A joint 2D NMR and theoretical investigation of Ca2+ binding loops III and IV of calmodulin

A 2D NMR NOESY spectrum of integral CaM in water(148 residues) reveals a series of downfield-shifted crosspeaks stemming from the NH protons of the Ca2(+)-binding loops III and IV. Their attribution, with the help of already assigned proton resonances of isolated tryptic fragments, was complemented by means of energy-minimizations on the Ca2+ complexes of loops III and IV. From these calculations, a set of two alternative, related conformations was obtained for each loop. The first type of conformation provides a coordination pattern for Ca2+ that is similar to that found in loop EF of parvalbumin. The computed interproton distances in both loops are fully compatible with the inferences from the sets of NOESY cross-peaks. Evidence is also provided for interloop interactions.

Theoretical design of novel, 4 base pair selective derivatives of mitoxantrone

Mitoxantrone (MTX) is a recently synthesized antitumor intercalative molecule, currently in use in chemotherapy. Previous theoretical computations showed that the base pair selectivity of MTX is limited to the sole two base-pair sequence making up the intercalation site. In order to further extend the recognition site, we undertook, by means of theoretical computations, the design of novel MTX derivatives, in which the terminal hydroxyl group of each side chain is esterified with oligopeptides. We compare in the present study the binding affinities of two derivatives, depsiGly-Lys(D) and depsiGly-Gly-Orn(L), for the palindromic sequences d(CCCGGG)2, d(GCCGGC)2, d(GGCGCC)2, and d(CGCGCG)2. Major groove binding of the oligopeptide arms was shown to be significantly more favourable than either minor groove binding, or binding to the sole phosphate groups. With the two arms adopting two antiparallel directions, two distinct arrangements were investigated in the major groove: (a) the two oligopeptides are brought closer together by means of two hydrogen bonds involving the backbone of their second residue in a beta-sheet like arrangement; (b) the two arms are remote from each other so as to reduce their mutual electrostatic repulsion. Whatever the disposition, the optimal binding configurations were invariably found to be those in which the cationic side chains of the terminal residues chelate N7/O6 of two successive guanines, whenever present on a given strand. A distinct energetical preference for arrangement (a) was obtained with the depsiGly-Gly-Orn(L) derivative. Replacement of the central Gly residue by a Cys one, as in the sequence depsiGly-Cys-Orn(L), was proposed subsequently, so as to further stabilize such a beta-sheet arrangement by means of a disulfide bridge between the two Cys residues. The two investigated compounds were shown to preferentially bind sequences d(CCCGGG)2 and d(GCCGGC)2, with a tetrameric core CCGG rather than sequences d(GGCGCC)2 and d(CGCGCG)2, with a tetrameric core GCGC.

Modelling of the binding specificity in the interactions of cationic porphyrins with DNA

A theoretical investigation is performed of the complexes of a tetracationic porphyrin, tetra-(4-N-methylpyridyl)-porphyrin, (T4MPyP), with the hexanucleotides d(CGCGCG)2 and d(TATATA)2, considering the possibility of both the intercalative and the groove binding interactions. These computations demonstrate that T4MPyP manifests a significant preference for intercalation in its complex with d(CGCGCG)2 but for non intercalative binding in the minor groove in its complex with d(TATATA)2. Such a dual binding behaviour of T4MPyP as a function of the sequence to which it is attached is fully consistent with available experimental data. It demonstrates that intercalation and groove binding may be viewed as two potential wells on a continuous energy surface. In agreement with experiment, the computations indicate that in the here considered case the deepest well is associated with intercalation.

A theoretical investigation of the base sequence preferences of monointercalating polymethylene carboxamide derivatives 9-aminoacridine

