Triad Conformation Research Articles

Generalized nonequilibrium Fermi's golden rule and its semiclassical approximations for electronic transitions between multiple states.

The nonequilibrium Fermi's golden rule (NE-FGR) approach is developed to simulate the electronic transitions between multiple excited states in complex condensed-phase systems described by the recently proposed multi-state harmonic (MSH) model Hamiltonian. The MSH models were constructed to faithfully capture the photoinduced charge transfer dynamics in a prototypical organic photovoltaic carotenoid-porphyrin-C60 molecular triad dissolved in tetrahydrofuran. A general expression of the fully quantum-mechanical NE-FGR rate coefficients for transitions between all pairs of states in the MSH model is obtained. Besides, the linearized semiclassical NE-FGR formula and a series of semiclassical approximations featuring Wigner and classical nuclear sampling choices and different dynamics during the quantum coherence period for the MSH model are derived. The current approach enables all the possible population transfer pathways between the excited states of the triad, in contrast to the previous applications that only addressed the donor-to-acceptor transition. Our simulations for two triad conformations serve as a demonstration for benchmarking different NE-FGR approximations and show that the difference between all levels of approximation is small for the current system, especially at room temperature. By comparing with nonadiabatic semiclassical dynamics, we observe similar timescales for the electronic population transfer predicted by NE-FGR. It is believed that the general formulation of NE-FGR for the MSH Hamiltonian enables a variety of applications in realistic systems.

Structural study of the uPA-nafamostat complex reveals a covalent inhibitory mechanism of nafamostat

Nafamostat mesylate (NM) is a synthetic compound that inhibits various serine proteases produced during the coagulation cascade and inflammation. Previous studies showed that NM was a highly safe drug for the treatment of different cancers, but the precise functions and mechanisms of NM are not clear. In this study, we determined a series of crystal structures of NM and its hydrolysates in complex with a serine protease (urokinase-type plasminogen activator [uPA]). These structures reveal that NM was cleaved by uPA and that a hydrolyzed product (4-guanidinobenzoic acid [GBA]) remained covalently linked to Ser195 of uPA, and the other hydrolyzed product (6-amidino-2-naphthol [6A2N]) released from uPA. Strikingly, in the inactive uPA (uPA-S195A):NM structure, the 6A2N side of intact NM binds to the specific pocket of uPA. Molecular dynamics simulations and end-point binding free-energy calculations show that the conf1 of NM (6A2N as P1 group) in the uPA-S195A:NM complex may be more stable than conf2 of NM (GBA as P1 group). Moreover, in the structure of uPA:NM complex, the imidazole group of His57 flips further away from Ser195 and disrupts the stable canonical catalytic triad conformation. These results not only reveal the inhibitory mechanism of NM as an efficient serine protease inhibitor but also might provide the structural basis for the further development of serine protease inhibitors.

Three-state harmonic models for photoinduced charge transfer.

A widely used strategy for simulating the charge transfer between donor and acceptor electronic states in an all-atom anharmonic condensed-phase system is based on invoking linear response theory to describe the system in terms of an effective spin-boson model Hamiltonian. Extending this strategy to photoinduced charge transfer processes requires also taking into consideration the ground electronic state in addition to the excited donor and acceptor electronic states. In this paper, we revisit the problem of describing such nonequilibrium processes in terms of an effective three-state harmonic model. We do so within the framework of nonequilibrium Fermi's golden rule (NE-FGR) in the context of photoinduced charge transfer in the carotenoid-porphyrin-C60 (CPC60) molecular triad dissolved in explicit tetrahydrofuran (THF). To this end, we consider different ways for obtaining a three-state harmonic model from the equilibrium autocorrelation functions of the donor-acceptor, donor-ground, and acceptor-ground energy gaps, as obtained from all-atom molecular dynamics simulations of the CPC60/THF system. The quantum-mechanically exact time-dependent NE-FGR rate coefficients for two different charge transfer processes in two different triad conformations are then calculated using the effective three-state model Hamiltonians as well as a hierarchy of more approximate expressions that lead to the instantaneous Marcus theory limit. Our results show that the photoinduced charge transfer in CPC60/THF can be described accurately by the effective harmonic three-state models and that nuclear quantum effects are small in this system.

Charge transfer rate constants for the carotenoid-porphyrin-C60 molecular triad dissolved in tetrahydrofuran: The spin-boson model vs the linearized semiclassical approximation.

