Solvent Friction Research Articles

Anomalous viscosity effect in the early stages of the ion-assisted adhesion/fusion event between lipid bilayers: A theoretical and computational study

The effect of viscosity on the encounter rate of two interacting membranes was investigated by combining a non-equilibrium Fokker-Planck model together with extensive Molecular Dynamics (MD) calculations. The encounter probability and stabilization of transient contact points represent the preliminary steps toward short-range adhesion and fusion of lipid leaflets. To strengthen our analytical model, we used a Coarse Grained MD method to follow the behavior of two charged palmitoyl oleoyl phosphatidylglycerol membranes embedded in a electrolyte-containing box at different viscosity regimes. Solvent friction was modulated by varying the concentration of a neutral, water-soluble polymer, polyethylene glycol, while contact points were stabilized by divalent ions that form bridges among juxtaposed membranes. While a naïve picture foresees a monotonous decrease of the membranes encounter rate with solvent viscosity, both the analytical model and MD simulations show a complex behavior. Under particular conditions, the encounter rate could exhibit a maximum at a critical viscosity value or for a critical concentration of bridging ions. These results seem to be confirmed by experimental observations taken from the literature.

Viscosity-dependent structural fluctuation of the M80-containing Ω-loop of horse ferrocytochrome c

To determine the effect of solvent viscosity on low-frequency local motions that control the slow changes in structural dynamics of proteins, we have studied the effects of solvent viscosity on the structural fluctuation of presumably the M80-containing Ω-loop by measuring the rate of thermally-driven CO-dissociation from a natively-folded carbonmonoxycytochrome c (NCO-state) in the 0.65–92.5cP range of viscosity at pH 7.0. At low viscosities (⩽8cP), the rate coefficient, kdiss for dissociation of CO from the NCO-state varies inversely with the viscosity, but saturates at high viscosities, suggesting that CO-dissociation reaction involves sequential stages that depend differently on solvent friction, i.e., solvent coupled and nonsolvent-coupled stages of the process. In the low viscosity regime (0.65⩽ηs⩽8.0cP), the rate-viscosity data were fitted to modified Kramers model, kdiss=[A′/(σ+ηs)n]exp(−ΔG/RT), which produced internal friction, σ=1.35cP (±0.88), which suggests that the speed of CO-dissociation from NCO at ηs⩽8.0cP is controlled by internal friction.

49 Nucleic acids in anhydrous media: G-quadruplex folding governed by Kramers rate theory

Structures formed by human telomere sequence (HTS) DNA are of interest due to G-quadruplex forming HTS DNA that has recently generated a tremendous interest due to its involvement in aging process and cancer. The present study examines HTS in anhydrous, exceptionally viscous deep eutectic solvent (DES), comprised of choline, chloride, and urea. Under these conditions, HTS adopts an extremely stable “parallel-propeller” form of G-quadruplex, consistent with the previously observed effects of diminished water activity. Additionally, the high solvent friction of DES slows the dynamics of HTS folding on the order of months, as opposed to milliseconds in aqueous solution, and allows the entrapment of kinetic intermediates. Moreover, analogous transition studies of the quadruplex converting from the aqueous buffer structure to the parallel form in 90% DES (w/v) and 40% PEG 200 (v/v) differ from hours to days scaling inversely with viscosity ∼ 1/ƞ1.4. This diffusion control over the HTS folding is consistent with Kramers rate theory and these findings highlight the necessity to consider the viscosity of intracellular conditions when exploring the structure dynamics of telomeres and drug binding interactions. Lastly, tuning solvent viscosity could prove useful in the future study of G-quadruplex dynamics, and applied DNA nano-technology, where time dependent structural transitions are desired.

Vibrationally resolved dynamics of the reaction of Cl atoms with 2,3-dimethylbut-2-ene in chlorinated solvents

Broadband transient infra-red absorption measurements are reported that contrast the dynamics of reaction of Cl atoms with 2,3-dimethylbut-2-ene and n-pentane in chlorinated solvents. In both cases, H-atom transfer produces HCl and a hydrocarbon radical, but the energy release in the former reaction is greater because of the formation of a resonance-stabilized allylic radical. The reaction of Cl atoms with n-pentane in solution in CH2Cl2 forms HCl exclusively in its lowest vibrational level (v = 0), and our measured rate coefficient agrees with a prior report by Sheps et al., J. Phys. Chem. A, 2006, 110, 3087. The time-dependence of the growth of intensity in the HCl fundamental absorption band shows two domains of reaction: a prompt rise is associated with reaction of the photolytically generated Cl atoms with n-pentane molecules lying within the first solvent shell, whereas a slower further rise is attributed to reaction following diffusion through the solution. For the reaction of Cl atoms with 2,3-dimethylbut-2-ene, these two domains of reaction are also observed, and fitting to a kinetic model incorporating these components gives bimolecular rate coefficients for formation of HCl of (1.7 ± 1.4) × 1010 M−1 s−1 in CDCl3 and (3.4 ± 1.2) × 1010 M−1 s−1 in CCl4. However, an additional transient absorption is observed ∼115 cm−1 lower in wavenumber than the fundamental HCl absorption band and is assigned to the v = 2 ← v = 1 hot band of HCl. The absorption by the nascent HCl(v = 1) peaks after ∼20 ps and subsequently decays to baseline levels because of vibrational relaxation, which is shown to be enhanced by energy transfer to 2,3-dimethylbut-2-ene. In solution in CDCl3, the fraction of HCl formed initially in v = 1 is determined to be 0.24 ± 0.04 and in CCl4 it is 0.15 ± 0.02. The branching to HCl(v = 1) for these reactions in solution is significantly lower than the 0.48 ± 0.06 fraction reported for reaction of Cl atoms with propene in the gas phase [Pilgrim and Taatjes, J. Phys. Chem. A, 1997, 101, 5776]. This comparison between similar bimolecular reactions in solution and in the gas phase therefore identifies changes to the dynamics caused by interaction with the solvent. Solvent friction experienced by the separating products of the reaction is considered more likely to be responsible for the lower vibrational excitation of the nascent HCl than is solvent modification of the topology of the potential energy surface in the vicinity of the transition state. Possible consequences of addition of Cl to the unsaturated bond in 2,3-dimethylbut-2-ene are also discussed.