Theoretical computations are performed of the comparative binding affinities of five polymethylene carboxamide derivatives of 9-aminoacridine to a series of double-stranded hexanucleotides. The purpose of this investigation is to ascertain whether minor groove recognition of a guanine base adjacent to the intercalation site can occur, and be preferentially stabilized, for a given length of the polymethylene side chain, encompassing from n = 2 up to n = 6 methylene groups. For that purpose, several representative sequences were investigated, in which intercalation of the 9-aminoacridine chromophore occurred at a central d(CpG) or d(TpA) step. Investigated were the self-complementary sequences d(CGCGCG)2, d(GCCGGC)2, d(TATATA)2 and d(ATTAAT)2, as well as the 'mixed' sequences d(ACTAAT) .d(ATTAGT) and d(TGTATA). d(TATACA). For n = 3 up to n = 6, such a recognition was enabled only when the guanine base was located downstream of the intercalation site, i.e. with steps d(CGG) and d(TAG). It occurred by means of a bidentate interaction involving, on the one hand, H(N2) and N3 of the base, and, on the other hand, the carbonyl oxygen and the cis amino hydrogen of the terminal formamide moiety of the ligand. Because of the flexibility of the side chain, however, alternative binding modes were also found to occur competitively, involving backbone-only interactions of the side chain. On the basis of the present computations, upon binding to the sequence d(GCCGGC)2, an optimal value of n = 5 could be derived, with the corresponding acridine derivative eliciting both a significant prevalence of the bidentate over backbone only binding mode, and the most favourable energy balance within the investigated series. This privileged value of n = 5 is fully consistent with the experimental results of Markovits et al. and Gaugain et al. The very flexibility of the side chain, however, hampered any preferential recognition of a triplet sequence with a downstream guanine, such as d(CGG) or d(TAG), to be elicited over sequences such as d(TAA), d(TAT) or d(TAC).

Modelling basic features of specificity in the binding of a dicationic steroid diamine to double-stranded oligonucleotides

An investigation of the intrinsically preferred binding modes of a steroid diamine, dipyrandium, to the double-stranded hexanucleotides d(TATATA)2, d(ATATAT)2, and d(CGCGCG)2 is carried out by the energy minimization procedure JUMNA. Several alternative binding modes are compared: groove binding in which the conformation of the oligonucleotide remains close to that of B-DNA, intercalation between base-pairs and interaction with variously kinked structures in which base pairs of dinucleoside steps open towards the groove in which the binding occurs. The favored binding configuration occurs at the d(TpA) step of the AT kinked nucleotides in which the kink opens the base pairs towards the minor groove. Thus, for the d(T1A2T3A4T5A6)2 sequences the preferred complexation involves the kink at the T3A4 step facing the cyclohexane rings A, B, and C of the ligand. For the d(A1T2A3T4A5T6)2 sequence, the kink occurs at the T2A3 step facing the cationic pyrrolidine ring linked to ring A. The binding of dipyrandium to d(CGCGCG)2 is found to be considerably less favourable than for either of the two (AT) sequences.

A theoretical investigation of the intercalative binding of 7-H pyrido[4.3C]carbazole chromophore into a d(CpG)2 minihelix

Theoretical computations are performed of the intercalative binding to a model d(CpG)2 minihelix of 7-H pyrido[4.3C]carbazole, the precursor of the antitumor bisintercalating drug ditercalinium. The conformations of the intercalation site are generated by the AGNAS procedure (algorithm to generate nucleic acid structures) of Miller and co-workers. The ligand-nucleotide interactions and the nucleotide conformational energies are computed with the SIBFA procedures (sum of interactions between fragments ab initio computed), which use formulas of empirical origin that reproduce ab initio SCF (self-consistent field) computations. Among the candidate intercalation sites most favored energetically, one has a pattern of conformational angles related to the one determined crystallographically by Sobell et al. in a series of x-ray structural studies of small intercalator-dinucleotide monophosphate complexes. Optimal values of the unwinding angle, found in the range of -12 degrees to -14 degrees, are consistent with available experimental data on DNA.

Modelling basic features of specificity in DNA-aureolic acid-derived antibiotic interactions

The nonintercalative groove binding of a simplified model of olivomycin, to sequences d(CGCGCGC)2, d(TATATAT)2, and d(CICICIC)2 is investigated. A significant preference is displayed for the minor groove of the d(CG) sequence. This is due predominantly to the formation of H-bonds between the hydroxyl groups on the aglycone of the drug and the 2-amino group of the central guanine of the oligonucleotide.

