platinum drugs are 
 a mainstay of cancer therapy . approximately 
 half of all cancer patients receiving chemotherapy cisplatin , carboplatin , and oxaliplatin 
 ( chart 1 )  three platinum complexes 
 approved by the us fda for the treatment of human cancer  
 are commonly applied to treat bladder , testicular , head and neck , 
 ovarian , colon , and small cell and nonsmall cell lung cancers . despite such widespread use , the cytotoxicity of these drugs is not limited to cancer cells , 
 and off - target activity results in emesis , alopecia , nausea , kidney 
 damage , myelosuppression , and peripheral neuropathy . moreover , many 
 tumors are either inherently resistant to the currently employed platinum - based 
 therapies or acquire resistance during treatment . in an attempt to 
 find molecules with improved potency , fewer side effects , and a novel 
 spectrum of activity , researchers have prepared thousands of platinum 
 complexes and tested them for anticancer activity . one strategy used 
 to address the foregoing issues is to devise complexes that depart 
 from the neutral , square - planar , dna - cross - linking cis - dia(m)mine a current manifestation explores 
 cationic monofunctional platinum(ii ) complexes , which bear only one labile ligand and form one bond to 
 the dna nucleobases . the significant difference in the interaction 
 of monofunctional complexes with dna compared to classical bifunctional 
 cross - linking compounds very likely contributes to the unique response 
 that phenanthriplatin , or cis-[pt(nh3)2(phenanthridine)cl ] ( chart 1 ) , elicits when used to treat cancer cells . studies with the monofunctional compound pyriplatin ( chart 1 ) , cis-[pt(nh3)2(pyridine)cl ] , reveal that little distortion of 
 the dna double helix is induced upon platination , and a similar situation is likely to be obtained with phenanthriplatin . this result is very different from the significant dna bending at 
 1,2-intrastrand cross - links that occurs following treatment with bifunctional 
 platinum agents such as cisplatin . anonleaving group ligands are 
 colored blue , and leaving group ligands are red . nonleaving group ligands are 
 colored blue , and leaving group ligands are red . crystallographic and biochemical studies have revealed the mechanism 
 by which pyriplatin exerts its anticancer activity , most likely transcription 
 inhibition followed by consequent apoptosis . structural studies suggest that the steric 
 bulk of the pyridine ring is crucial for activity . this hypothesis provides an explanation for the contrast 
 between the activity of pyriplatin and the inactivity previously observed 
 for monofunctional compounds such as [ pt(nh3)3cl ] and [ pt(dien)cl ] . a systematic variation of the n - heterocyclic 
 ring am in compounds of the form cis-[pt(nh3)2(am)cl ] resulted in the discovery of the 
 far more potent analog phenanthriplatin . in a preliminary 
 screen of cultured human cancer cells , phenanthriplatin displayed 
 significantly greater cytotoxicity than cisplatin and showed a pattern 
 of activity distinct from that of either cisplatin / carboplatin or 
 oxaliplatin . a more detailed understanding of the spectrum of activity 
 was gained by analyzing the cytotoxicity of phenanthriplatin in the 
 nci60 panel of cancer cells . the pattern of cell killing was uncorrelated 
 with that of any other platinum agent in the nci database . unlike 
 pyriplatin , the asymmetry of the phenanthridine ring renders phenanthriplatin 
 chiral . that phenanthriplatin can exist as two distinct enantiomers 
 is of potential importance because the two enantiomers may display 
 different pharmaceutical activity . initial experiments revealed 
 that phenanthriplatin binds dna and that it does so in a covalent , 
 rather than intercalative , manner . moreover , 
 studies with e. coli , analogous to those initially 
 performed to investigate the mechanism of action of cisplatin , corroborate 
 the hypothesis that the interaction of phenanthriplatin with dna is 
 responsible for its anticancer effects . to gain more insight into the nature of the interaction of 
 phenanthriplatin 
 with dna , we prepared small molecule complexes that model its reactions 
 with guanosine residues ( chart 2 ) . the n7 position of guanine is the most nucleophilic among 
 dna bases 
 and , as a result , it is the primary binding site for platinum agents . the model complexes cis-[pt(nh3)2(r - gua)(am)](otf)2 , where r - gua is 
 a 9-alkylguanine , am is phenanthridine , and otf is trifluoromethanesulfonate 
 or triflate , were therefore prepared using the triflate salt of phenanthriplatin 
 as a synthetic precursor . the chirality of phenanthriplatin combined 
 with coordination to r - gua creates diastereomers , the nature of which 
 was investigated by x - ray crystallography and nmr spectroscopy . here we report the results of an investigation of these diastereomeric 
 analogs of phenanthriplatin ( chart 2 ) , which 
 were prepared to investigate whether the two phenanthriplatin enantiomers 
