Influence on proline-specific enzymes of a substrate containing the thioxoaminoacyl-prolyl peptide bond

Rational design of thioamide peptides as selective inhibitors of cysteine protease cathepsin L

Aberrant levels of cathepsin L (Cts L), a ubiquitously expressed endosomal cysteine protease, have been implicated in many diseases such as cancer and diabetes. Significantly, Cts L has been identified as a potential target for the treatment of COVID-19 due to its recently unveiled critical role in SARS-CoV-2 entry into the host cells. However, there are currently no clinically approved specific inhibitors of Cts L, as it is often challenging to obtain specificity against the many highly homologous cathepsin family cysteine proteases. Peptide-based agents are often promising protease inhibitors as they offer high selectivity and potency, but unfortunately are subject to degradation in vivo. Thioamide substitution, a single-atom O-to-S modification in the peptide backbone, has been shown to improve the proteolytic stability of peptides addressing this issue. Utilizing this approach, we demonstrate herein that good peptidyl substrates can be converted into sub-micromolar inhibitors of Cts L by a single thioamide substitution in the peptide backbone. We have designed and scanned several thioamide stabilized peptide scaffolds, in which one peptide, RS 1A, was stabilized against proteolysis by all five cathepsins (Cts L, Cts V, Cts K, Cts S, and Cts B) while inhibiting Cts L with >25-fold specificity against the other cathepsins. We further showed that this stabilized RS 1A peptide could inhibit Cts L in human liver carcinoma lysates (IC50 = 19 μM). Our study demonstrates that one can rationally design a stabilized, specific peptidyl protease inhibitor by strategic placement of a thioamide and reaffirms the place of this single-atom modification in the toolbox of peptide-based rational drug design.

Opportunities and challenges in the synthesis of thioamidated peptides

Chemical modifications of peptides hold great promise for modulating their pharmacological properties. In the last few decades amide to thioamide substitution has been widely explored to modulate the conformation, non-covalent interactions, and proteolytic stability of peptides. Despite widespread utilization, there are some potential limitations including epimerization and degradation under basic and acidic conditions, respectively. In this chapter, we present the synthetic method to build thio-precursors, their site-specific incorporation onto a growing peptide chain, and troubleshooting during the elongation of thioamidated peptides. This highly efficient, rapid, and robust method can be used for positional scanning of the thioamide bond.

Studies of Thioamide Effects on Serine Protease Activity Enable Two-Site Stabilization of Cancer Imaging Peptides

Thioamide substitutions in peptides can be used as fluorescence quenchers in protease sensors and as stabilizing modifications of hormone analogs. To guide these applications in the context of serine proteases, we here examine the cleavage of several model substrates, scanning a thioamide between the P3 and P3' positions, and identify perturbing positions for thioamide substitution. While all serine proteases tested were affected by P1 thioamidation, certain proteases were also significantly affected by other thioamide positions. We demonstrate how these findings can be applied by harnessing the combined P3/P1 effect of a single thioamide on kallikrein proteolysis to protect two key positions in a neuropeptide Y-based imaging probe, increasing its serum half-life to >24 h while maintaining potency for binding to Y1 receptor expressing cells. Such stabilized peptide probes could find application in imaging cell populations in animal models or even in clinical applications such as fluorescence-guided surgery.

One-Atom Substitution Enables Direct and Continuous Monitoring of Histone Deacylase Activity

We developed a one-step direct assay for the determination of histone deacylase (HDAC) activity by substituting the carbonyl oxygen of the acyl moiety with sulfur, resulting in thioacylated lysine side chains. This modification is recognized by class I HDACs with different efficiencies ranging from not accepted for HDAC1 to kinetic constants similar to that of the parent oxo substrate for HDAC8. Class II HDACs can hydrolyze thioacylated substrates with approximately 5-10-fold reduced k_cat values, which resembles the effect of thioamide substitution in metallo-protease substrates. Class IV HDAC11 accepts thiomyristoyl modification less efficiently with an ∼5-fold reduced specificity constant. On the basis of the unique spectroscopic properties of thioamide bonds (strong absorption in spectral range of 260-280 nm and efficient fluorescence quenching), HDAC-mediated cleavage of thioamides could be followed by ultraviolet-visible and fluorescence spectroscopy in a continuous manner. The HDAC activity assay is compatible with microtiter plate-based screening formats up to 1536-well plates with Z' factors of >0.75 and signal-to-noise ratios of >50. Using thioacylated lysine residues in p53-derived peptides, we optimized substrates for HDAC8 with a catalytic efficiency of >250000 M^-1 s^-1, which are more than 100-fold more effective than most of the known substrates. We determined inhibition constants of several inhibitors for human HDACs using thioacylated peptidic substrates and found good correlation with the values from the literature. On the other hand, we could introduce N-methylated, N-acylated lysine residues as inhibitors for HDACs with an IC₅₀ value of 1 μM for an N-methylated, N-myristoylated peptide derivative and human HDAC11.