Joint experimental and theoretical investigation of the comparative DNA binding affinities of intercalating anthracycline derivatives

The comparative binding affinities for poly(dA-dT) and poly(dG-dC) of novel antitumor anthracyclines are reported. The data concern, besides the parent compound adriamycin (ADM), 4-demethoxy 6-deoxy 6-aminodaunomycin (II), 9-deoxy-ADM (III), 4-demethyl-6-O-methyl-ADM (IV), and 3'-deamino-3'-hydroxy-4'-epi-ADM (IV). Theoretical computations are performed in parallel for their comparative binding affinities to model double-stranded hexanucleotides, d(GCGCGC)2, d(TATATA)2, and d(CGTACG)2, using the SIBFA (sum of interactions between fragments computed ab inito) procedure. The computations reproduce in a very satisfactory manner the most salient features of the experimental comparative binding affinities. These encompass, in particular, a higher affinity for the d(TATATA)2 oligomer of II than that of ADM, despite the absence of the 14-OH substituent in II, a marked reversal of the CG versus TA sequence selectivity of the neutral compound V, favoring the d(CGCGCG)2 oligomer over the d(TATATA)2 one; and the deleterious effect incurred on the binding affinities by the presence of an O-methyl substituent at position 6 of the chromophore.

Energetics and stereochemistry of DNA complexation with the antitumor AT specific intercalators tilorone and m-AMSA

Computations by the SIBFA method on the intercalative interaction energies of tilorone and m-AMSA with B-DNA representative oligonucleotides account for the specificity of these antitumor drugs for AT sites and minor groove intercalation. In tilorone this specificity is due to the strong preference of the side chains for the minor groove, which overcomes the preference of the chromophore for a GC intercalation site. In m-AMSA the specificity is due to the combined preference of both the chromophore and the anilino side chain for AT intercalation site and minor groove, respectively. o-AMSA is shown to manifest a similar (although significantly less pronounced specificity) as m-AMSA but a higher affinity for DNA. A comparison of the energetics and stereochemistry of intercalative binding to DNA of m-AMSA (AT minor groove specific) and 9-aminoacridine-4-carboxamide (GC major groove specific), which possess the same chromophore and differ only by the nature and position of the side chains, shows the possibility of important variations in the intercalative behaviour of chromophoric drugs as a function of the substituent groups attached to them.

Groove selectivity in the interaction of 9-aminoacridine-4-carboxamide antitumor agents with DNA

Theoretical quantitative evaluation of the intercalative binding to DNA of the new antitumor drug 9-aminoacridine-4-carboxamide indicates that, in contradiction with a recently proposed model, the compound should show specificity for interaction with the major (and not minor) groove of GC sequences.

A theoretical study of the intercalative binding of the anti-tumour drug anthrapyrazole to double-stranded oligonucleotides

Theoretical computations have been performed on the intercalative binding of anthrapyrazole to the double-stranded tetranucleotides d(GCGC)2 and d(ATAT)2, intercalation taking place in the central pyrimidine-(3',5')-purine sequences, with an unwinding angle of 29 degrees. The complex is principally stabilized by hydrogen-bonds of the two side chains with the ionic oxygens of the two central phosphates. In marked contrast with the corresponding complexes of the related compound mitoxantrone, the N7 atoms of the two central purines do not participate in hydrogen-bonding interactions with the dimethylamino fragments of the side chains. Side chain A of the anthrapyrazole, which is on the imino nitrogen side, is located farther away from the core of the major groove, closer to the phosphate backbone. Compared to the mitoxantrone complexes, there is a considerable decrease in the difference of interaction energy between the d(GCGC)2 and the d(ATAT)2 complexes. This is in line with the experimental results indicating that, compared to mitoxantrone, anthrapyrazole manifests little or no GC sequence selectivity.

Theoretical studies of the binding of trifluoperazine derivatives to site (82-93) of calmodulin: effect of lengthenings of the methylene linker chain on the binding affinity

A theoretical study was performed of the comparative binding affinities to fragment (82-93) of calmodulin (CaM) of trifluoperazine (TFP) and three derivatives, in which the methylene chain linking the phenothiazine ring and the piperazinium group was lengthened by addition of one to three methylenes. The backbone of the oligopeptide was held in the alpha-helical conformation. The computations were performed with the SIBFA procedures (sum of interactions between fragments computed ab initio), which use empirical formulas based on ab initio self-consistent field computations. The interaction energy is the sum of the intermolecular phenothiazine derivative-oligopeptide interaction energy and of the separate intramolecular energy variations of the ligand, on the one hand, and of the oligopeptide, on the other hand, upon relaxing the conformations of side chains Glu 84, Glu 87, Phe 89 and Phe 92 due to complex formation. All three derivatives were found to display a higher binding affinity than did TFP itself, an optimal affinity being found for a four- and a five-methylene linker chain. In as much as fragment (82-93) of CaM is a plausible candidate receptor site for phenothiazines, these results imply that two such compounds should be endowed with a significantly greater anti-CaM activity than TFP itself.

