Methylamide Research Articles

Macrocyclization studies and total synthesis of cyclomarin C, an anti-inflammatory marine cyclopeptide

The studies on macrocyclization at possible sites toward the total synthesis of cyclomarin C are described. The results showed that both Trp and Phe derivatives involved in the target could not be the terminals of the final linear peptide precursors. Additionally, preparation of corresponding dipeptides with an N-methyl amide bond is not favorable in the synthesis of linear precursors. Site d was finally proved a proper site for the cyclopeptide formation, and the corresponding head-to-tail macrocyclization was achieved under mild conditions and gave repeatable and satisfactory yields.

Novel [2‘,5‘-Bis-O-(tert-butyldimethylsilyl)-β-d-ribofuranosyl]- 3‘-spiro-5‘ ‘-(4‘ ‘-amino-1‘ ‘,2‘ ‘-oxathiole-2‘ ‘,2‘ ‘-dioxide) Derivatives with Anti-HIV-1 and Anti-Human-Cytomegalovirus Activity

New [2',5'-bis-O-(tert-butyldimethylsilyl)-beta-d-ribofuranosyl]-3'-spiro-5' '-(4''-amino-1'',2''-oxathiole-2'',2''-dioxide) (TSAO) derivatives substituted at the 4' '-amino group of the spiro moiety with different carbonyl functionalities have been designed and synthesized. Various synthetic procedures, on the scarcely studied reactivity of the 3'-spiroaminooxathioledioxide moiety, have been explored. The compounds were evaluated for their inhibitory effect on both wild-type and TSAO-resistant HIV-1 strains, in cell culture. The presence of a methyl ester (10) or amide groups (12) at the 4''-position conferred the highest anti-HIV-1 activity, while the free oxalyl acid derivative (11) was 10- to 20-fold less active against the virus. In contrast, the presence at this position of (un)substituted ureido or acyl groups markedly diminished or annihilated the anti-HIV-1 activity. Surprisingly, some of the target compounds also showed inhibition of human cytomegalovirus (HCMV) replication at subtoxic concentrations. This has never been observed previously for TSAO derivatives. In particular, compound 26 represents the first TSAO derivative with dual anti-HIV-1 and -HCMV activity.

Different Weak C−H···O Contacts in N-Methylacetamide-Water System: Molecular Dynamics Simulations and NMR Experimental Study

An all-atom N-methylacetamide (NMA) model and a refined SPC water model have been adopted for molecular dynamics simulations. 1H NMR chemical shifts of water, as well as the relative chemical shifts of the NMA methyl group and amide hydrogen atom in the NMA-water system, are measured and calculated over an entire composition range at different temperatures: 308, 323, 338, and 353 K. NMA molecules can act as a hydrogen bonding acceptor or a donor competing with water molecules. Interestingly, the two methyl groups in the NMA molecule are found to have different capabilities in forming weak C−H···O contacts in the mixtures from the radial distribution functions (RDFs). Also, temperature-dependent NMR results of the different methyl groups show excellent agreements with the MD simulations.

Synthesis and conformational analysis of 9,10-bis-aminomethyl-11,12-dicarboxy-dibenzobarrelene derivatives

The synthesis of bis-γ-amino acid dibenzobarrelene derivatives (9,10-bis-aminomethyl-11,12-bis-carboxy-dibenzobarrelene) is presented. Bromomethylation of anthracene followed by azide substitution gave 9,10-bis-azidomethylanthracene. Azide reduction, N-Boc protection, and Diels–Alder cycloaddition in DMAD furnished the protected 9,10-bi-aminomethyl-11,12-dicarboxy-dibenzobarrelene derivative, which was further converted into the bis-γ-amino acid methyl ester, the N-Boc-protected bis-γ-amino methyl amide, and a bis-γ-lactam. Monte Carlo simulations and X-ray analysis of the 9,10-substituted dibenzobarrelenes revealed an exposed hydrophobic surface surrounded by amino and carboxy groups.

Infrared-induced conformational isomerization and vibrational relaxation dynamics in melatonin and 5-methoxy-N-acetyl tryptophan methyl amide.