Fluorescent Probes for Studying Thioamide Positional Effects on Proteolysis Reveal Insight into Resistance to Cysteine Proteases

Thioamide substitutions of the peptide backbone have been shown to reduce proteolytic degradation, and this property can be used to generate competitive protease inhibitors and to stabilize peptides toward degradation in vivo. Here, we present a straightforward sensor design that allows a systematic study of the positional effects of thioamide substitution by using real-time fluorescence. Thioamide scanning in peptide substrates of five papain family cysteine proteases demonstrates that a thioamide at or near the scissile bond can slow proteolysis in all cases, but that the magnitude of the effects varies with position and protease in spite of high sequence homology. Mechanistic investigation of papain proteolysis reveals that the thioamide effects derive from reductions in both affinity (KM ) and turnover number (kcat ). Computational modeling allows these effects to be understood based on disruption of key enzyme-substrate hydrogen bonds, providing a model for future rational use of thioamides to confer cysteine protease resistance.

Biosynthesis and Chemical Applications of Thioamides

Thioamidation as a posttranslational modification is exceptionally rare, with only a few reported natural products and exactly one known protein example (methyl-coenzyme M reductase from methane-metabolizing archaea). Recently, there has been significant progress in elucidating the biosynthesis and function of several thioamide-containing natural compounds. Separate developments in the chemical installation of thioamides into peptides and proteins have enabled cell biology and biophysical studies to advance the current understanding of natural thioamides. This review highlights the various strategies used by Nature to install thioamides in peptidic scaffolds and the potential functions of this rare but important modification. We also discuss synthetic methods used for the site-selective incorporation of thioamides into polypeptides with a brief discussion of the physicochemical implications. This account will serve as a foundation for the further study of thioamides in natural products and their various applications.

Thioamide Substitution Selectively Modulates Proteolysis and Receptor Activity of Therapeutic Peptide Hormones

Peptide hormones are attractive as injectable therapeutics and imaging agents, but they often require extensive modification by mutagenesis and/or chemical synthesis to prevent rapid in vivo degradation. Alternatively, the single-atom, O-to-S modification of peptide backbone thioamidation has the potential to selectively perturb interactions with proteases while preserving interactions with other proteins, such as target receptors. Here, we use the validated diabetes therapeutic, glucagon-like peptide-1 (GLP-1), and the target of clinical investigation, gastric inhibitory polypeptide (GIP), as proof-of-principle peptides to demonstrate the value of thioamide substitution. In GLP-1 and GIP, a single thioamide near the scissile bond renders these peptides up to 750-fold more stable than the corresponding oxopeptides toward cleavage by dipeptidyl peptidase 4, the principal regulator of their in vivo stability. These stabilized analogues are nearly equipotent with their parent peptide in cyclic AMP activation assays, but the GLP-1 thiopeptides have much lower β-arrestin potency, making them novel agonists with altered signaling bias. Initial tests show that a thioamide GLP-1 analogue is biologically active in rats, with an in vivo potency for glycemic control surpassing that of native GLP-1. Taken together, these experiments demonstrate the potential for thioamides to modulate specific protein interactions to increase proteolytic stability or tune activation of different signaling pathways.