A tentative model of the intercalative binding of the neocarzinostatin chromophore to double-stranded tetranucleotides

Theoretical computations are performed of the intercalative binding of the neocarzinostatin chromophore (NCS) with the double-stranded oligonucleotides d(CGCG)2, d(GCGC)2, d(TATA)2 and d(ATAT)2. Minor groove binding is preferred over major groove binding. It is found that the long axis of the stacked naphtoate ring lies approximately parallel to the long axis of the base pairs of the intercalation site. The galactosamine ammonium group interacts with specific sites of the groove (O2/N3 of bases 2 and O1' of sugar S3), whereas the dodecadyine ring system wraps around the groove towards the backbone. An overall AT versus GC preference is derived. Intercalation in a central purine-(3', 5')-pyrimidine sequence appears to be preferred over that in a central pyrimidine-(3', 5')-purine sequence.

A theoretical study of anthracene and phenanthrene derivatives acting as A-T specific intercalators

Theoretical computations are performed on the comparative A-T versus G-C binding selectivities of two DNA intercalating molecules recently synthesized by Wilson et al. These are derivatives of phenanthrene and anthracene with side chains containing an hydroxy group bound to its C alpha carbon and a cationic amino group bound to its C beta carbon. We have optimized the binding energies of these phenanthrene and anthracene derivatives (1 and 2, respectively) to the double-stranded tetramers d(ATAT)2 and d(GCGC)2, the intercalation occurring in the central pyrimidine (3'-5') purine sequence. The sum of the intercalator-oligonucleotide intermolecular interaction energy plus the conformational energy variation of the intercalator upon binding were computed by the SIBFA procedures, which use empirical formulas based on ab initio SCF computations. Both compounds are found to bind more favourably to the AT sequence than to the GC one. Moreover, the affinity of 1 for the AT oligomer is computed to be larger than that of 2, whereas conversely that of 2 is larger than that of 1 for the GC oligomer. The AT versus GC binding selectivity of 1 is significantly larger than that of 2. These results are in excellent agreement with the experimental findings of Wilson et al. However, contrary to the suggestion of these authors the alpha-hydroxy group of the side chain of the intercalators does not seem to play a decisive role in determining the A-T specificity.

A theoretical study of the comparative binding affinities of daunomycin derivatives to a double-stranded oligomeric DNA. Proposal for new high affinity derivatives

Theoretical computations were performed on the comparative binding affinities of daunomycin (DM, 1) and seven derivatives related to the double-stranded oligonucleotide d(CGATCG)2. The compounds investigated were 4-demethoxy DM (2), and its beta-anomer (3), 4-demethoxy-7,9-bis-epi DM (4) and its beta anomer (5), a derivative with glucosamine instead of daunosamine (6), and two additional hypothetical DM derivatives in which the cationic NH3+ group of the daunosamine moiety is replaced by either a CH2--NH3+ group (7) or a CH2CH2NH3+ group (8), so as to indicate the effect on the binding affinity of interposing one- or two-methylene groups between the sugar and the cationic charge. The conformational angles of the hexanucleotide are fixed in values found in the representative crystal structure of the d(CGTACG)2-DM complex. The intermolecular drug-hexanucleotide interaction energies and the conformational energy changes of the drug upon binding are computed and optimized in the framework of the SIBFA procedure (sum of interactions between fragments computed ab initio), which uses empirical formulas based on ab initio SCF computations. The overall binding affinity ordering of compounds 1-6 compares satisfactorily with the ordering of available experimental affinity constants. The binding affinities of compounds 7 and 8, for which no experimental results seem to be available yet, are predicted to be significantly higher than those of the parent compound DM, with the greatest affinity found for 7. Because of the overall correlation between binding affinity of anthracyclines to DNA and their antitumor activity, these last two compounds deserve an exploration of their chemotherapeutic efficiency.