 can be resolved on the physiological time scale . consideration 
 of its dynamics is crucial for interpreting the conformational isomerism 
 observed with the 9-alkylguanine model complexes . evidence in both 
 the solid state and solution phase indicates that , when phenanthriplatin 
 reacts with guanine , diastereomeric selection occurs among the possible 
 conformational isomers that can form . the origin of this selection 
 has been identified , as described in this article . the synthesis and characterization 
 of the compounds under discussion 
 are presented in the supporting information along with crystallographic details and specifications of the instruments 
 used for physical measurements . h nmr spectra were acquired over a temperature 
 range 
 for 24 to investigate potential 
 fluxional behavior exhibited by these compounds . the rate of exchange 
 at the temperature at which two peaks coalesce was estimated using 
 eq 1 , in which k is the rate 
 of exchange at the coalescence temperature and  is the 
 difference in hz between the two signals in the low temperature ( slow 
 exchange ) limit . this estimate was used to inform the initial guess 
 in a full line shape analysis . simulated spectra were fit to experimental spectra at the different 
 temperatures , and the corresponding rate constants were extracted . these rate constants were used to construct eyring plots and determine 
 activation parameters.1 deuterated 
 dmso solutions of 6 and 8 , both of which 
 are 9-methylguanine complexes , were sparged with n2 for 
 2 min and then sealed in an nmr tube under a blanket of nitrogen . briefly , for each sample , a h nmr spectrum was 
 acquired following preirradiation at the frequency of the methylguanine 
 h8 ( h8 g ) resonance . the preirradiation power was chosen 
 so as to just saturate the h8 g signal and eliminate it 
 from the spectrum . a second spectrum was then acquired with identical 
 parameters , but with the saturation frequency set to a region downfield 
 and devoid of signals . no cross peaks between h8 g and any of the 
 phenanthridine protons were observed in a noesy experiment . rotating - frame 
 overhauser effect ( roesy ) spectra were collected on samples prepared 
 in deoxygenated acetone - d6 . a mixing time 
 of 200 ms was employed with a 90 pulse of 11 s . the complex cation of 6 was constructed in gaussview , and the planes 
 of the phenanthridine and 9-ethylguanine ligands were set perpendicular 
 to the coordination plane . two conformers were investigated , one in 
 which the h6 proton of the phenanthridine ligand ( h6p ) 
 and h8 g were on the same side of the platinum coordination 
 plane and the other in which they were on opposite sides . the zero order relativistic approximation ( zora ) , along with the 
 attendant tzv - zora basis set , was applied to treat relativistic effects . the resolution of the identity approximation 
 and the appropriate auxiliary basis set were used to accelerate computations . the stationary nature of the structures obtained from geometry optimizations 
 was confirmed using numerical frequency calculations . optimizations 
 were conducted in either the gas phase or in solution by using an 
 implicit conductor - like screening model ( cosmo ) . to model aqueous 
 solvation , the dielectric constant of the polarizable continuum was 
 set to 80.400 and the refractive index to 1.3300 . monofunctional 
 complexes 
 having the formula cis-[pt(nh3)2(am)cl ] , where am is an n - heterocyclic 
 ligand , have previously been obtained as nitrate salts by treating 
 cisplatin , cis-[pt(nh3)2cl2 ] , with 1 equiv of silver nitrate followed by an equivalent 
 of am . alternatively , cis-[pt(nh3)2(am)cl]cl can be obtained 
 by heating cisplatin with am to displace one of the chloride ligands . a major problem with these methods , however , 
 is that neither silver - mediated halide abstraction nor direct ligand 
 substitution proceeds selectively at just one coordination site . as 
 a result , in addition to the desired cis-[pt(nh3)2(am)cl ] complex , appreciable amounts 
 of cis-[pt(nh3)2(am)2 ] form together with unreacted cisplatin . here we use silver triflate to prepare 7 and 1 , triflate salts of pyriplatin and phenanthriplatin , respectively , 
 to provide a much wider range and degree of solubility in organic 
 solvents . as a result , addition of acetone to the residue that remains 
 following removal of dmf from the synthesis mixtures of 1 and 7 dissolves the triflate salts of cis-[pt(nh3)2(am)cl ] and cis-[pt(nh3)2(am)2 ] . unreacted 
 cisplatin , on the other hand , does not dissolve in acetone and can 
 be removed by filtration . when ether is layered onto the acetone filtrate , 
 crystals of cis-[pt(nh3)2(am)cl]otf 
 deposit over the course of a few days . these crystals can be harvested 
 before the more soluble cis-[pt(nh3)2(am)2]otf2 precipitates , providing access 
 to analytically pure material . spectroscopic characterization 
 of these complexes was consistent 
 with that which had been previously reported for the nitrate salts 