Charge transfer rate constants were calculated for the carotenoid-porphyrin-C60 (CPC60) molecular triad dissolved in explicit tetrahydrofuran. The calculation was based on mapping the all-atom anharmonic Hamiltonian of this system onto the spin-boson Hamiltonian. The mapping was based on discretizing the spectral density from the time correlation function of the donor-acceptor potential energy gap, as obtained from all-atom molecular dynamics simulations. Different spin-boson Hamiltonians were constructed for each of the possible transitions between the three excited electronic states in two different triad conformations. The rate constants of three possible transitions were calculated via the quantum-mechanically exact Fermi's golden rule (FGR), as well as a progression of more approximate expressions that lead to the classical Marcus expression. The advantage of the spin-boson approach is that once the mapping is established, the quantum-mechanically exact FGR and the hierarchy of approximations are known in closed form. The classical Marcus charge transfer rate constants obtained with the spin-boson Hamiltonians were found to reproduce those obtained from all-atom simulations with the linearized semiclassical approximation, thereby confirming the equivalence of the two approaches for this system. Within the spin-boson Hamiltonian, we also found that the quantum-mechanically exact FGR rate constants were significantly enhanced compared to the classical Marcus theory rate constants for two out of three transitions in one of the two conformations under consideration. The results confirm that mapping to the spin-boson model can yield accurate predictions for charge transfer rate constants in a system as complex as CPC60 dissolved in tetrahydrofuran.

The structures of Arabidopsis Deg5 and Deg8 reveal new insights into HtrA proteases.

Plant Deg5 and Deg8 are two members of the HtrA proteases, a family of oligomeric serine endopeptidases that are involved in a variety of protein quality-control processes. These two HtrA proteases are located in the thylakoid lumen and participate in high-light stress responses by collaborating with other chloroplast proteins. Deg5 and Deg8 degrade photodamaged D1 protein of the photosystem II reaction centre, allowing its in situ replacement. Here, the crystal structures of Arabidopsis thaliana Deg5 (S266A) and Deg8 (S292A) are reported at 2.6 and 2.0 Å resolution, respectively. The Deg5 trimer contains two calcium ions in a central channel, suggesting a link between photodamage control and calcium ions in chloroplasts. Previous structures of HtrA proteases have indicated that their regulation usually requires C-terminal PDZ domain(s). Deg5 is unique in that it contains no PDZ domain and the trimeric structure of Deg5 (S266A) reveals a novel catalytic triad conformation. A similar triad conformation is observed in the hexameric structure of the single PDZ-domain-containing Deg8 (S292A). These findings suggest a novel activation mechanism for plant HtrA proteases and provide structural clues to their function in light-stress response.

Effects of Single Nucleotide Polymorphisms on Human N-Acetyltransferase 2 Structure and Dynamics by Molecular Dynamics Simulation

BackgroundArylamine N-acetyltransferase 2 (NAT2) is an important catalytic enzyme that metabolizes the carcinogenic arylamines, hydrazine drugs and chemicals. This enzyme is highly polymorphic in different human populations. Several polymorphisms of NAT2, including the single amino acid substitutions R64Q, I114T, D122N, L137F, Q145P, R197Q, and G286E, are classified as slow acetylators, whereas the wild-type NAT2 is classified as a fast acetylator. The slow acetylators are often associated with drug toxicity and efficacy as well as cancer susceptibility. The biological functions of these 7 mutations have previously been characterized, but the structural basis behind the reduced catalytic activity and reduced protein level is not clear.Methodology/Principal FindingsWe performed multiple molecular dynamics simulations of these mutants as well as NAT2 to investigate the structural and dynamical effects throughout the protein structure, specifically the catalytic triad, cofactor binding site, and the substrate binding pocket. None of these mutations induced unfolding; instead, their effects were confined to the inter-domain, domain 3 and 17-residue insert region, where the flexibility was significantly reduced relative to the wild-type. Structural effects of these mutations propagate through space and cause a change in catalytic triad conformation, cofactor binding site, substrate binding pocket size/shape and electrostatic potential.Conclusions/SignificanceOur results showed that the dynamical properties of all the mutant structures, especially in inter-domain, domain 3 and 17-residue insert region were affected in the same manner. Similarly, the electrostatic potential of all the mutants were altered and also the functionally important regions such as catalytic triad, cofactor binding site, and substrate binding pocket adopted different orientation and/or conformation relative to the wild-type that may affect the functions of the mutants. Overall, our study may provide the structural basis for reduced catalytic activity and protein level, as was experimentally observed for these polymorphisms.