The conformational isomerization dynamics of melatonin and 5-methoxy N-acetyltryptophan methyl amide (5-methoxy NATMA) have been studied using the methods of IR-UV hole-filling spectroscopy and IR-induced population transfer spectroscopy. Using these techniques, single conformers of melatonin were excited via a well-defined NH stretch fundamental with an IR pump laser. This excess energy was used to drive conformational isomerization. By carrying out the infrared excitation early in a supersonic expansion, the excited molecules were re-cooled into their zero-point levels, partially re-filling the hole created in the ground state population of the excited conformer, and creating gains in population of the other conformers. These changes in population were detected using laser-induced fluorescence downstream in the expansion via an UV probe laser. The isomerization quantum yields for melatonin show some conformation specificity but no hint of vibrational mode specificity. In 5-methoxy NATMA, no isomerization was observed out of the single conformational well populated in the expansion in the absence of the infrared excitation. In order to study the dependence of the isomerization on the cooling rate, the experimental arrangement was modified so that faster cooling conditions could be studied. In this arrangement, the pump and probe lasers were overlapped in space in the high density region of the expansion, and the time dependence of the zero-point level populations of the conformers was probed following selective excitation of a single conformation. The analysis needed to extract isomerization quantum yields from the timing scans was developed and applied to the melatonin timing scans. Comparison between the frequency and time domain isomerization quantum yields under identical experimental conditions produced similar results. Under fast cooling conditions, the product quantum yields were shifted from their values under standard conditions. The results for melatonin are compared with those for N-acetyl tryptophan methyl amide.

Conformers of the peptides glycine-tryptophan, tryptophan-glycine and tryptophan-glycine-glycine as revealed by double resonance laser spectroscopy

The peptides Trp-Gly, Gly-Trp and Trp-Gly-Gly were investigated by UV–UV and IR–UV hole burning spectroscopy. Solid samples of the three peptides were vaporised into an argon jet by laser desorption. The IR–UV spectra of different conformers of the peptides were assigned by comparison with the IR–UV spectra of tryptophan [Snoek et al., Phys. Chem. Chem. Phys., 2001, 3, 1819], the free peptide bond in N-acetyl tryptophan methyl amide [Dian et al., J. Chem. Phys., 2002, 117, 10688] and ab initio calculations performed at the DFT B3LYP 6-31G(d,p) level. Apart from an NH⋯NH2 interaction, the peptide backbone of one conformer of each dipeptide is unfolded. The second conformer of Gly-Trp shows a COOH⋯OC hydrogen bond and the second conformer of Trp-Gly-Gly a hydrogen bond between the peptide backbone and the NH group of the indole ring.

The dynamics of conformational isomerization in flexible biomolecules. II. Simulating isomerizations in a supersonic free jet with master equation dynamics.

Infrared-induced conformational isomerization of N-acetyl-tryptophan methyl amide is studied theoretically using a microcanonical Rice-Ramsperger-Kassel-Marcus description of the rates on potential energy surfaces calculated using the AMBER and OPLS-AA force fields. The results are compared with the experimental data from Dian et al. in the preceding paper [J. Chem. Phys. 120, 133 (2004)]. An exhaustive search of the potential energy surfaces locates all minima and transition states on these surfaces. A simple model is proposed for the vibrational cooling, and an appropriate cooling rate is chosen as the standard conditions for the master equation simulations by comparison with experiment. The two potential energy surfaces differ in the relative energies of minima and the heights of key transition states, leading to differences in the dominant pathways and rates of conformational isomerization. The predicted quantum yields depend sensitively on the cooling rate, varying from the slow cooling limit in which equilibrium populations are achieved to the fast quenching limit in which conformational isomerization is shut off. The excitation energy is varied from 5 to 20 kcal mol(-1). At the lowest energies, isomerization is completely quenched, while at the highest energies, equilibrium conditions are achieved. In between these extremes, the quantum yields are sensitive to the excitation energy, and can be used to locate the rate-limiting barriers to isomerization.

The dynamics of conformational isomerization in flexible biomolecules. I. Hole-filling spectroscopy of N-acetyl tryptophan methyl amide and N-acetyl tryptophan amide.