 of these cations . the assignment of the peaks in the h 
 nmr spectrum of 1 was carried out using a combination 
 of cosy and noesy spectra ( figure s4 ) . the nmr spectra for these compounds ( figure s19s22 ) show a multitude of peaks 
 arising from different rotamers that interconvert slowly on the nmr 
 time scale at room temperature . the syntheses of 5 , 6 , and 8 were achieved by treating either 1 or 7 with an additional equivalent of silver 
 triflate and adding the appropriate 9-alkylguanine . as described above , the h nmr spectra 
 of 24 exhibit peak 
 multiplicity as a result of slow to intermediate exchange between 
 conformational isomers . upon heating , these signals broaden , coalesce , 
 and finally sharpen as the rate of exchange increases . although some regions of the 
 spectra have complex overlapping features , other regions show well - resolved 
 peaks and coalescence events . the line shapes of portions of the h nmr spectra of 24 that 
 showed well - defined , baseline - resolved coalescing signals were simulated . the simulated spectra were fit to the experimental data by varying 
 the rate constant . a two - site model was used for 2 and 4 , and a four - site model was used for 3 . an example 
 of the simulated and experimental data for 4 is shown 
 in figure 1 . data for all compounds can be 
 found in the supporting information ( figures 
 s23s25 ) . experimental and simulated line shapes of portions of 
 the h nmr spectrum of 4 . the temperature 
 at which 
 the experimental data were collected and the rate constant used to 
 generate the simulation are shown next to each set of data . the variation in rate constant with temperature can be used 
 to 
 determine the activation parameters for the interconversion . the enthalpy 
 and entropy of activation , h and s respectively , are 
 obtained from the first order rate constants using an eyring analysis 
 ( figure 2 ) . eyring plots for all the coalescence 
 events simulated are presented in the supporting 
 information . the gibbs free energy of activation at a given 
 temperature t , gt , can be obtained using eq 2.23the values of g298.15 for compounds 24 are collected in table 1 . using eq 3 , where kb is boltzmann s constant , h is planck s 
 constant , and r is 
 the universal gas constant , the rate constant at a given temperature , the lifetime 
 of the molecule in a given conformational state at temperature t , t , for the process 
 can be obtained by taking the inverse of the rate constant . the errors for g298.15 are obtained by standard propagation of the errors 
 of h and s obtained from the least - squares linear 
 regression eyring plots . the errors should be taken only as estimates 
 of the true errors and are presented to indicate the relative precision 
 with which the different determinations were made . the approximate 
 nature of the error estimate arises from the fact that this analysis 
 treats data across a logarithmic scale on equal footing . moreover , 
 it equally weights the rate constants from near the coalescence point , 
 which are more accurate , and rate constants obtained far from coalescence , 
 which are less accurate . the error propagation was not carried out 
 for the lifetimes because only modification by physical constants 
 is involved . eyring plot of the conformational isomerism of 4 using 
 the data from figure 1 . pertinent crystallographic data 
 for 1 , 5 , 7 , and 8 are summarized in table s1 . the crystal structure of 1 ( figure 3 ) shows the cis-[pt(nh3)2(phenanthridine)cl ] cation to be similar 
 in structure to that present in the structure of the nitrate salt , with bond lengths and angles falling within expected 
 ranges . the geometry of the primary coordination sphere in both structures 
 is essentially identical with an rmsd of 0.023  . the most significant 
 differences are found in the orientation of the phenanthridine rings 
 as shown in figure s30 . in the nitrate 
 salt , the phenanthridine plane does not contain the line 
 connecting the platinum atom and the nitrogen atoms of the phenanthridine 
 and the trans ammine . this line instead forms an 
 angle of 18 with the phenanthridine plane . in the 
 triflate salt , this angle is only 10. an important aspect of 
 the structure of the complex is that the asymmetry of the phenanthridine 
 ligand about the platinum coordination plane produces a chiral molecule . molecular diagrams of the platinum complexes 
 from the crystal structures 
 of 1 , 5 , 7 , and 8 with thermal ellipsoids drawn at the 50% probability level . color 
 code : n blue , o red , c gray , cl green , pt magenta , h open circles . the chirality originates about 
 the bond between the platinum center 
 and the phenanthridine nitrogen atom ( pt  np ) and can be classified according to the conventions of axial 
 chirality . np vector , the platinum coordination 
 plane lies in front of the perpendicular plane of the phenanthridine 
 ring . the direction in which the front ring needs to be rotated 