A novel analog of antimicrobial peptide Polybia-MPI, with thioamide bond substitution, exhibits increased therapeutic efficacy against cancer and diminished toxicity in mice

Polybia-MPI (MPI), a short cationic α-helical antimicrobial peptide, exhibited excellent anticancer activity and selectivity in vitro in our previous studies. To improve its in vivo application, we synthesized an analog (MPI-1) of MPI by replacing the C terminal amide -[CO-NH(2)] with thioamide -ψ[CS-NH(2)]. Although there is just one atom difference, the MPI-1 exhibited some surprising properties. In vitro studies revealed that MPI-1 exhibited relatively high lytic activity over MPI, whereas its stability to enzymatic degradation in serum was improved remarkably. Despite the enhanced toxicity in vitro, MPI-1 exhibited significantly lower mortality to mice than MPI at 75 mg/kg. Importantly, in vivo anticancer activity study indicated that MPI-1 could remarkably suppress the growth of sarcoma xenograft tumors more efficiently than MPI. Therefore, the significantly improved anticancer activity and predominantly lower in vivo toxicity might allow MPI-1 to be a good candidate for future anticancer treatment.

Transient non-native hydrogen bonds promote activation of a signaling protein

Phosphorylation is a common mechanism for activating proteins within signaling pathways. Yet, the molecular transitions between the inactive and active conformational states are poorly understood. Here we quantitatively characterize the free-energy landscape of activation of a signaling protein, nitrogen regulatory protein C (NtrC), by connecting functional protein dynamics of phosphorylation-dependent activation to protein folding and show that only a rarely populated, pre-existing active conformation is energetically stabilized by phosphorylation. Using nuclear magnetic resonance (NMR) dynamics, we test an atomic scale pathway for the complex conformational transition, inferred from molecular dynamics simulations (Lei et al., 2009). The data show that the loss of native stabilizing contacts during activation is compensated by non-native transient atomic interactions during the transition. The results unravel atomistic details of native-state protein energy landscapes by expanding the knowledge about ground states to transition landscapes.

A photoswitchable thioxopeptide bond facilitates the conformation-activity correlation study of insect kinin

Thioxopeptide bond psi[CS-N], a nearly isosteric modification of the native peptide bond, was introduced into insect kinin active core pentapeptide to evaluate the impact of backbone cis/trans photoswitching on bioactivity. The thioxo analog Phe(1)-Tyr(2)-psi[CS-N]-Pro(3)-Trp(4)-Gly(5)-NH(2) (psi[CS-N](2)-kinin), was synthesized by Fmoc solid-phase peptide strategy. The reversible photoswitching property was characterized via spectroscopic methods and HPLC, which showed that the cis conformer increased from 15.7 to 47.7% after 254 nm UV irradiation. A slow thermal reisomerization (t(1/2) = 40 min) permitted us to determine the cockroach hindgut myotropic activity of the thioxopeptide in the photostationary state. The results indicated that the activity increased significantly after UV irradiation and recovered to the ground level after thermal re-equilibration. In the present study, by utilizing the phototriggered isomerization in a specific position of peptide backbone, we revealed that the cis psi[CS-N](2)-kinin conformer is the active conformation when interacting with kinin receptor on cockroach hindgut.

Force-Induced Prolyl Cis−Trans Isomerization in Elastin-like Polypeptides

Elastin-like polypeptides (ELPs) are stimulus-responsive polymers that contain repeats of five amino acids, Val-Pro-Gly-Xaa-Gly (VPGXG), where Xaa is a guest residue that can be any amino acid with the exception of proline. While studying the conformational mechanics of ELPs over a range of solvent conditions by single-molecule force spectroscopy, we noticed that some force-extension curves showed temperature-independent, extensional transitions that could not be fitted with a freely jointed chain or worm-like chain model. Here we show that the observed molecular elongation results from the force-induced peptidyl-prolyl cis-trans isomerization in prolines, which are repeated every fifth residue in the main chain of ELPs. Control experiments with poly(L-proline) demonstrate the similarity of the conformational transition between poly(L-proline) and ELPs. In contrast, the force-extension behavior of poly(L-lysine) showed no deviation in the relevant force range. Force-extension curves in hysteresis experiments showed an elongational difference between extension and relaxation pathways that suggests that the cis conformational state of the prolines could be exhausted on the time scale of the experiment. We present further computational evidence for this mechanism by Monte Carlo simulation of the force-extension behavior using an elastically coupled, two-state model. We believe ours is the first demonstration of force-induced prolyl cis-trans isomerization in proline-containing polypeptides. Our results suggest that single-molecule force spectroscopy could provide an alternate means to assay this important conformational transition in polypeptides.

