopic grooves in duplexes. 28 Glaser chemical examples of ' la Coupe du Roi ' polysectioned ensemble 'unit cells' illustrated by the 1/2-turn translational repeat unit of the polyriboadenylic acid duplex and the 1/3-turn of the Curdlan polysaccharide triplex. 29. Chemical examples of the last ' la Coupe du Roi ' bisection steps at which the chirality of the ensemble is set: stepwise ligation of Ag(I)/Ni(I) ions to odd-parity units of bidentate ligands in J. M. Lehn's 21-symmetry metal helicates.

C. Symmetry in NMR Spectroscopy, General and Special Positions in Crystals, and in M. C. Escher's Periodic Drawings. points covered: 1. Review: Eight symmetry operations: four of the 1st kind (identity, rotation, translation, and screw displacement) and four of the 2nd kind reflection, inversion, rotatory-reflection, and glide reflection). 2. Review: The three operations found only in space groups all involve 'translation'. 3. Symmetry in NMR spectroscopy. 4. in low viscosity isotropic liquids: molecules undergo rapid tumbling by Brownian motion and fast topomerizations of non-summetry equivalent nuclei can occur via rapid rotation around single bonds. These motions give rise to a 'time-averaged' environment throughout the NMR sample tube vis-à-vis the external magnetic field. 5. Solid-state cp/mas NMR spectra are more complex due to the confines of the crystal lattice. 6. Symmetry non-equivalent methylene protons in a simple molecule such as 2-bromo-propane 1,3-diol give rise to complicated sub-spectra. 7. Two nuclei give the same NMR signal ( i.e. they are ' isochronous ') if and only if: (a) the nuclei can be exchanged by a symmetry operation of the 1st or 2nd kind, OR (b) they are exchanged by a rapid topomerization on the NMR time-scale. 8. Review: Topic (topos) relationships are comparisons of sub-regions of an object or molecule. 9. Review: Internal comparison : if the sub-regions are in the same object or molecule. 10. Review: External comparison : if the sub-regions are in different objects or molecules. 11. Review: Homotopism in NMR : if the sub-regions are exchanged by a symmetry operation of the Prof. Robert Glaser, Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel 4

1st kind, and can never be differentiated either in an achiral or a chiral environment. 13. Review: Enantiotopism in NMR : if the sub-regions are exchanged by a symmetry operation of the 2nd kind, and cannot be differentiated in an achiral environment, but can be differentiated in a chiral environment since they then become 'diastereotopic'. 14. Review: Diastereotopism in NMR : if the sub-regions contain the same number and types of atoms AND they are ligated together with the same 'constitution', e.g. methyl groups in menthol. They can always be differentiated either in an achiral or a chiral environment. 15. Review: Heterotopism in NMR : if the sub-regions contain the same number and types of atoms AND they are NOT ligated together with the same 'constitution', e.g. methyl groups in 2- butanol. They can always be differentiated either in an achiral or a chiral environment. 16. Desymmetrization is lowering the symmetry. 17. Review: Chirotopicity : chiral local environments occupied of atoms in 'general or special' positions of symmetry of the 1st kind. 18. Review: Achirotopicity : achiral local environments occupied of atoms in 'special' positions of symmetry of the 2nd kind. 19. Review: What something is" or 'The properties that that something exhibits' ( i.e. its topic relationship) depends on 'where it is located'. 20. Introduction to crystal lattices : M. C. Escher's periodic drawing of chiral asymmetric fish and chiral asymmetric frogs is broken down into 'repeat' units [ i.e. 'unit cells'] which fill space by the translation operations. 21. Every point group must contain at least the ' identity ' operation. 22. Every space group must contain at least the ' identity ' and the ' translation ' operations. 23. Chiral point or space groups ONLY contain elements which are the symmetry operations of the 1st kind. 24. 'General positions of symmetry ' in an extended array [ e.g. crystals, Escher's periodic drawings] are positions whose local site symmetry symmetry is only the identity and translation operations. Since both of these operations are of the 'first kind', these positions are ' chirotopic'. 25. 'General positions of symmetry' in a molecule are positions [either occupied by an atom or in space] whose local site symmetry symmetry is only the identity operation. Also these positions are ' chirotopic'. 26. 'Special positions of symmetry' in an extended array are positions whose local site symmetry is a symmetry operation OTHER THAN identity or translation. These operations can be either of the 1st or of the 2nd kind. 27. 'Unit cell' : the repeat unit of the lattice or array that is duplicated only by the translation operation. 28. 'Asymmetric unit': that portion of the unit cell which is used to generate the whole unit cell using all the symmetry operations found in the unit cell. 29. If only the identity and translation operations exist in the lattice or array, then the ENTIRE content of the unit cell is the 'asymmetric unit'. 30. The asymmetric unit within the unit cell need not contain 'contiguous' parts of an object. 31. 'Z-parameter ': the number of formula units in the unit cell. 32. If the unit cell contains only the identity and translation operations, then subunits within either the fish or the frog CANNOT be interchanged and the formula unit is one complete fish and one complete frog, i.e. Z = 1. 33. M. C. Escher's periodic drawing of chiral asymmetric fish and chiral asymmetric birds related by two-fold [ Cn] rotation axes. The drawing is broken down into unit cells. 34. Since the fish and bird objects are ' asymmetric ' [literally containing no symmetry what-so-ever] then the 'special position of 2-fold rotation symmetry' at which the Cn-axes reside MUST be between like objects and NOT within them, i.e. the asymmetric objects may NOT occupy the Prof. Robert Glaser, Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel 5