 so as to have the priority substituent of the front plane coincide 
 with the priority substituent of the back plane dictates the stereochemistry . clockwise rotation is denoted p and counterclockwise 
 rotation m. the different enantiomers of phenanthriplatin 
 are shown in figure 4 . ( a ) the enantiomers of 
 phenanthriplatin and ( b ) the 
 convention used to classify them . in part b , the complex is viewed 
 along the ammine  platinum  phenanthridine 
 vector . the coordination plane is shown as a darkened line , and the 
 phenanthridine plane as a dashed line . the structure of 7 ( figure 3 ) , the triflate salt of pyriplatin , also displays 
 expected bond lengths 
 and angles . in this complex , unlike 1 , the line connecting 
 the platinum atom and the nitrogen atoms of the pyridine and the trans ammine is essentially contained by the plane of the n - heterocycle . the ring deviates significantly , however , 
 from a perpendicular orientation with respect to the platinum coordination 
 plane . the dihedral angle of 60 between the pyridine and the 
 coordination plane is consistent with the angle of 56 observed 
 for trans-[pt(pyridine)2cl2 ] . details of the determination of the structure of a nonmerohedral 
 trilling of this latter compound are presented in the supporting information . given the lack of steric 
 or electronic factors to enforce strict perpendicularity between the 
 pyridine ring and the coordination plane , the angle in both cases 
 is most likely dictated by crystal packing interactions . the 
 solution and refinement of the structure of 8 ( figure 3 ) proceeded smoothly , except for the presence of 
 a void about the 0 , 0 , 1/4 special position containing disordered 
 electron density . the density could not be successfully modeled as 
 either a molecule of dmf , diethyl ether , or a 1:1 disorder of the 
 two , any of which would be consistent with the 41 e within the void . one dmf molecule disordered across each of 
 the 4 voids of 228  within the unit cell ( z = 8) would be consistent with the combustion analysis 
 results obtained from this material . the pyridine and 9-methylgunanine 
 rings are both canted in the same direction by 19 and 23 , 
 respectively . the structure of 5 , the product of 
 the reaction of 
 activated 1 with 9-ethylguanine , was also solved ( figure 3 ) . the salt crystallized along with one water molecule , 
 located on the 2-fold proper rotation axis , and one disordered acetone 
 molecule . the presence of 0.5 equiv of water is consistent with the 
 elemental analysis of this compound . the acetone present in the structure 
 is not observed during combustion analysis and is probably removed 
 during the vacuum drying of the substance . the phenanthridine 
 ligands lie parallel to the ac plane , and the 9-ethylguanine 
 ligands and the platinum coordination plane lie perpendicular to this 
 crystallographic plane . the phenanthridine and 9-ethylguanine 
 ligands of the square - planar complex cation the former is oriented essentially perpendicular 
 to the coordination plane but the latter is canted by 23 , such 
 that the guanine carbonyl oxygen approaches the ammine coordinated cis to it . preirradiation 
 at the frequency of the h8 g signal in solutions of 6 and 8 induced perturbations in the signals 
 of those protons that interact with h8 g in a through - space 
 manner . in the difference spectrum of 8 , obtained as 
 described above , negative peaks were seen arising from the ch3 protons of the 9-methyl group as well as the ortho hydrogen atoms of the pyridine ring ( figure 
 s26 ) . in the difference spectrum of 6 , a negative 
 peak was again seen for the ch3 protons of the 9-methyl 
 group . negative peaks were also observed for h6 and h7 of the phenanthridine 
 ring ( figure s27 ) owing to their close 
 proximity to h8 g . the roesy spectrum of 6 also 
 confirms the presence of the through - space interaction between h8 g and h6 of the phenanthridine ring ( figure 
 s28 ) . geometry optimizations were performed 
 on two conformational isomers of 5 that are related by 
 a 180 rotation about the pt np bond . in calculations 
 in which the starting geometry had both n - heterocyclic 
 ligands perpendicular to the coordination plane , optimization did 
 not significantly alter the geometries of either of the conformers , 
 which exhibited a negligible difference in strain energy . an overlay 
 of the optimized structures of both conformers is shown in figure 5 . overlay of the molecular mechanics optimized geometries 
 obtained 
 by setting the aromatic ligands perpendicular to the platinum coordination 
 plane and rotating 180 about the platinum more rigorous geometry optimization 
 using dft methods reproduced the canting of the alkylguanine ligand 
 that was observed in the crystal structure of 5 ( figure 6 ) . in the gas phase , the distance of 2.72  
 between the carbonyl oxygen and the ammine nitrogen is sufficiently 
 small to propose the presence of an intramolecular hydrogen bond . the calculations were also carried out with implicit aqueous solvation . 