A nearly isosteric photosensitive amide-backbone substitution allows enzyme activity switching in ribonuclease s

psi[CS-NH]4-RNase S, a site specific modified version of RNase S obtained by thioxylation (O/S exchange) at the Ala4-Ala5- peptide bond, was used to evaluate the impact of protein backbone photoswitching on bioactivity. psi[CS-NH](4)-RNase S was yielded by recombination of the S-protein and the respective chemically synthesized thioxylated S-peptide derivative. Comparison with RNase S revealed similar thermodynamic stability of the complex and an unperturbed enzymatic activity toward cytidine 2',3'-cyclic monophosphate (cCMP). Reversible photoisomerization with a highly increased cis/trans isomer ratio of the thioxopeptide bond of psi[CS-NH](4)-RNase S in the photostationary state occurred under UV irradiation conditions (254 nm). The slow thermal reisomerization (t(1/2) = 180 s) permitted us to determine the enzymatic activity of cis psi[CS-NH](4)-RNase S by measurement of initial rates of cCMP hydrolysis. Despite thermodynamic stability of cis psi[CS-NH](4)-RNase S, its enzymatic activity is completely abolished but recovers after reisomerization. We conclude that the thioxopeptide bond modified polypeptide backbone represents a versatile probe for site-directed photoswitching of proteins.

cis/trans Photoisomerization of Secondary Thiopeptide Bonds

The reversible cis/trans photoisomerization of secondary thiopeptide bonds has been systematically studied with UV-visible absorption, capillary electrophoresis, 1H NMR spectroscopy, and circular dichroism methods. It was found that the concentration of the cis conformers could be increased from less than 1 % in the thermal equilibrated solution to up to 20 % in the photostationary state. The rotational barriers of the thiopeptide bond and the pH dependence of the isomerization rates were also studied. The quantum yields of the trans-->cis and the cis-->trans processes were determined from photokinetic analysis.

Dipeptidyl-peptidase IV from bench to bedside: an update on structural properties, functions, and clinical aspects of the enzyme DPP IV

Dipeptidyl-peptidase IV/CD26 (DPP IV) is a cell-surface protease belonging to the prolyloligopeptidase family. It selectively removes the N-terminal dipeptide from peptides with proline or alanine in the second position. Apart from its catalytic activity, it interacts with several proteins, for instance, adenosine deaminase, the HIV gp120 protein, fibronectin, collagen, the chemokine receptor CXCR4, and the tyrosine phosphatase CD45. DPP IV is expressed on a specific set of T lymphocytes, where it is up-regulated after activation. It is also expressed in a variety of tissues, primarily on endothelial and epithelial cells. A soluble form is present in plasma and other body fluids. DPP IV has been proposed as a diagnostic or prognostic marker for various tumors, hematological malignancies, immunological, inflammatory, psychoneuroendocrine disorders, and viral infections. DPP IV truncates many bioactive peptides of medical importance. It plays a role in glucose homeostasis through proteolytic inactivation of the incretins. DPP IV inhibitors improve glucose tolerance and pancreatic islet cell function in animal models of type 2 diabetes and in diabetic patients. The role of DPP IV/ CD26 within the immune system is a combination of its exopeptidase activity and its interactions with different molecules. This enables DPP IV/CD26 to serve as a co-stimulatory molecule to influence T cell activity and to modulate chemotaxis. DPP IV is also implicated in HIV-1 entry, malignant transformation, and tumor invasion.

Expression, purification, and kinetic characterization of full-length human fibroblast activation protein

Human fibroblast activation protein (FAP), an integral membrane serine protease, was produced in insect cells as a hexa-His-tagged protein using a recombinant baculovirus expression system. Two isoforms of FAP, glycosylated and nonglycosylated, were identified by Western blotting using an anti-His-tag antibody and separated by lectin chromatography. The glycosylated FAP was purified to near homogeneity using immobilized metal affinity chromatography and was shown to have both postprolyl dipeptidyl peptidase and postgelatinase activities. In contrast, the nonglycosylated isoform demonstrated no detectable gelatinase activity by either zymography or a fluorescence-based gelatinase activity assay. The kinetic parameters of the dipeptidyl peptidase activity for glycosylated FAP were determined using dipeptide Ala-Pro-7-amino-trifluoromethyl-coumarin as the substrate. The k(cat) is 2.0 s(-1) and k(cat)/K(m) is 1.0 x 10(4) M(-1) s(-1) at pH 8.5. The pH dependence of k(cat) reveals two ionization groups with pK(a1) of 7.0 and pK(a2) of 11.0. The pH profile of k(cat)/K(m) yields similar results with pK(a1) 6.2 and pK(a2) 11.0. The neutral pK(a1) is associated with His at the active site. The basic pK(a2) might be contributed from an ionization group that is not involved directly in catalysis, instead associated with the stability of the active site structure.