'special position of 2-fold rotation symmetry '. This special position is ' chirotopic ', since the two-fold axis of symmetry is an operation of the first kind. 35. Special positions of symmetry may be occupied by an object if and only if the object itself has the SAME symmetry as that of the special position. 36. Unit cells with additional symmetry operations other than identity and translation have asymmetric units containing only a fraction of the entire contents of the unit cell. 37. In M. C. Escher's periodic drawing of chiral asymmetric fish and chiral asymmetric birds related by two-fold [ Cn] rotation axes, Z =2 and the asymmetric unit is only one complete fish and one complete bird while the cell contents contain two asymmetric birds and two asymmetric fish. 38. M.C. Escher's periodic drawing of achiral bees, birds, bats, and butterflies: since the objects all contain a σ−plane of reflection, they can occupy the special positions of mirror [ m] symmetry, and they do in this picture. Now these special positions are ' achirotopic ' since reflection is an operation of the second kind. 39. Occupancy of special positions by molecules [or the above-mentioned Escher objects] means that the unit cell volume shrinks since the asymmetric unit contains 'chiral half-objects' rather than chiral or achiral 'complete objects'. 40. A Z-value expected for a particular space group might mean two molecules in the asymmetric unit where each one is occupying a special position of symmetry. 41. Within 15 minutes of mounting the crystal, the Z = 4 value for the Pbca space [instead of the expected Z = 8 value] showed that L. Paquette had synthesized the 1,6-dimethyl-dodecahedrane molecule rather than the expected 1,5-dimethyl constitutional isomer. 42. Magnetic Equivalence in NMR spectroscopy : a practical application of the symmetry principles illustrated by M. C. Escher's periodic drawings. 43. Magnetic Equivalence: ' isogamy ' (equal mating)/ anisogamy (unequal mating)- how a 3rd nucleus 'views' or is 'mated to' two symmetry equivalent neighbors. First order A 2X2 versus severely second order AA'XX' four-spin systems.