 as expected , the interaction between the carbonyl oxygen atom and 
 the ammine nitrogen atom is attenuated , but the resulting on 
 distance of 2.86  suggests that the interaction persists even 
 in the presence of highly polar solvents . the canting of the guanine 
 ring in ( a ) the crystal structure and 
 ( b ) the dft - optimized structure of 5 . color code : n blue , 
 o red , c gray , pt magenta , h atoms omitted for clarity . ( c ) schematic 
 representation of the dihedral angle formed between the platinum coordination 
 plane ( black ) and the phenanthridine ring of 5 in the 
 mm calculations ( red ) , the crystal structure ( blue ) , the dft calculations 
 with implicit water solvation ( purple ) , and the dft calculations in 
 the gas phase ( green ) . studies with pyriplatin 
 showed that this compound has a spectrum of activity that differs 
 significantly from those of the clinically employed platinum anticancer 
 drugs . the low potency of pyriplatin 
 prompted a search for molecules that maintain this distinct spectrum 
 of activity , but display higher activity . phenanthriplatin was developed 
 as the result of a systematic variation of the n - heterocyclic 
 amine ligand , am , of cis-[pt(nh3)2(am)cl ] . it was found 
 to be 740-fold more potent that cisplatin across a variety 
 of cell lines , and the distinct spectrum of activity was maintained . in an effort to better understand the mechanism of anticancer action 
 of phenanthriplatin , we sought to investigate the structures 
 of the adducts formed by phenanthriplatin and analogs 
 of guanosine . for these studies , the triflate salt of the cis-[pt(nh3)2(am)cl ] cation 
 was prepared to facilitate subsequent synthetic steps . in the process 
 of analyzing the solid state structure of the compound , however , we 
 realized an aspect of the structure of this complex that had previously 
 gone unnoticed : in the solid state , the complex cation is chiral . the centrosymmetric space group of the structure requires that both 
 enantiomers be present in the crystal in equal portions . the chirality 
 arises from the asymmetry of the phenanthridine ring about the coordination 
 plane to which it is perpendicular . different enantiomers of pharmaceutics typically 
 display different activity because these molecules interact with biological 
 systems that are inherently chiral . one example of direct relevance 
 to the field of platinum anticancer research is oxaliplatin , which 
 contains only one enantiomer in the form that is marketed for clinical 
 use . the r , r isomer of trans - diaminocyclohexaneoxalatoplatinum(ii ) 
 has greater activity than that of the enantiomeric s , s form . if phenanthriplatin 
 is indeed a racemic mixture of two stable enantiomers , then one might 
 have significantly different activity than the other . it is 
 crucial to realize , however , that the two enantiomers can 
 interconvert via rotation about pt  np . if rotation 
 about this bond is rapid at ambient or physiological temperatures , 
 then the complex is effectively achiral . as will be discussed below , 
 in addition to implications for the enantiomeric resolution of the 
 compound , asymmetry of the phenanthridine ligand about 
 the platinum coordination plane also has implications for its interaction 
 with dna . investigation of the phenanthridine rotation 
 is similar in nature to studies that have been carried out on models 
 and retro - models of 1,2-d(gpg ) intrastrand cross - links formed by bifunctional 
 platinum compounds . this similarity arises 
 from the fact , as will become important below , that both the phenanthridine 
 ligand and guanine derivatives coordinate in a manner that is asymmetric 
 about the platinum coordination plane . the chirality present in models 
 of the intrastrand [ pt(nh3)2{d(gpg ) } ] adduct 
 was first recognized by cramer et al . the phenomenon has subsequently been investigated in great detail 
 by marzilli , natile , and co - workers , among others . dynamic nmr spectroscopy is a method ideally suited 
 to investigate 
 whether rotation about a chemical bond is hindered . np in phenanthriplatin 
 interconverts two enantiomers with identical nmr properties , however , 
 and so this dynamic process will induce no change in the line shape 
 of the spectrum , regardless of the rate at which it occurs . accordingly , 
 the room temperature h nmr spectrum of 1 shows 
 a single set of well - defined resonances . the signals in the aromatic 
 region are particularly sharp , and the breadth of the signals arising 
 from the two ammines is due to a combination of quadrupolar relaxation 
 from n and coupling to csa - broadened pt . 