Activation loop phosphorylation‐independent kinase activity of human protein kinase C ζ

Atypical protein kinase C zeta (PKCzeta) plays an important role in cell proliferation and survival. PKCzeta and its truncated form containing only the kinase domain, CATzeta, have been reported to be activated by the phosphorylation of threonine 410 in the activation loop. We expressed both the full length PKCzeta and CATzeta in a baculovirus/insect cell over-expression system and purified the proteins for biochemical characterization. Ion exchange chromatography of CATzeta revealed three species with different levels of phosphorylation at Thr-410 and allowed the isolation of the CATzeta protein devoid of phosphorylation at Thr-410. All three species of CATzeta were active and their activity was not correlated with phosphorylation at Thr-410, indicating that the kinase activity of CATzeta did not depend solely on activation loop phosphorylation. Tyrosine phosphorylation was detected in all three species of CATzeta and the full length PKCzeta. Homology structural modeling of PKCzeta revealed a conserved, predicted-to-be phosphorylated tyrosine residue, Tyr-428, in the close proximity of the RD motif of the catalytic loop and of Thr-410 in the activation loop. The structural analysis indicated that phospho-Tyr-428 would interact with two key, positively-charged residues to form a triad conformation similar to that formed by phospho-Thr-410. Based on these observations, it is possible that the Thr-410 phosphorylation-independent kinase activity of CATzeta is regulated by the phosphorylation of Tyr-428. This alternative mode of PKCzeta activation is supported by the observed stimulation of PKCzeta kinase activity upon phosphorylation at the equivalent site by Abl, and may be involved in resistance to drug-induced apoptosis.

Insights into the substrate specificity of a novel snake venom serine peptidase by molecular modeling

The cDNA encoding BthaTL, a serine peptidase from the venom of the snake Bothrops alternatus, was cloned and sequenced. The deduced primary structure shows over 62% of identity with snake venom thrombin-like enzymes (SVTLEs), molecules with high substrate specificity toward different natural substrates. Indeed, a phylogenetic reconstruction by two different methods clustered this enzyme close to other SVTLEs. These enzymes generally affect the hemostatic system in several ways, and therefore are used as tools in pharmacology and clinical diagnosis. A three-dimensional model of BthaTL was built by homology modeling using TSV-PA ( Trimeresurus stejnegeri venom plasminogen activator) crystal structure as template. BthaTL model showed that the typical catalytic triad conformation of serine peptidases was preserved. The calcium coordination ligands were absent or adopt an unfavorable conformation, preventing interactions with metals. On the other hand, the Asp97–Arg174 saline bridge of TSV-PA was not found and its specificity determinant Phe193 is replaced by a Gly in BthaTL. The substitution of essential residues in the neighborhoods of the catalytic site cleft of BthaTL indicates that these two proteins do not share the same enzymatic specificity, what means that BthaTL will probably not activate plasminogen. Such observations may be helpful in the understanding of the molecular mechanism for substrate specificity of these enzymes.

Specific protein dynamics near the solvent glass transition assayed by radiation-induced structural changes.

The nature of the dynamical coupling between a protein and its surrounding solvent is an important, yet open issue. Here we used temperature-dependent protein crystallography to study structural alterations that arise in the enzyme acetylcholinesterase upon X-ray irradiation at two temperatures: below and above the glass transition of the crystal solvent. A buried disulfide bond, a buried cysteine, and solvent exposed methionine residues show drastically increased radiation damage at 155 K, in comparison to 100 K. Additionally, the irradiation-induced unit cell volume increase is linear at 100 K, but not at 155 K, which is attributed to the increased solvent mobility at 155 K. Most importantly, we observed conformational changes in the catalytic triad at the active site at 155 K but not at 100 K. These changes lead to an inactive catalytic triad conformation and represent, therefore, the observation of radiation-inactivation of an enzyme at the atomic level. Our results show that at 155 K, the protein has acquired--at least locally--sufficient conformational flexibility to adapt to irradiation-induced alterations in the conformational energy landscape. The increased protein flexibility may be a direct consequence of the solvent glass transition, which expresses as dynamical changes in the enzyme's environment. Our results reveal the importance of protein and solvent dynamics in specific radiation damage to biological macromolecules, which in turn can serve as a tool to study protein flexibility and its relation to changes in a protein's environment.