D. What Something Is Depends Upon Where It Is points covered: 1. Review: general versus special positions of symmetry. 2. D(+)-tartaric acid staggered rotamers: permutation of local environments, solution-state 13 C NMR isochronicity of methine carbons and also of carboxyl carbons due to dynamic homotopic site averaging at the fast exchange limit for rotamer interconversion. 3. Crystalline D(+)-tartaric acid: anisochronicity of methine carbons and also of carboxyl carbons in diastereotopic environments due to occupancy of a general position in the P21 space group crystal. 4. Meso -tartaric acid staggered rotamers: permutation of local environments, solution-state 13 C NMR isochronicity of methine carbons and also of carboxyl carbons due to dynamic enantiotopic site averaging at the fast exchange limit for rotamer interconversion. 5. General position occupancy of meso -tartaric acid desymmetrizes it into an asymmetric chiral molecule in the crystal. 6. Racemus and tartaric acids. Hemihedralism in quartz crystals. 7. Enantiomorphous hemihedral faces in quartz. 8. 'Racemic Compounds ' exist in achiral crystals 9. (2 RS ,3 RS )-tartaric acid racemic compound. Prof. Robert Glaser, Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel 6

10. X-ray diffraction photos show 'reciprocal space'. B-DNA fiber diffraction photo. 11. Symmetry limitations in the solid-state. Only C2-, C3-, C4-, and C6- rotational symmetry axes are allowed. 12. Penrose diagrams, aperiodic arrays, and quasicrystals. 13. The solution-state Ih-symmetry of dodecahedrane, and the I-symmetry of viral capsids cannot be expressed in the crystalline state, since C5-symmetry axes cannot exist, and thus the structures are desymmetrized. 14. Crystalline racemates, racemic compounds versus conglomerates. 15. Symmetry equivalence and identity are ideal phenomena. Symmetry arguments. 16. 'Same conformations ' versus ' identical geometries '. The same conformation but non-identical geometry of the nefopam HCl racemic compound versus that in the chiral crystal. 17. Review: asymmetric unit versus the unit cell. 18. Review: multiple object asymmetric units smaller than the entire unit cell. 19. 'Same' versus 'identical'. 20. Two molecules in the asymmetric unit. 21.Two molecules in the asymmetric unit of an achiral crystal with the same conformation. 22. Two molecules in the asymmetric unit of an achiral crystal with different conformations. 23. Chiral point and space groups contain only symmetry operations of the First Kind. Reference and ideal mirror-image structure can never coexist with the same chiral space group crystal. 24. Two molecules in the asymmetric unit of a chiral crystal with the same configuration and the same conformation. 25. Two molecules in the asymmetric unit of a chiral crystal with the same configuration and the different conformations. 26. Two molecules in the asymmetric unit of a chiral crystal with different configurations and the same conformation. 27. Two molecules in the asymmetric unit of a chiral crystal with different configurations and different conformations. 28. Pseudo-symmetry and Avnir's ' continuous chirality measurement ' (CCM).

E. Dynamic Stereochemistry, Phase Isomerism, and Nanomachines points covered: 1. Bevel gears and bis(9-triptycyl)-Z molecules. 2. Correlated motion in bis(9-triptycyl)-Z nanomachines. 3. Correlated motion affords phase isomerism. 4. Mechanics of linear versus cyclic gear trains. 5. Cogwheeling circuit and permutational isomers. 6. Graph theory: the hexagonal Calley diagram. Cogwheeling circuit for the ' d'-isomer. 7. Cogwheeling circuit for the ' l' isomer is also different. 8. Phase isomers can NOT interconvert by free cogwheeling motion. 9. Residual characteristics of the 'group' and 'residual isomerism'. 10. Diaryl-Z propeller ground-state geometry. 11. Diaryl-Z propellers: aromatic ring geometry in helicity-change transition-states. 12. Diaryl-Z propellers: ' 0-, 1-, and 2-Ring Flip ' transition-state site exchange. 13. Correlated motion in diaryl-Z propellers. 14. Binary descriptors for permutational isomers. 15. The cogwheeling circuit for structure (000). Prof. Robert Glaser, Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel 7

16. None of the 1-ring flip structures in a cogwheeling circuit is the of another. 17. A real life example: dynamic NMR of an isopropyl edge-labeled diaryl-Z propeller. 18. 1-Ring flip cogwheeling mechanism ' threshold mechanism '. 19. 2-Ring flip mechanism transition-state still affords two methyl signals via the higher energy mechanism. 20. 2-Ring flip mechanism transition-state 'C' explains only one methyl signal in the higher temperature mechanism. 21. DNMR: lower and higher temperature correlated motion. 22. Think complicated thoughts.