The conformational isomerization dynamics of N-acetyl tryptophan methyl amide (NATMA) and N-acetyl tryptophan amide (NATA) have been studied using the methods of IR-UV hole-filling spectroscopy (HFS) and IR-induced population transfer spectroscopy (IR-PTS), which were developed for this purpose. Single conformations of these molecules were selectively excited in well-defined NH stretch fundamentals. This excess energy was used to drive conformational isomerization. By carrying out the infrared excitation early in a supersonic expansion, the excited molecules were recooled into their zero-point levels, partially refilling the hole created in the ground state population of one of the conformers, and creating gains in population in other conformers. These changes in population were detected using laser-induced fluorescence downstream in the expansion. In HFS, the IR wavelength is fixed and the UV laser tuned in order to determine where the population went following selective infrared excitation. In IR-PTS, the UV is fixed to monitor the population of a given conformation, and the IR is tuned to record the IR-induced changes in the population of the monitored conformer. Besides demonstrating the capability of the experiment to change the downstream conformational population distribution, the IR-PTS scans were used to extract two quantitative results: (i) The fractional populations of the conformers in the absence of the infrared, and (ii) the isomerization quantum yields for each of the six unique amide NH stretch fundamentals (three conformers each with two amide groups). The method for obtaining quantum yields is described in detail. In both NATMA and NATA, the quantum yields show modest conformational specificity, but only a hint of vibrational mode specificity. The prospects for the hole-filling technique for providing insight into energy flow in large molecules are discussed, leaving a more detailed theoretical modeling to the adjoining paper [Evans et al. J. Chem. Phys. 120, 148 (2004)].

Neutralization–reionization of ions produced by electrospray: Instrument design and initial data

A tandem mass spectrometer is described for neutralization–reionization studies of ions produced by electrospray ionization. The instrument consists of a high current electrospray source, an electrodynamic ion funnel, and an octopole ion guide that transport the ions into the high vacuum region of the tandem mass spectrometer. The ions are accelerated to 8250 eV and submitted to collisional neutralization and reionization, followed by deceleration, separation by kinetic energy and/or mass, and detection. The performance of the ionizer and ion transfer elements is optimized as a function of background pressure and the applied rf and dc potentials. Examples are given for neutralization–reionization mass spectra of protonated adenine, heterocycles, and β-alanine- N-methyl amide.

Synthesis and activity of 5′-Uridinyl dipeptide analogues mimicking the amino terminal peptide chain of nucleoside antibiotic mureidomycin A

A series of 5′-uridinyl dipeptides were synthesised which mimic the amino terminal chain of nucleoside antibiotic mureido omycin A. Aminoacyl-β-alanyl- and aminoacyl- N-methyl-β-alanyl- dipeptides were attached either via an ester linkage to the 5′-hydroxyl of uridine, or via an amide linkage to 5′-amino-5′-deoxyuridine. The most active inhibitor of Escherichia coli phospho-MurNAc-pentapeptide translocase (MraY) was 5′- O-( l-Ala- N-methyl-β-alanyl)-uridine ( 13l), which also showed 97% enzyme inhibition at 2.35 mM concentration, and showed antibacterial activity at 100 μg/mL concentration against Pseudomonas putida. Both the central N-methyl amide linkage and a 5′ uridine ester linkage were required for highest biological activity. Enzyme inhibition was shown to be competitive with Mg 2+. It is proposed that the primary amino terminus of the inhibitor binds in place of the Mg 2+ cofactor at the MraY active site, positioned via a cis- N-methyl amide linkage.

A Convenient Protocol for Selective Cleavage of 2‐Hydroxy Acid Amides. Application to Semisynthesis of the Cyclic Heptapeptide Aza HUN‐7293.

A two-step protocol for the first chemoselective cleavage of 2-hydroxy acid amides has been developed. Mesylation of the model substrate 2-(hydroxypropionylamino)-4-methylpentanoic acid methyl ester (11) followed by treatment with N-ethylthiourea (13) allows cleavage of 2-hydroxy acid amides under smooth conditions. Successful application of this methodology to the open-chain transesterification product 15 (methylester) of the cyclic heptadepsipeptide HUN-7293, a potent inhibitor of inducible cell adhesion molecule expression, delivered the corresponding hexapeptide 18 with unprotected N-terminus in 70−75% yield. This result demonstrates that the protocol developed even works in the presence of an ester and several methylated and unmethylated amide bonds. Finally, a sequence of ligation of methyl d-dehydroglutaminate (20) to the C-terminus of the saponification product 21, followed by the degradation protocol and ring closure, allowed chemical “point mutation” at the DGCN site affording the aza analogue o...

Kinetics and mechanism of the cyclization of omega-(p-nitrophenyl)-hydantoic acid amides: steric hindrance to proton transfer causes a 10(4)-fold change in rate.