Stereoselective Nature of Graft Copolymers Based on Poly(vinyl chloride): Synthesis of PVC-g-PMMA and PVC-g-PIB

Graft polymers from poly(vinyl chloride) with side chains of poly(methyl methacrylate) and poly(isobutylene) obtained by free radical and cationic techniques, respectively, were synthesized. The graft products were separated from the resulting mixtures of homo- and copolymers by fractional precipitation or extraction. The graft copolymers were characterized by infrared spectra, elemental analysis, NMR, and osmometric determinations. The quantitative microstructural analysis after grafting reaction has been followed by 13C NMR spectroscopy. From the results it is concluded that the modification of PVC by graft reactions through free radical and cationic polymerization proceeds by a stereoselective mechanism in that a fraction of isotactic triads is involved. Comparing the results obtained herein and those previously obtained in the nucleophilic substitution on PVC it is concluded that the highly reactive GTTG– isotactic triad conformation which is likely only at mmr tetrad has an apparent implication on the reactivity of the graft reaction. This conclusion was confirmed on the basis of the FT-IR and the thermal stability of the graft copolymers. This work provides some new insight into the prominent role of the microstructure-dependent local conformation on the polymer properties.

Influence of tacticity on the thermal degradation of PVC: 8. A comprehensive study of the local isotactic GTTG− conformation dependence of the mechanism of initiation

Some new attempts to demonstrate the superiority of the GTTG − conformation of isotactic triads relative to other labile structures containing allylic or tertiary chlorine in inducing the thermal degradation of poly(vinyl chloride) (PVC) are presented. They concern: (i) the degradation of polymers after nucleophilic substitution in two types of solvent involving the removal of the GTTG − arrangement in a quite different way; (ii) the degradation of PVC blends with a few additives which, as proved by Fourier transform infrared ( FTi.r.) spectroscopy, are able to interact specifically with the mmr structure adopting GTTG − TT conformation at the end of isotactic sequences; (iii) the degradation of PVC after the disappearance of GTTG − conformations as a result of the GTTG − TT x → GTGTTT irreversible conformational change provoked by stretching of films at temperatures near the glass transition; and (iv) the degradation of PVC after FTi.r. checked changes in content of GTTG − conformation, induced by annealing at temperatures between 70 and 120°C. The correlations so obtained give evidence of the PVC thermal-instability-determining role of the GTTG − isotactic triad conformation.

A verification by FTIR spectroscopy of the effect of some local chain conformations on the specific molecular interactions of solvents, esters, and polyesters with PVC

AbstractThe molecular interactions of poly(vinylchloride) (PVC) with some solvents [cyclohexanone (CH), methyl ethyl ketone (MEK) and N‐methylpirrolidone (MP)], esters [dioctylphthalate (DOP) and butyl stearate (BuSt)], and polyesters [poly(ethylene adipate) (PEA) and poly(ε‐caprolactone) (PCL)] have been investigated by FTIR spectroscopy. In all cases the band of the carbonyl group is found to shift to lower frequencies, but significant differences between the solvent and the esters, whether polymeric or not, are evidenced. For PVC‐solvent systems, the shift proves to increase linearly as PVC/solvent ratio increases, what suggests that only a definite number of polymer sites is involved. From the slopes of the straight lines this effect of composition is shown to increase in the order MP < MEK < CH, i.e., as the basicity of the solvent decreases. In contrast, for the PVC‐esters or polyester blends, a nonlinear behavior consisting of two distinct interaction processes, is obtained. The increase of shift as PVC/ester ratio increases is faster in the first process for all PVC‐ester systems and it is particularly enhanced for BuSt and, to a lesser extent, for DOP. Instead, during the second process, that increase is of little significance for BuSt relative to DOP and PCL. These results account for the saturation of the polymer structures that are capable of interacting, at different rates depending on the type of ester. Besides, the whole number of those structures appears to be lower than in the case of solvents.The results are discussed on the ground of, on one side, the mechanism of nucleophilic substitution on PVC, in the same solutions and blends, which, as found previously, is of a stereospecific nature, and, on the other, the electron‐donor‐acceptor concept (EDA) and the hard‐soft‐acid‐base concept (HSAB) as applied to both the interacting agents (solvents and esters) and the isotactic GTGT and GTTG− triad conformations as well as the heterotactic GTTT. In the light of the resulting conclusions it is suggested that: (i) the linear behavior shown by the solvents obeys the solvent ability to ensure a dynamic equilibrium between the two possible conformations of ‐mmr‐ sequence, i.e., GTGTTT and GTTG−TT, through the preferential interaction with the little likely GTTG− conformation, the content of which happens so to be constant as long as there are ‐mmr‐ sequences in solution; (ii) the nonlinear behavior of PVC‐ester or polyester binary systems reveals a nonequilibrium situation and so the conformational change GTGTTT ⇒ GTTG−TT, which is highly hindered, will occur occasionally depending on the ester nature. This enables one to attribute the fast and the slow interaction processes to the permanent GTTG−TT conformations derived from the polymerization and to the same conformations formed as the result of the conformational changes, respectively.Strong support for the above novel finding that PVC … OC interaction is of a local conformational nature is given by two additional investigations. First, a similar study with a PVC sample prepared at −50°C, shows that the carbonyl band shifts of CH and PCL are appreciably lower than those of PVC prepared at 70°C. The same holds for the blendof PCL with the latter PVC sample after substitution reaction (0.6%) at −15°C in CH with sodium benzenethiolate (NaBT). Since the PVC obtained at −50°C and the 0.6% substituted polymer exhibit a lower content of both permanent GTTG−TT conformations ad ‐mmr‐ sequence, these results agree with expectatins and confirm the above suggestions. Secondly, the changes in the CCl stretchign frequencies of PVC with increasing amounts of solvent or ester, as extensively studied, clearly indicate the occurrence of the aforementioned conformational change, and so they are consistent with our proposals as to the actual conformational nature of PVC…OC interactions. © 1995 John Wiley & Sons, Inc.