 to investigate rotation about the pt  np bond , the 
 rotamers must be rendered diastereomeric . this goal was accomplished 
 by replacing the two ammine ligands with the enantiomerically pure r , r - diaminocyclohexane ( dach ) chelate . the c2 symmetry of this ligand ensures 
 that the coordination sites trans to the two coordinated 
 nitrogen atoms are chemically equivalent . in the room temperature h nmr spectrum of 2 , the signals arising from 
 the protons on the aromatic ring are all cleanly doubled ( figures 1 and s19 ) . moreover , 
 raising the temperature of the sample induces broadening , coalescence , 
 and subsequent sharpening of the signals ( figure 
 s19 ) . this behavior is consistent with the presence of two 
 rotamers that interconvert rapidly on the nmr time scale at elevated 
 temperature . the conformations of these two rotamers are equivalent 
 to those of the enantiomers of phenanthriplatin , i.e. m and p , giving rise to ( r , r)m and ( r , r)p diastereomers . detailed information about 
 the energetics of the rotamer interconversion 
 can be obtained by simulating the experimental line shapes obtained 
 at different temperatures . the fit of the simulation to the experimental 
 data can be optimized by varying k , the rate constant 
 for interconversion . these k values obtained at different 
 temperatures can be used to construct an eyring plot from which the 
 activation parameters for the interconversion can be extracted . the 
 gibbs free energy of activation for the interconversion of the ( r , r)m and ( r , r)p diastereomers of 2 at room temperature , g298.15 , was 70 kj mol , and the corresponding 
 rate constant at this temperature was 3.2 s. the 
 inverse of this rate constant reveals that 63% of a diastereomerically 
 pure sample of 2 would racemize within about 300 ms of 
 dissolution at ambient conditions . the validity 
 of this conclusion , however , rests on the accuracy with which 2 models the structure of 1 . the most significant 
 difference of relevance to the discussion at hand is in the n n 
 angle formed by either the ammines of 1 or the dach of 2 . the former , obtained from the crystal structure of 1 , is 89.1 , and the latter , from the crystal structure 
 of oxaliplatin , is 83.5. the smaller angle enforced 
 by the chelator relieves the steric interactions that provide a barrier 
 to rotation of the phenanthridine ring . the barrier obtained 
 with 2 , therefore , provides an upper estimate to the rate of rotation in phenanthriplatin . np in 
 phenanthriplatin will be sufficiently slower that enantiomeric 
 resolution may be possible . to obtain a diastereomeric analog 
 of 1 in which the 
 n  pt  n angle formed by the ammines is left unperturbed , 4 was prepared . the addition of a second pt np bond as a center of chirality creates ( p , m ) , ( m , p ) , ( p , p ) , and ( m , m ) 
 conformational isomers . the ( p , p ) and ( m , m ) designations describe 
 a meso compound , and the ( p , m ) and 
 ( m , p ) rotamers are enantiomers . 