Thioxo amino acid pyrrolidides and thiazolidides: new inhibitors of proline specific peptidases

Aminopeptidase P (APP), dipeptidyl peptidase II (DP II), dipeptidyl peptidase IV (DP IV) and prolyl oligopeptidase (POP) are proline specific peptidases. Hence, they are able to cleave peptide bonds containing the imino acid proline. Amino acid pyrrolidides (Pyrr) and thiazolidides (Thia) are well-known product analogue inhibitors of DP IV and POP. For the first time we describe the influence of a thioxo amide bond, incorporated into these compounds, on the inhibition of the proline specific peptidases. Taking into account the substrate specificity of these peptidases, we have synthesized Xaa-psi[CS-N]-Pyrr and Xaa-psi[CS-N]-Thia of the amino acids Ala, Phe, Val and Ile. The inhibition constants were determined for the above mentioned proline specific peptidases isolated from different sources. As a result, the serine proteases DP II, DP IV and POP were inhibited competitively, whereas metal-dependent APP displayed a linear mixed-type inhibition with inhibition constants up to 10(-4) M. Thioxylation of Xaa-Pyrr and Xaa-Thia led to a slight decrease of inhibition of DP IV and POP compared to Xaa-Pyrr and Xaa-Thia, though the inhibition constants were still in the range up to 10(-7) M. As Xaa-Thia exist as two isomers, we investigated isomer specific inhibition with regard to DP IV. Thus, our studies have revealed that DP IV was only inhibited by the Z isomer of the Xaa-psi[CS-N]-Thia. For the first time, Xaa-Pyrr and Xaa-Thia were characterized as inhibitors of DP II with inhibition constants in the micromolar range. In contrast to DP IV inhibition, the Xaa-psi[CS-N]-Pyrr and Xaa-psi[CS-N]-Thia have proven to be more potent inhibitors of DP II than the corresponding Xaa-Pyrr and Xaa-Thia. Thus, these Xaa-psi[CS-N]-Thia are new potent inhibitors especially suitable for DP II with K(i) values ranging in the upper nanomolar concentration.

Fibroblast activation protein, a dual specificity serine protease expressed in reactive human tumor stromal fibroblasts

Proteolytic degradation of extracellular matrix (ECM) components during tissue remodeling plays a pivotal role in normal and pathological processes including wound healing, inflammation, tumor invasion, and metastasis. Proteolytic enzymes in tumors may activate or release growth factors from the ECM or act directly on the ECM itself, thereby facilitating angiogenesis or tumor cell migration. Fibroblast activation protein (FAP) is a cell surface antigen of reactive tumor stromal fibroblasts found in epithelial cancers and in granulation tissue during wound healing. It is absent from most normal adult human tissues. FAP is conserved throughout chordate evolution, with homologues in mouse and Xenopus laevis, whose expression correlates with tissue remodeling events. Using recombinant and purified natural FAP, we show that FAP has both dipeptidyl peptidase activity and a collagenolytic activity capable of degrading gelatin and type I collagen; by sequence, FAP belongs to the serine protease family rather than the matrix metalloprotease family. Mutation of the putative catalytic serine residue of FAP to alanine abolishes both enzymatic activities. Consistent with its in vivo expression pattern determined by immunohistochemistry, FAP enzyme activity was detected by an immunocapture assay in human cancerous tissues but not in matched normal tissues. This study demonstrates that FAP is present as an active cell surface-bound collagenase in epithelial tumor stroma and opens up investigation into physiological substrates of its novel, tumor-associated dipeptidyl peptidase activity.