Additional lectures not part of the minicourse-

F. Introduction to CP/MAS NMR Spectroscopy. points covered: 1. Differences between solid-state and solution-state NMR: the < 3cos 2θ–1> geometric mathematical factor in quantum mechanical equations is effectively time-averaged to zero via rapid isotropic molecular tumbling due to Brownian motion in low viscosity liquids where angle θ is the angle between the vector r between two through-space nuclear magnetic dipoles and the external magnetic field, H 0]. However, there is only limited very small amplitude thermal motion in the crystal lattice. 2. Problem no. 1: Severe line-broadening due to through-space magnetic dipolar interactions . 3. NMR line broadening due to these through-space magnetic dipolar interactions is directly related to 1/T 2 [the spin-spin relaxation time] and quantum mechanical equations describing this phenomenon also involve the same the <3cos 2θ–1> geometric mathematical factor. In solids, this term is not time-averaged to zero. 4. Experiments in solid simulating isotropic motion: rapid rotation of solid samples about an axis having angle β to the external magnetic field H 0, where angle β is time-averaged to 54.7° (the ‘magic angle ’), and thus: 3cos 2(54.7°)–1 = 0. 5. Problem number 1 still not solved: MAS (magic angle spinning) can only attenuate the homonuclear and heteronuclear severe through-space magnetic dipolar interaction problem affording large magnitude line-broadening, since the sample cannot be mechanically spun fast enough to attain the necessary time-averaging of angle β to 54.7°. 6. Solution to problem no. 1: Remove homonuclear magnetic dipolar interactions by removing simply removing any homonuclear neighbors. Work only with dilute spins, i.e. study those such as 13 C, 15 N, 31 P, 29 Si which do not have homonuclear neighbors. ‘Knock out’ the usually ubiquitous 1H heteronuclear neighbors by massively decoupling the 1H nuclei (need high power amplifiers). 7. Problem no. 2: Line-broadening from Shielding Anisotropy : Shielding constants give us the well-known chemical shifts, and these depend upon the orientation of the nucleus in the applied magnetic field, H 0. Again, due to rapid Brownian motion reorientation in low viscosity liquids, this effect is efficiently averaged (i.e. one observes an ‘average orientation’). But this does not happen in solids since the particle sizes are very very far from those approaching molecular dimensions. As a result, the same nucleus exhibits a number of orientations, each one with its shielding constant, and thus giving rise to Chemical Shift Anisotropy (CSA) line-broadening. 8. Solution to problem number 2: now, since the magnitude of the line-broadening arising from this problem is much much smaller than that from the through-space magnetic dipolar Prof. Robert Glaser, Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel 8