 hindered rotation about the pt np bonds would , therefore , 
 give rise to two sets of signals . using a dynamic nmr analysis analogous 
 to that described above for 2 , activation parameters 
 can be extracted ( table 1 , figure s25 ) . in addition to adding a second chiral center , 
 however , the second phenanthridine ligand may introduce 
 steric bulk that inhibits pt  np rotation to a degree 
 greater than in 1 . to assess the influence of the cis disposition 
 of two phenanthridine rings on pt  np rotation , 3 was prepared . the set of conformers listed for 3 is also present for 4 , but the presence of the r , r - dach ligand renders all four rotamers 
 diastereomeric : ( r , r)(p , m ) , ( r , r)(m , p ) , ( r , r)(p , p ) , and ( r , r)(m , m ) . dynamic 
 nmr spectroscopy again reveals the activation parameters for interconversion 
 ( table 1 ) through a simulation of the line 
 shapes of the four distinct sets of h nmr signals that 
 coalesce on heating ( figure s24 ) . a comparison of the results obtained from the dynamic nmr experiments 
 on 24 is shown in scheme 1 . upon transitioning from 2 to 3 , the addition of the extra phenanthridine ligand 
 raised the barrier to rotation , but the lifetime only changed by a 
 factor of 1.7 . exchanging the chelating dach present in 3 for the two ammine ligands in 4 increased the lifetime 
 by an order of magnitude . even so , the lifetime of a given diastereomer 
 of 4 is about 5 s. importantly , unlike 2 , 4 provides an upper estimate of the barrier to rotation 
 in phenanthriplatin . the results with 3 and 2 indicate that transitioning from 4 to 1 will lower the barrier to phenanthridine rotation . np bond in phenanthriplatin is rapid on the pharmacological 
 time scale and that enhanced activity can not be obtained by isolating 
 and administering one of the isolated enantiomers . the rotation about 
 this bond does , however , play a significant role in the interaction 
 of 1 with dna . the interaction of 
 phenanthriplatin with dna can be modeled by preparing 
 complexes with the formula cis-[pt(nh3)2(phenanthridine)(9-alkylguanine ) ] . compounds 6 and 5 ( chart 2 ) were prepared using 9-methyl- and 9-ethylguanine , 
 respectively . this mode of coordination is corroborated 
 by the shift in the h8gh nmr signal observed 
 on binding to the platinum . as a result 
 of this coordination geometry , the bond between the platinum center and the 
 n7 of the guanine ( pt  ng ) acts as a center of chirality 
 in the same manner as pt  np . multiplicity in the h signals was expected to arise due to the slow interconversion 
 of ( pp , mg ) , 
 ( mp , pg ) , ( pp , pg ) , and ( mp , mg ) , where the 
 subscripts indicate the coordinate bond to either phenanthridine 
 ( p ) or 9-alkylguanine ( g ) . surprisingly , however , only a single 
 set of signals was observed ( figure s8 ) . cooling an nmr sample to 60 c did not induce any decoalescence 
 or broadening ( figure s22 ) . in these molecules , 
 therefore , either intramolecular rotation is occurring significantly 
 more rapidly than in complexes 14 or one particular set of conformers is preferentially formed . note 
 that , for instance , ( pp , pg ) and ( mp , mg ) are enantiomers and would produce identical nmr spectra . through - space dipolar interactions between nmr active nuclei provide 
 an ideal means with which to probe three - dimensional molecular structure 
 in solution . before investigating the more complex phenanthridine - containing 
 compounds , however , a simpler pyriplatin analog was prepared and studied . 
 in 8 , the pyridine ring is symmetric about the platinum 
 coordination plane and so the platinum  pyridine bond is not 
 a center of chirality . the chirality about pt  ng produces enantiomers but not diastereomers . in the crystal structure 
 of 8 , there are 2 sets of protons that are close enough 
 to suggest that a nuclear overhauser effect ( noe ) will occur involving 
 h8 g : the ch3 of the 9-methyl group ( 2.52  ) 
 and the ortho protons from the pyridine ring ( 3.56 
  ) . the methyl group provides a particularly convenient internal 
 standard because the rigidity of the guanine ring ensures that the 
 methyl protons will stay in close proximity to h8 g regardless 
 of the relative orientation of the pyridine and guanine rings . a h saturation transfer nmr experiment was carried out with 
 preirradiation at the frequency of the h8 g signal and revealed 
 a through - space interaction between h8 g and the 9-methyl 
 protons as well as the ortho protons of the pyridine 
 ring ( figure s26 ) . in the ( mp , pg ) isomer of 6 , as well as the enantiomeric ( pp , mg ) , h8 g and h6p are on the same side of the coordination plane . 
 in ( pp , pg ) 
 and ( mp , mg ) , which are diastereomers of the first pair , h8 g and 
 h4p are on the same side of the platinum coordination plane . 