Probing substrate backbone function in prolyl oligopeptidase catalysis--large positional effects of peptide bond monothioxylation

Site-specific effects on the catalytic activity of prolyl oligopeptidase from human placenta were studied using oligopeptide substrates in which a peptide bond has been replaced by a thioxo peptide bond. Two series of tetrapeptide-4-nitroanilides, Ala-Gly-Pro-Phe-NH-Np and Ala-Ala-Pro-Phe-NH-Np, along with all possible monothioxylated derivatives, were synthesised and k(cat) and Km values were determined for proteolytic cleavage at the Pro-Phe bond. Regardless of either Gly or Ala in the P2 subsite, tetrapeptides were rendered uncleavable by thioxylation at the Pro-Phe linkage. As a result, Ala-Xaa-Pro-psi[CS-NH]-Phe-NH-Np (Xaa = Gly or Ala) displayed competitive inhibition with Ki-values of 12 microM and 44 microM, respectively. Furthermore, in controlling proteolytic susceptibility of the substrates, cooperation of the P3-P2 thioxylation site and the side chain at the P2 subsite was obtained. Thioxylation at this position enhanced k(cat)/Km fivefold in the Gly series, but led to a 1.7-fold decrease in the Ala series of substrates. With respect to the Xaa-Pro peptide bond, all of the substrates underwent cis/trans isomerisation, thus presenting two stable conformers to the protease. However, the magnitudes of the isomerisation constants suggested that neither isomerisation rates nor cis/trans equilibria can explain the effect of thioxylation on the steady-state constants of proteolysis.

Amino monothio acids in solid-phase synthesis of peptide thioamides

Peptides containing backbone thioamides (endothiopeptides) have been synthesized utilizing thioacylation under solid-phase conditions. The thioacylations were performed by activating N-protected amino monothio acids with the phosphorus-containing coupling reagent 6-nitrobenzotriazol-l-yloxytris(pyrrolidino)phosphoniu m hexafluorophosphate (PyNOP). This method avoids the use of P4S10-based O/S-exchange reagents, and it is thus amendable to amino acids with side-chain amides. Synthesis of endothio analogs of biologically active peptide such a pGlu-psi [CSNH]-His-Pro-NH2 (TRH) and Leu-Gln-psi [CSNH]-Leu-Lys demonstrated this feature. Proton and carbon NMR spectra of the TRH analog verified the sequential position of its thioamide function. Compatibility of endothiopeptides with allylprotecting groups was studied, and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was evaluated as a substitute for piperidine.

A new mechanism in serine proteases catalysis exhibited by dipeptidyl peptidase IV (DP IV)--Results of PM3 semiempirical thermodynamic studies supported by experimental results

Herein we present results of semiempirical molecular orbital calculations employing the PM3 molecular model. The compounds studied are related to substrates of the serine protease dipeptidyl peptidase IV (DP IV). Our goal was the thermodynamic characterization of the DP IV-enzyme-catalyzed reaction pathway. A new mechanism of serine proteases catalysis is presented. We found that a tetrahedral intermediate can be stabilized by the formation of an oxazolidine ring with the nonscissile P2-P1 peptide bond. In this way, the negative charge of the tetrahedral intermediate around the scissile bond is transferred to the carbonyl oxygen atom of the preceding peptide bond. This negative charge can be compensated by a proton transfer from the positively charged N-terminus to this oxygen atom. It is shown that the positively charged N-terminus is the driving force in this particular serine protease mechanism of catalysis. The mechanism is supported by observed secondary hydrogen isotope effects on the C alpha proton for an alanine residue in the P2 position. We suggest a trans-cis isomerisation around the P2-P1 peptide bond in the final step of the acylation and cleavage of the substrates. The results obtained by our theoretical calculations are compared with several experimental findings supporting the suggested mechanism.

Separation of cis/trans isomers of a prolyl peptide bond by capillary zone electrophoresis

On capillary electrophoresis of the chemically pure thioxo peptide Ala-Phe-psi[CS-N]-Pro-Phe-4-nitroanilide a peak splitting was observed at a capillary temperature of 25 degrees C. By contrast, the oxo peptide analogue exhibits a single, sharp peak under these conditions. Both peaks of the thioxo compound coincided gradually when the temperature was increased to 60 degrees C. Peak fusion was reverted by cooling down the heated sample. This behavior could be attributed to the electrophoresis-mediated separation of the cis/trans prolyl bond isomers of the thioxo peptide, allowing data of this conformational equilibrium to be determined. Derived from computational data about molecular volume and the hydration energy of low-energy cis and trans isomeric structures, the more rapid migration of the cis form in comparison to trans may be explained by structural parameters.