interactions, now the available spin-rates of MAS are fast enough to solve this. 13 9. Problem no. 3: Low sensitivity due to long C spin-lattice relaxation times, T 1, in the solid- state. 10. In solution-state NMR, one has to wait 5T 1 [spin-lattice relaxation] time periods to rebuild the initial Boltzmann distribution of nuclear populations in the two energy states for nuclei of spin- 1/2. This must be done prior to the application of the next pulse of radio-frequency energy required for the application of multiple pulses needed to effectively average out the random noise in the spectrum. The T 1 time constant is related to the reorientation rate of the target nucleus, and the most efficient situation for T 1 relaxation is when excited state nuclei reorientate by Brownian motion and rotation at (1/T 1) rates equal to the Larmor frequency [frequency proportional to energy difference between the two energy states of the nucleus]. Since nuclei are trapped within the confines of the crystal lattice, they reorientate at these favorable rates, and their T 1 time constants can thus be 100’s of seconds long. 11. Solution to problem number 3: cross polarization - apply energy to the protons and then prevent them from being able to effectively relax by a process called a spin-lock . Then, under this condition, simultaneously apply energy separately to both the proton and 13 C nuclei so that the two magnetic fields are equal in strength [ Hartmann-Hahn matching condition ]. Now, the 13 C nuclei absorb some of the proton energy by this indirect process and their long T1 time constants are no longer relevant. Cross polarization (CP) takes advantage of the shorter T1 time constants for protons. 12. Introduction to some typical solid-state NMR experiments: standard cp/mas; Variable Amplitude Cross Polarization (VACP ); SELTICS ( Sideband El imination by Temporary Interruption of Chemical Shift); TOSS ( Total elimination of Spinning-Side Bands). 13. Introduction to some solid-state spectral editing experiments. 14. Non-Quaternary and Non-Methyl Suppression ( NQS ): based on inefficient magnetic dipolar relaxation for quaternary carbon atoms [they have no protons ligated to them], and methyl carbons [they preferably relax by a spin-rotation mechanism where the methyl group rotates about the C—CH 3 bond]. Result: see quaternary and methyl carbon signals, and usually not those from methylene or methine. 15. Single T 1 delay period after a population inversion: Of all the carbon nuclei with protons ligated to them, the methyl groups can still rotate [reorientate] about the C—CH 3 bond in solids (although at lower rates than in solution). The appropriate T 1 delay period is chosen so that methyl carbon signals completely relax. Result: observe quaternary, methylene, and methine methyl carbon signals, and usually not those from methyls. 16. Cross-Polarization/ Polarization Inversion ( CPPI ): The cross-polarization is performed under special conditions giving rise to spectra containing only inverted phase signals from methylene carbons (so-called methylene carbon only spectrum). 17. Multiple signals for each nucleus: CP/MAS NMR can show if there are two (or more) symmetry unrelated molecules in the asymmetric unit of the crystal. These molecules may have the same or different molecular conformations. In either case, one observes a doubling of the number of lines in the cp/mas spectrum.

G. Solid-State CP/MAS NMR Spectroscopy and Stereochemical Studies on Polymorphism in Drugs. points covered: Prof. Robert Glaser, Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel 9

1. Crystalline polymorphism involves the existence of multiple forms of crystals. Since these different forms are not related by any symmetry operation, they are ‘different’ crystals and all of their properties (including cp/mas NMR spectra) are different [the only question remaining is just how different are they]. Among these properties is the ‘rate of dissolution’ which is intimately related to the phenomenon of ‘ bioavailability ’ of solid pharmaceuticals. Another important property is the crystalline-state stability upon tabletation as well as the stability upon long-term storage of the tablets. 2. Polymorphs may be patented!. 3. 'Concomitant polymorphs' are those that exist in the same vessel. 4. Solid-State CP/MAS NMR can: a. Characterize a polymorph, even an amorphous one (X-ray crystallography requires crystalline material). b. Detect the active ingredient’s polymorphic form inside a tablet. c. Ascertain that the tabletation process has not changed the polymorphic form of the active ingredient. d. Be used as evidence that your company is NOT producing a competitor’s patented polymorph. e. Provide insight as to the polymorphic form in a competitor’s tablet, assuming the appropriate standards are available. 5. Is the spectrometer sensitive enough to ascertain differences in the cp/mas spectra of two or more polymorphs. Hard to predict since ‘a symmetry argument’ is used to state that polymorphic crystals are ‘different’. But, symmetry arguments never predict the magnitude of the difference. 6. 'Excipients ': all the material in the tablet that is not the ‘active ingredient’. 7. 13 C CP/MAS NMR spectrum of a tablet shows ‘windows’ between or to the sides of those arising from the excipient. Since most excipients contain fairly large amounts of starch (a polysaccharide), one can readily observe aromatic and carbonyl carbons, as well as some of the aliphatic region. 8. Comparison of the cp/mas spectrum of the tablet versus standards of known polymorphic forms can enable the determination of the polymorphic form inside the tablet. This can be accomplished even for relatively ‘low’ (3-5%) loadings of active ingredient. 9. CP/MAS of accelerated aging studies (high relative humidity, elevated storage temperatures) can show if the polymorphic form is stable under these conditions. 10. If the active ingredient contains a 31 P nucleus, then data acquisition is a relatively fast process as opposed to the markedly slower period of time needed for the standard 13C nucleus. 11. Quality control: by preparation of bone-fide weighed mixtures of polymorphic forms, cp/mas can sometimes detect the presence of polymorphic impurities at the level of 1%, provided that there are the appropriately located ‘windows’ in the spectrum where signals are present from only one polymorph. 12. CP/MAS Polymorphism studies on Sertraline HCl (Pfizer’s Zoloft® ), a highly specific Serotonin reuptake inhibitor which is a highly prescribed antidepressant drug (2001 sales $2.3 bil.). Zoloft® is similar in action to Prozac® . 13. Solution-state NMR (DMSO-d6 or CD 2Cl 2) gives no hint of a preferred conformation for the axial-methylammonium group ligated to the cyclohexenyl-type ring [actually a benzannelated cyclohexyl]. CP/MAS 13 C NMR of conformational polymorphs containing either + antiperiplanar (ca. 180°) or synclinal (ca. 60°) CH2–CH– NH–CH3 dihedral angles are different + and show a marked gamma-gauche effect for the CH 2–CH–N methylene carbon chemical shift. Comparison of the CP/MAS chemical shifts with those from the solutions clearly shows that the Prof. Robert Glaser, Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel 10