 only those protons that are on the same side of the platinum coordination 
 plane as h8 g are expected to undergo through - space interactions 
 with this nucleus . when a saturation transfer experiment analogous 
 to that described for 8 was conducted with 6 , an interaction was observed between h8 g and the 9-methyl 
 protons , as expected , and with the h6p proton . moreover , an interaction was also seen with h7p , further confirming that the ( mp , pg)/(pp , mg ) enantiomeric pair is preferentially formed 
 in solution over the ( pp , pg)/(mp , mg ) pair . a though - space interaction was also observed in the 
 roesy spectrum ( figure s28 ) . a crystal 
 structure of 5 was solved ( figure 3 ) , and it also showed only the presence of ( mp , pg ) and ( pp , mg ) . the centrosymmetry of the 
 space group c2/c in which the complex 
 crystallized requires the presence of both enantiomers . molecular 
 mechanics calculations were initially performed to interrogate 
 the origin of the diastereoselectivity exhibited by 5 and 6 . calculations in which the aromatic ligands were 
 set perpendicular to the coordination plane ( figure 5 ) revealed little energetic difference between the two diastereomeric 
 forms . inspection of the crystal structure of 5 , however , 
 reveals that the guanine ligand is not perpendicular to the coordination 
 plane . rather , it cants so as to direct the carbonyl oxygen toward 
 the cis - coordinated ammine . the on 
 distance of 3.19  is too long to constitute a formal intramolecular 
 hydrogen bond in the solid state , but such an interaction may occur 
 in solution . a similar interaction between the carbonyl of a pyriplatin - platinated 
 guanosine and the cis ammine of the platinum complex 
 was observed in the crystal structure of pyriplatin - platinated dodecamer 
 duplex dna . dft geometry optimization 
 was able to reproduce the canting of 
 the 9-alkylguanine . in the optimized gas phase geometry , inclusion of implicit aqueous solvation weakens 
 the interaction somewhat , but the on separation 
 remains short at 2.86  . the origin of the diastereoselectivity 
 appears to result from this canting of the guanine ligand , which relieves 
 steric congestion over one face of the platinum complex . one side 
 of the phenanthridine ligand has a greater degree of steric 
 bulk in the vicinity of the platinum center than the other , as illustrated 
 in figure 6 . in the favored diastereomer , this 
 bulkier portion of the phenanthridine is directed toward 
 the vacant space formed by the canting of the guanine . it remains 
 to be seen whether the conformational preference is maintained in 
 full length dna polymers or during complexation with proteins that 
 recognize platinum lesions . as a final comment n 
 bonds in phenanthriplatin were not rapid , then different 
 diastereomeric forms of the platinum adduct could be kinetically trapped 
 regardless of the energetic preference that might exist for one conformer . 
 instead , the unhindered bond rotation established above allows the 
 complex to assume the thermodynamically favorable conformation , regardless 
 of the manner in which it initially binds to the 9-alkylguanine . the solid state structure of phenanthriplatin 
 highlights the fact 
 that asymmetry of the phenanthridine ligand about the 
 platinum coordination plane results in chirality . the use of model compounds with diastereomeric rotamers 
 provides evidence that rotation about this bond in phenanthriplatin 
 is rapid , eliminating any need to isolate and administer an enantiomerically 
 pure compound . rapid rotation about the this bond also prevents the 
 kinetic resolution of diastereomers in complexes of the type cis-[pt(nh3)2(phenanthridine)(9-alkylguanine ) ] , which mimic the interaction of phenanthriplatin 
 with dna . the ( mp , pg ) and ( pp , mg ) diastereomers observed in both solution and the solid state are 
 those in which the h8 g and h6p are located on 
 the same side of the platinum coordination plane . the experimental 
 data described above indicate that interaction between the guanine 
 carbonyl and cis - coordinated ammine determines the 
 preferential diastereomer formation .
the monofunctional platinum complex cis-[pt(nh3)2cl(am)]+ , 
 also known as phenanthriplatin , 
 where am is the n - heterocyclic base phenanthridine , 
 has promising anticancer activity . unlike bifunctional compounds such 
 as cisplatin 
 , phenanthriplatin can form only one covalent bond to 
 dna . 
 another distinguishing feature is that phenanthriplatin is chiral . 
 rotation about the pt 
 n bond of the phenanthridine ligand racemizes 
 the complex , and the question arises as to whether this process is 
 sufficiently slow under physiological conditions to impact its dna - binding 
 properties . here 
 we present the results of nmr spectroscopic , x - ray 
 crystallographic , molecular dynamics , and density functional theoretical 
 investigations of diastereomeric phenanthriplatin analogs in order 
 to probe the internal dynamics of phenanthriplatin . 
 these results 
 reveal that phenanthriplatin rapidly racemizes under physiological 
 conditions . 
 the information also facilitated the interpretation of 
 the nmr spectra of small molecule models of phenanthriplatin - platinated 
 dna . 
 these studies indicate , inter alia , that one diastereomeric form 
 of the complexes cis-[pt(nh3)2(am)(r - gua)]2 + , where r - gua is 9-methyl- or 9-ethylguanine , 
 is preferred over the other , the origin of which stems from an intramolecular 
 interaction between the carbonyl oxygen of the platinated guanine 
 base and a cis - coordinated ammine . 
 the relevance 
 of this finding to the dna - damaging properties of phenanthriplatin 
 and its biological activity is discussed .