Influence of tacticity on the thermal degradation of PVC. Part 7—Further approaches to the conformational mechanism through a temperature effect study

The thermal degradation of three different PVC samples has been studied at temperatures ranging from 110 to 200°C. Polymers 1 and 3 were prepared in bulk with 2,2′-azodiisobutyronitrile as initiator at 90 and 20°C, respectively, so as to obtain different isotactic contents and therefore different contents of the GTTG − isotactic triad conformation. Sample 2 resulted from a stereospecific nucleophilic substitution reaction on sample 1 at −15°C. As shown elsewhere, such a treatment involves the disappearance of GTTG − isotactic triads without other significant changes in structure taking place. The content of GTTG − isotactic triads should thus be expected to decrease from sample 1 to sample 3. The Arrhenius plots for the initial degradation rates, make evident the occurrence of two distinct competing degradation processes with activation energies of 7·0 and 19·8 kcal/mol. Their respective contributions to the overall degradation is shown to depend upon the isotactic content of the polymer such that the higher the isotactic content, the higher the polymer fraction involved in the process of lower activation energy. On the other hand, the UV-visible spectra of the polymers after degradation to similar degrees, demonstrate that the main absorption, i.e. the most abundant polyene length, shifts towards shorter wavelengths as the degradation temperature increases. On the basis of previous results which allow a correlation between definite long polyenes with the fraction of isotactic triads that adopt the GTTG − conformation during the polymerization, the above behaviour is shown to prove that the latter conformation is responsible for the degradation process of lower activation energy. Such a conclusion is strongly supported by the behaviour of sample 2, relative to sample 1, in both degradation kinetics and polyene distribution in the degraded polymers. As a whole, the results are fresh support for our prior work proving that the GTTG − isotactic triad conformation is the instability-determining structure in PVC.

Specific polymer-solvent interaction and stereospecificity of nucleophilic substitution reaction of PVC

The substitution reaction of PVC with sodium benzenethiolate is studied in a series of solvents in comparison to cyclohexanone as considered in earlier work. As expected from the known influence of solvent on the nucleophile reactivity, the reaction extent is observed to be higher as the solvent polarity increases. In contrast, the stereoselective character of the reaction, as demonstrated for cyclohexanone solution, is found to hold only for solvents containing C=O or-O-function. The others deviate thoroughly from this behaviour. The results are discussed on the grouds of prior findings. They tentatively suggest that the occurrence of the definite conformational changes that are responsible for the stereoselective mechanism, strongly depends on some specific interactions of the solvent with isotactic GTTG and heterotactic GTTT triad conformations.

Stereospecificity of substitution on PVC

The kinetics of substitution reaction on PVC with sodium benzenethiolate at temperatures from-30°C to 5°C is studied and compared to that of earlier work at temperatures from 5°C to 70°C. The kinetic parameters indicate the occurrence of two overlapping processes having activation energy of 10 and 17 Kcal/mol. Their attribution to the isotactic GTTG− and the heterotactic GTTT triad conformations respectively, is argued on the basis of recent results. A few of substitution experiments were repeated in the presence of diglyme or crown ether. They clearly demonstrate that kinetic features are only dependant on the microstructure of the polymer.