+ same synclinal (ca. 60°) CH2–CH– NH–CH3 dihedral angle rotamer is the overwhelming major contributor to the time-averaged solution-state structure. 14. 'Pseudo-polymorphs ': term used if the molecular content of the unit cells are not exactly the same, e.g. an anhydrate is compared with a hydrate. 15. CP/MAS Pseudo-Polymorphism studies on Scopolamine HBr anhydrate versus sesquihydrate forms: The anticholinergic drug Scopolamine HBr has extended versus compact extreme conformations in the tropic acid ester moiety. These conformations primarily differ in the solvent exposure of the tropic acid phenyl group. In the compact conformation [seen by X-ray crystallography of the sesquihydrate form grown from water], one face of the phenyl group is underneath the scopine amino-alcohol bicyclic moiety while the other phenyl face and the methylol (CH 2OH) are solvent exposed. In the extended conformation conformation [seen by X-ray crystallography of the anhydrate form grown from ethanol/acetone], both phenyl faces are readily solvent-exposed, while the methylol is now underneath the scopine moiety. . 13 16. Comparison of the C solution-state NMR (D 2O or CD 2Cl 2) spectra with those of the CP/MAS for the two pseudo-polymorphic forms shows that the major conformational contributor to the weighted time-averaged structure in aqueous medium is the compact structure while that for the less polar CD 2Cl 2 medium is the extended structure. 17. Neither pseudo-polymorph is listed in the US Pharmacopeiea or the EUP , but rather a ‘trihydrate’ form. CP/MAS 13 C NMR (5.0 kHz spin-rate) clearly shows that the ‘trihydrate’ [from two different commercial sources] is actually a mixture of polymorphic forms, one of which is the sesquihydrate. 18. Using a lead nitrate ‘internal thermometer’ experiment: at 5.0 kHz spin-rate, the cooling effect of the spin-air overcomes the heating resulting from the rotor friction. Increasing the spin-rate from 5.0 kHz to 13.5 kHz heats the rotor and results in a solid-state phase change whereby only signals from the sesquihydrate are observed. 19. Within the capped (sealed) rotor, the sample is ‘metastable’ since lowering the spin-rate back down to 5.0 kHz did not regenerate the three component system until some time between greater than 2 days storage at ambient temperature and less than 3 weeks. . 20. Comparison of the x-ray structures of the isostructural HBr sesquihydrate and the HCl 1.66 Hydrate crystals shows that an additional site for water binding exists on a special position of C2-rotational symmetry, and it can be partially occupied by a third water molecule which hydrogen-bonds to two adjacent halide anions to enable a maximum of dihydrate stoichiometry. The remaining water in the ‘trihydrate’ form is physically absorbed due to the dilequesant nature of scopolamine hydrohalide salts.

Bibliography:

Eliel, E. L.; Wilen, S. H.; Mander, L. N. Stereochemistry of Organic Compounds , Wiley- Interscience: New York, 1994, chapters 2-4.