A theoretical investigation on the sequence selective binding of mitoxantrone to double-stranded tetranucleotides

Theoretical computations are performed on the comparative binding energetics of mitoxantrone (MX), a newly synthesized intercalating anthraquinone antitumor drug, to six representative double-stranded tetranucleotides: d(GCGC)2, d(CGCG)2, d(ATAT)2, d(TATA)2, d(GTGT), d(ACAC), and d(CCGG)2. The computations are performed with the SIBFA procedure, which uses empirical formulas based on ab initio SCF computations. The best binding configuration of mitoxantrone locates its two side chains in the major groove. A considerable preference is elicited for intercalation of the chromophore ring in a pyrimidine (3'-5') purine sequence rather than the isomeric purine (3'-5') pyrimidine sequence. Contrary to the situation encountered with "simple" intercalators, in which this preference is generally attributed solely to differences in the energies of unstacking necessary to generate the intercalation sites, the preference is dictated in MX to a large extent by the intermolecular interaction energy term. This result is imposed by the interactions of the side chains of MX with the oligonucleotide.

A theoretical study of the binding of phenothiazine derivatives to residues 82-93 of calmodulin

A theoretical study was performed of the interaction of four phenothiazine derivatives, promethazine, promazine, trifluopromazine, and trifluoperazine, with a fragment (82-93) of calmodulin, held in the alpha-helical conformation. The computations were performed in the framework of the SIBFA 2 procedure (sum of interactions between fragments computed ab initio), which uses analytical formulas based on ab initio self-consistent field computations. The interaction energy is the sum of the intermolecular phenothiazine-oligopeptide interaction energy and of the separate intramolecular energy variations of the phenothiazine and of the side chains of the oligopeptide upon complex formation. The ordering of interaction energies of the four investigated phenothiazines parallels the ordering of their experimentally measured affinities for calmodulin, with a maximum affinity for trifluoperazine. The principal features of the trifluoperazine complex are a short hydrogen bond between the piperazinium proton and one anionic oxygen of Glu 87, and hydrophobic interactions between the piperazinium ring and Val 91 and between the methylene chain and Ala 88, together with partial insertion of the phenothiazine ring and the--CF3 substituent between Phe 89 and Phe 92.

A theoretical investigation on the sequence selective binding of adriamycin to double-stranded polynucleotides

Theoretical computations are performed on the structural and energetical factors involved in the sequence selective binding of adriamycin (ADM) to five self-complementary double-stranded hexanucleotides. Among the two regularly alternating hexanucleotides d (TATATA)2 and d (CGCGCG)2, a stronger binding is predicted for the former. The strongest complex is computed, however, for the mixed hexanucleotide d (CGTACG)2, containing the intercalation site between two CG base pairs and an adjacent TA base pair. The overall sequence preference is the result of an intricate interplay of sequence preferences of the constituents in particular of daunosamine and the 9-OH substituent. Altogether, the selective base pair recognition by adriamycin cannot be defined in terms of the two base pairs implicated in the intercalation site alone but must be expressed in terms of a triplet of base pairs.

A theoretical investigation on the sequence selective binding of daunomycin to double-stranded polynucleotides

Theoretical computations are performed on the structural and energetical factors involved in the sequence selective binding of daunomycin (DNM) to six representative self-complementary double-stranded hexanucleotides: d(CGTACG)2,d(CGATCG)2,d(CITACI)2, d(TATATA)2, d(CGCGCG)2 and d(TACGTA)2. The conformational angles of the hexanucleotides are fixed in values found in the representative crystal structure of the d(CGTACG)2-DNM complex. The intermolecular DNM-hexanucleotide interaction energies and the conformational energy changes of DNM upon binding are computed and optimized in the framework of the SIBFA procedure, which uses empirical formulas based on ab initio SCF computations. Among the two regularly alternating hexanucleotides, d(TATATA)2 and d(CGCGCG)2, a stronger binding is predicted for the former, in agreement with experimental results obtained with poly(dA-dT).poly(dA-dT) and poly(dG-dC).poly(dG-dC). Altogether, however, among the six investigated sequences, the strongest complexes are computed for the mixed hexanucleotides d(CGATCG)2 and d(CGTACG)2, containing the intercalation site between two CG base pairs and an adjacent TA base pair. This situation may be related to the increased affinity of DNM for GC rich DNA's and to the situation in the crystal structure of the DNM-d(CGTACG)2 complex. Analysis of the intrinsic base sequence preferences expressed by the individual constituents of DNM, namely the daunosamine side chain, the chromophore ring and its two 9-hydroxyl and 9-acetoxy substituents, reveals that the overall sequence preference found is the result of a rather intricate interplay of intrinsic sequence preferences, in particular at the level of daunosamine and the 9-hydroxyl substituent.(ABSTRACT TRUNCATED AT 250 WORDS)