The pH-rate profiles for the cyclization of primary 2,3-dimethyl and 2,2,3-trimethyl-hydantoinamides (2-UAm and 3-UAm respectively) differ strikingly from those for the cyclizations of the corresponding N-methylated amides 2-MUAm and 3-MUAm; which are dominated by the water reaction, spanning some 6 pH units. For the cyclization of UAm the plateau extends over no more than two pH units. The difference is due to the slower base-catalyzed cyclization of the N-methylamides. The solvent kinetic isotope effect for this hydroxide-catalyzed reaction is close to 1.2, consistent with a slow protonation by water of the amino-group of the negatively charged tetrahedral intermediate. General base catalysis was observed with bases of pKBH up to 8. The Brønsted beta are compatible with a hydrogen bonding mechanism for the GBC. In the gem-dimethyl compounds 3 the leaving group is flanked by substituents on both sides. The N-methyl group in 3-MUAm hinders frontal access of the proton, causing a 14000 fold decrease in rate. This is only 3800 fold in the compound with one methyl group at position 2.

Hydride stretch infrared spectra in the excited electronic states of indole and its derivatives: Direct evidence for the 1πσ* state

The hydride stretch infrared spectra of indole, indole-H2O, 3-methyl indole, 3-methyl indole-H2O, the main conformer of tryptamine (TRA), two conformers of N-acetyl tryptophan amide (NATA), and three conformers of N-acetyl tryptophan methyl amide (NATMA), have been recorded in the electronically excited singlet states using excited-state fluorescence-dip infrared spectroscopy. NATA and NATMA are methyl-capped dipeptides of tryptophan that have conformational flexibility and exhibit sensitivity in their electronic spectra to the conformation of the dipeptide backbone. In the indole monomer, the indole NH stretch fundamental at the S1 origin is shifted from its ground-state value (3525 cm−1) to 3478 cm−1. The corresponding band in the indole-H2O complex appears at 3387 cm−1, shifted by a similar amount from its ground-state position (3436 cm−1). Higher vibronic levels within 1500 cm−1 of the S1 origin, which have been identified previously [B. J. Fender et al., Chem. Phys. Lett. 239, 31 (1995)] as being 1Lb or 1La in character, all show similar excited state indole NH stretch absorptions. The corresponding spectra in 3-methyl indole, 3-methyl indole-H2O, TRA, and in the C5 conformers of NATA and NATMA all are missing the indole NH stretch absorption. In its place, a broad background absorption appears, spread over the entire 2800-3800 cm−1 region. In these molecules, other CH stretch or amide NH stretch absorptions remain sharp, appearing in their expected frequency ranges. Finally, the C7 conformations of NATA and NATMA, which possess an intramolecular hydrogen bond in the dipeptide backbone, have all infrared transitions washed out, replaced by a stronger broad background absorption. The entire data set can be explained by the presence of an excited 1πσ* state which is dissociative along the indole NH stretch coordinate, as recently predicted by Sobolewski and Domcke [Chem. Phys. Lett. 315, 293 (1999)]. In the weak coupling case (indole, indole-H2O), the gap between the 1πσ* state and the S1 origin is too large to be reached by infrared excitation. The selective loss of the indole NH stretch in the intermediate coupling case reflects the strong coupling of the 1Lb state NH stretch (v=1) level to the 1πσ* state, which is dissociative along the NH stretch coordinate. In the NATA and NATMA C7 conformers, an inversion of ordering of the electronic states occurs, pushing the 1La state below the 1Lb origin, and strengthening the coupling of all hydride stretch vibrational levels to the 1πσ* dissociative continuum. These results highlight the important influence of the conformation of the polypeptide backbone on the photophysics of tryptophan in polypeptides.

Phthalimide as a chromophoric tag in the circular dichroism determination of absolute configuration of α-aminoacid amides and dipeptides. A case of a dipeptide isostructurality

Both N-phthaloyl aminoacid amides and methyl amides as well as N-phthaloyl dipeptide methyl esters give Cotton effects of opposite signs at around 220 and 240nm. The signs of these Cotton effects are directly correlated with the absolute configuration of the N-phthaloyl substituted stereogenic center: for l-configuration the Cotton effect at 240nm is positive and the Cotton effect at 220 is negative. The X-ray analysis shows that the diastereomeric l,d and l,lN-phthaloyl alanylvaline methyl esters form isostructural orthorombic crystals due to the conformational flexibility of the molecules and a similar space requirements of the methoxycarbonyl and isopropyl groups.