Influence of structure on the photo-degradation of PVC. Part IV—A conclusive approach to the mechanism of photo-oxidation and photo-dehydrochlorination

The photo-oxidation and the photo-dehydrochlorination of a series of PVC samples having different isotactic contents have been studied in the presence of oxygen. The build-up of carbonyl groups was found to decrease as the isotactic content was increased. Conversely, the dehydrochlorination quantum yield appeared to increase linearly as the isotactic content increased. It is proposed that the former result is due to a less favoured hydrogen abstraction reaction at the isotactic triads, resulting from a higher shielding of the hydrogen atoms at CHCl groups. The latter result is associated with both the occurrence of some chlorine atoms of low dissociation energy located at a definite type of isotactic triad and the favoured hydrogen abstraction reaction at the CH 2 group in syndio and heterotactic triads. The conclusions are discussed on the basis of the local conformations of the various triads. In particular, the role of the isotactic GTTG − triad conformation in the mechanisms above is emphasized.

Stereospecific functionalization of poly(vinyl chloride)

AbstractThe substitution reaction of PVC with sodium dimethyldithiocarbamate (NaDC) and sodium azide (NaN3) was performed in cyclohexanone and dimethylformamide at various temperatures. The degree of compositional heterogeneity was estimated and found to be important up to conversions of about 0,5%, but to vanish afterwards. The evolution of unreacted iso, hetero and syndiotactic triads was measured by 13C NMR spectroscopy. For conversions of at least up to 12%, the content of both the isotactic and heterotactic triads was found to decrease, relative to the syndiotactic ones, but at different relative rates depending on the nucleophile. From these results, together with the kinetic ones, it is concluded that the reaction proceeds by a stereoselective mechanism. The results are discussed on the basis of a hypothesis concerning the role of the local triad conformations in PVC that contain the TT isotactic diad conformation. The results enable the selective substitution of PVC with NaDC and NaN3, which may be used as a useful tool to prepare specific graft copolymers based on PVC.

Contribution to the study of labile structures in PVC

A kinetic treatment that takes into account the occurrence of two degradation processes through labile and stable chlorine atoms has been developed. By means of simulation of dehydrochlorination curves, the degradation rate and the kinetic parameters may be calculated provided that the content of labile structures is known and that both processes obey the Arrhenius equation. Assuming that, as deduced in previous work, the labile structures are a definite type of isotactic triad, the content of which may be easily determined, the individual degradation rates at various temperatures were calculated for a PVC prepared at 90°C (sample A). The sum of the calculated rates for the stable and the labile processes agrees quite well with the experimental overall degradation rates. On the other hand, the degradation rates for another PVC of lower content of labile isotactic triads (sample B), were calculated from those of polymer A taking into account the differences in content of labile isotactic triads. The values obtained in this way were found to agree with the experimental overall rates obtained for sample B at the same temperatures of degradation. The results provide quantitative support of earlier conclusions about the salient role of certain isotactic triad conformations in the degradation of PVC.

Polyene sequence distribution in modified poly(vinyl chloride) after thermal degradation

AbstractThe products resulting from the reaction of PVC with sodium benzenethiolate were degraded to 0,3% at 180°C in the solid state and at 160°C in solution in trichlorobenzene. The polyene distribution of the polymers after degradation was studied by both UV‐visible and resonance Raman spectroscopies, as a function of the degree of substitution. The results show that there are two types of behaviour: that of the PVC sample prior to the substitution reaction together with the samples modified up to a definite degree of substitution which depends on the starting isotactic content, and that of samples with higher degrees of substitution. The former group exhibits not only a steady improvement in thermal stability but also a preferential formation of polyenes of 7 – 9 double bonds whose concentration decreases with increasing degree of substitution. Conversely, for the second group of samples the thermal stability decreases with the degree of substitution and no specific absorption bands are observed. On the basis of earlier work on the selective substitution of the isotactic GTTG and heterotactic TTTG triads during the first stage of the reaction, the present results show that the bands at 393, 416, and 437 nm are related to specific polyenes which result from initiation by the above quoted conformations in PVC, a conclusion for which confirmatory evidence was obtained by resonance Raman spectroscopic examination of the samples. There is, therefore, clear evidence for the occurrence of two distinct degradation mechanisms, one involving initiation by the unstable triad conformations and the other via random initiation at stable and normal structures. To this may be added the initiation by defect structures, which have been extensively documented in the literature.