A theoretical study of the nonintercalative binding of berenil and stilbamidine to double-stranded (dA-dT)n oligomers

The nonintercalative binding of two diarylamidines , berenil and stilbamidine , to the minor groove of double-stranded (dA-dT)n oligomers in the B-DNA conformation was investigated by performing theoretical computations of their intermolecular interaction energies with the groove. The method consists of an additive procedure developed previously in this laboratory using empirical formulae based on ab initio computations. The objective was to assess the extent to which the particular structure of each diarylamidine bears on its binding mode and affinity to the minor groove. The results show that the intrinsically preferred configurations of the two compounds are markedly different. Owing to its slightly curved shape, berenil interacts with the groove predominantly through its concave side, the binding occurring principally with sites (O2, O1) belonging to two thymidines on the opposite strands. The binding of stilbamidine involves a more limited number of hydrogen-bonding interactions, although an appreciably large number of interatomic distances between its hydrogens and sites on the groove (O2, N3, O1) falls in the range 2.7-3.1 A. Each side of stilbamidine with respect to its long axis faces a distinct strand of DNA. The importance of the electrostatic contribution of the binding energy of the two diarylamidines is underlined. Preferential binding of berenil rather than of stilbamidine occurs only at the level of a complete helical turn of phosphates in (dA-dT)n. The energy difference increases significantly upon further buildup of phosphates. These results can be interpreted in terms of the molecular electrostatic potential in the grooves.

A theoretical study on the specificity of tetramethylammonium versus monomethylammonium binding to the active site of a phosphorylcholine antibody

In order to account for the specificity of binding of tetramethylammonium (TMA) versus monomethylammonium (MMA) to the combining site of phosphorylcholine specific immunoglobulin IgA H-8, theoretical computations are performed on the interaction energies of these cations with varying combinations of amino acid side chains, present or suspected to be present at that site. The dehydration of the cations, which represents a prerequisite for their binding is about 20 kcal/mol more difficult for MMA than for TMA. The interaction energies with the binding site are somewhat higher for MMA than for TMA. For some combinations of the amino acid side chains, their difference is smaller than the difference in the dehydration energy. Such combinations ensure preferential binding of TMA.

A theoretical study of the interaction of guanine and cytosine with specific amino acid side chains

Quantum-mechanical computations are performed on the in vacuo and in water interactions between the purine bases guanine and cytosine and the side chains of the amino acids arginine, lysine, glutamic acid and glutamine. The results predict that while guanine should be the more strongly interacting base both in vacuo and in water, lysine should be the most strongly interacting amino acid in vacuo and arginine the most strongly interacting amino acid in water solvent. The theoretical results on the interactions in water agree satisfactorily with experimentation.

Intermolecular chelation of two serine phosphates by Ca2+ and Mg2+. A theoretical structural investigation

The modes of interaction of Ca2+ and Mg2+ with 11 pre-selected conformations of serine phosphate (SP) are investigated by using an additive procedure based on ab initio Self Consistent Field computations for the calculation of intermolecular interaction energies. Possible models for the arrangements, SP-Ca2+-SP and SP-Mg2+-SP, are investigated. The comparison between the binding energetics of Mc2+ and Ca2+ to one and two serine phosphates is discussed. It appears that some specific arrangements, SP-M2+-SP (M2+ =Ca2+ or Mg2+), are able to account for the displayed marked selectivity of phosphatidylserine for Ca2+, in keeping with the distinctive features of this complex in model membranes.