The infrared and ultraviolet spectra of single conformations of methyl-capped dipeptides: N-acetyl tryptophan amide and N-acetyl tryptophan methyl amide

A combination of methods, including laser-induced fluorescence excitation, fluorescence-dip infrared (FDIR) spectroscopy, and UV-UV hole-burning spectroscopy, have been used to study the infrared and ultraviolet spectra of single conformations of two methyl-capped dipeptides: N-acetyl tryptophan amide (NATA) and N-acetyl tryptophan methyl amide (NATMA). Density functional theory calculations predict that all low-energy conformers of NATA and NATMA belong to one of two conformational families: C5, with its extended dipeptide backbone, or C7eq, in which the dipeptide backbone forms a seven-membered ring joined by a H bond between the ψ-amide NH and the φ-amide carbonyl groups. In NATA (NATMA), the LIF spectrum has contributions from two (three) conformers. FDIR spectroscopy has been used to record infrared spectra of the individual conformers over the 2800–3600 cm−1 region, free from interference from one another. The NH stretch region provides unequivocal evidence that one of the conformers of NATA is C5, while the other is C7eq. Similarly, in NATMA, there are two C5 conformers, and one C7eq structure. Several pieces of evidence are used to assign spectra to particular C5 and C7eq conformers. NATA(A) and NATMA(B) are both assigned as C5(AP) structures, NATA(B) and NATMA(C) are assigned as C7eq (ΦP), and NATMA(A) is assigned as C5(AΦ). In both molecules, the C5 structures have sharp vibronic spectra, while the C7eq conformers are characterized by a dense, highly congested spectrum involving long progressions that extend several hundred wave numbers to the red of the C5 S1–S0 origins. N-acetyl tryptophan ethyl ester (NATE), which can only form C5 conformers, shows only sharp transitions in its LIF spectrum due to four C5 conformers, with no evidence for the broad absorption due to C7eq. This provides direct experimental evidence for the importance of the peptide backbone conformation in controlling the spectroscopic and photophysical properties of tryptophan.

Tandem Oxidation Processes: The Direct Conversion of Activated Alcohols into Esters and Amides.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Post-synthesis incorporation of a lipidic side chain into a peptide on solid support.

A new strategy for the synthesis of lipopeptides has been developed. Using Weinreb (N-methoxy, N-methyl) amide as an aldehyde function precursor on the side chains of Asp or Glu residues, this new strategy avoids the synthesis of a lipidic amino acid residue before its incorporation in the peptide sequence. The aldehyde generated on the solid support can react with ylides leading to unsaturated or saturated side chains or with various nucleophiles to yield non-coded amino acid residues incorporated into the sequence.

Ab initio conformational space study of model compounds of O-glycosides of serine diamide.

Relative stabilities of rotamers of the N-acetyl-O-(2-acetamido-2-deoxy-alpha-D-galactopyranosyl)-L-seryl-N'-methyl amide (1) and eleven analogous molecules containing beta-galactose, alpha- and beta-mannose, alpha- and beta-glucose, and L-threonine were calculated to learn whether they could explain the natural preference for 1 in linkages between the carbohydrate and protein in glycoproteins. The lowest energy rotamers of four O-glycoside models of serine diamide were identified with a Monte Carlo search coupled with molecular mechanics (MM2*). These rotamers were further optimized with an ab initio level of theory (HF/6-31G(d)). Subsequently, B3LYP/6-31 + G(d) single point energies were calculated for the most stable HF structures. The most favorable interactions are present in 1 and its glucose analogue. The monosaccharide for the carbohydrate antenna is anchored to the serine residue with an AcNH...O=C-NHMe hydrogen bond in the most stable rotamers. The mannose analogue and the beta-anomers are considerably less stable according to the MM2* and especially to the ab inito energy values. The three analogues have HF/6-31 G(d) energies which are 4-6 kcal mol-1 higher; the single point B3LYP/6-31 + G(d)//HF/6-31 G(d) calculations yield preferences of 3-5 kcal mol-1 for 1. The most stable L-threonine analogues show a behaviour very similarly to the corresponding serine analogues. The ZPE and thermal correction components of the calculated delta H298 and delta G298 values are relatively small (< 0.4 kcal mol-1). However, the T delta S298 term can be as large as 2.6 kcal mol-1. The entropy terms stabilize the alpha-anomers relative to beta-anomers, and ManNAc relative to GalNAc. The largest stabilization effect is observed for one of the rotamers of the alpha-anomer of ManNAc.

Synthesis of anthelmintically active N-methylated amidoxime analogues of the cyclic octadepsipeptide PF1022A.

The N-methylated amidoxime analogues of the cyclic octadepsipeptide PF1022A represent novel derivatives with activity against Trichinella spiralis Owen and Nippostrongylus brasiliensis Lane in vitro and against parasitic nematodes in mice and sheep. Some of them show better activity against Hymenolepis nana Siebold, Heterakis spumosa Schneider and Heligmosomoides polygyrus Dujardin in mice than the natural product PF1022A. In particular an improved efficacy against Haemonchus contortus Rudolphi and Trichostrongylus colubriformis Giles in sheep compared to the potent cyclic octadepsipeptide PF1022A and its mono-thionated derivative has been observed. Here we report on a specific modification at the N-methyl amide linkage by using the mono-thionated PF1022A, resulting in novel anthelmintically active backbone analogues of PF 1022A.

Cyclosporins: structure-activity relationships for the inhibition of the human MDR1 P-glycoprotein ABC transporter.

Cyclic undecapeptide cyclo-[MeBmt(1)-Abu(2)-MeGly(3)-MeLeu(4)-Val(5)-MeLeu(6)-Ala(7)-D-Ala(8)-MeLeu(9)-MeLeu(10)-MeVal(11)], the immunosuppressive and antifungal antibiotic cyclosporin A (CsA), was reported to interfere with the MDR1 P-glycoprotein (Pgp), a transmembranous adenosine 5'-triphosphate binding cassette (ABC) transporter with phospholipid flippase or "hydrophobic vacuum cleaner" properties that mediate multidrug resistance (MDR) of cancer cells. By use of photoaffinity-labeled cyclosporins and membranes from Pgp-expressing cells, it was recently shown that in vitro, Pgp molecules could bind a large cyclosporin domain involving residues 4-9 as well as the side chain of residue 1. Tumor cell MDR can also be reversed by a product more distantly related to cyclosporin with the structure [Thr(2), Leu(5), D-Hiv(8), Leu(10)]-CsA (SDZ 214-103). In a standardized assay that measures Pgp function in vivo (on intact live cells) by the Pgp-mediated efflux of the calcein-AM Pgp substrate and uses human lymphoblastoid MDR-CEM (VBL(100)) cells as highly resistant Pgp-expressing cells, SDZ 214-103 was found to be one of the most active Pgp inhibitors among naturally occurring cyclosporins, with an IC(50) of 1.6 microM in an assay where CsA gives an IC(50) of 3.4 microM. Using the in vivo assay, 60, mostly natural, cyclosporin analogues were analyzed to establish structure-activity relationships (SAR). Our SAR are compatible with the in vitro-defined Pgp binding domain model and further disclose that in vivo Pgp inhibition is favored by larger hydrophobic side chains on cyclosporin residues 1, 4, 6, and 8 and a smaller one on residue 7, although with no effect on the residue 5 side chain; moreover, larger hydrophobic side chains on other residues 2, 3, 10, and 11 (outside the in vitro-defined Pgp binding domain) also favor the eventual inhibition of Pgp function. The N-desmethylation of any of the seven N-methylated amides, as naturally occurring in numerous cyclosporins, regularly leads to a decreased Pgp inhibitory activity (Pgp-InhA), up to its abrogation if it occurs at residues 4 and 9. Nevertheless, despite unfavorable use of [Thr(2)] and [Leu(10)] residues, all [D-Hiv(8)] analogues whose lead is SDZ 214-103 show a large Pgp-InhA. The SAR for Pgp inhibition by cyclosporins are thus very complex. Because CsA and SDZ 214-103 show largely different conformations when free in solution, but remarkably similar ones when bound to the cytosolic cyclophilins, SAR for Pgp inhibition must similarly include requirements for occurrence of suitable conformers for insertion in the cell membrane, sufficient conformational plasticity for gaining access to Pgp binding sites, and an adequate conformer structure there to achieve such binding with a high enough affinity and possibly escape from sequestration on cyclophilins.