Human Telomere Sequence DNA in Water-Free and High-Viscosity Solvents: G-Quadruplex Folding Governed by Kramers Rate Theory

Structures formed by human telomere sequence (HTS) DNA are of interest due to the implication of telomeres in the aging process and cancer. We present studies of HTS DNA folding in an anhydrous, high viscosity deep eutectic solvent (DES) comprised of choline choride and urea. In this solvent, the HTS DNA forms a G-quadruplex with the parallel-stranded ("propeller") fold, consistent with observations that reduced water activity favors the parallel fold, whereas alternative folds are favored at high water activity. Surprisingly, adoption of the parallel structure by HTS DNA in the DES, after thermal denaturation and quick cooling to room temperature, requires several months, as opposed to less than 2 min in an aqueous solution. This extended folding time in the DES is, in part, due to HTS DNA becoming kinetically trapped in a folded state that is apparently not accessed in lower viscosity solvents. A comparison of times required for the G-quadruplex to convert from its aqueous-preferred folded state to its parallel fold also reveals a dependence on solvent viscosity that is consistent with Kramers rate theory, which predicts that diffusion-controlled transitions will slow proportionally with solvent friction. These results provide an enhanced view of a G-quadruplex folding funnel and highlight the necessity to consider solvent viscosity in studies of G-quadruplex formation in vitro and in vivo. Additionally, the solvents and analyses presented here should prove valuable for understanding the folding of many other nucleic acids and potentially have applications in DNA-based nanotechnology where time-dependent structures are desired.

Peptide Chain Dynamics in Light and Heavy Water: Zooming in on Internal Friction

Frictional effects due to the chain itself, rather than the solvent, may have a significant effect on protein dynamics. Experimentally, such "internal friction" has been investigated by studying folding or binding kinetics at varying solvent viscosity; however, the molecular origin of these effects is hard to pinpoint. We consider the kinetics of disordered glycine-serine and α-helix forming alanine peptides and a coarse-grained protein folding model in explicit-solvent molecular dynamics simulations. By varying the solvent mass over more than two orders of magnitude, we alter only the solvent viscosity and not the folding free energy. Folding dynamics at the near-vanishing solvent viscosities accessible by this approach suggests that solvent and internal friction effects are intrinsically entangled. This finding is rationalized by calculation of the polymer end-to-end distance dynamics from a Rouse model that includes internal friction. An analysis of the friction profile along different reaction coordinates, extracted from the simulation data, demonstrates that internal as well as solvent friction varies substantially along the folding pathways and furthermore suggests a connection between friction and the formation of hydrogen bonds upon folding.

Viscosity Dependence of Intra- and Intermolecular Diels–Alder Reactions

The kinetics of the bimolecular Diels-Alder reaction between anthracene-9-carbinol and N-ethyl maleimide have been studied in a series of pyridinium-based ionic liquids with the tetrafluoroborate ([BF(4)](-)) and the bis(trifluorosulfonimide) ([NTf(2)](-)) anions and the viscosity dependence of the rate constants have been compared with the results for the intramolecular Diels-Alder reaction of (E)-1-phenyl-4-[2-(3-methyl-2-butenyloxy)benzylidene]-5-pyrazolone. The comparison leads to the surprising observation that the intermolecular reaction is less susceptible to viscosity variations of the ionic liquids as compared to the intramolecular reaction. The observed similarities and differences emphasize the complicated nature of solvent friction on the kinetics of cycloaddition reactions. The results are explained by considering the bimolecular and intramolecular processes as cases of translational and rotational diffusion, respectively. Plausible indicators of microviscosity consistent with the kinetic data for the Diels-Alder reactions are briefly discussed along with the constraints involved in designing a general microviscosity scale.

Photoisomerization dynamics of 3,3′-diethyloxadicarbocyanine iodide in ionic liquids: Breakdown of hydrodynamic Kramers model

Photoisomerization dynamics of 3,3'-diethyloxadicarbocyanine iodide (DODCI) has been examined in a series of 1-alkyl-3-methylimidazolium (alkyl = methyl, ethyl, propyl, butyl, and hexyl) bis(trifluoromethylsulfonyl)imides by measuring its fluorescence lifetimes and quantum yields. This study has essentially been undertaken to find out whether the process of photoisomerization of DODCI in ionic liquids is different compared to that observed in conventional solvents such as alcohols. Activation energy of the reaction has been attained with the aid of isoviscosity plots and was found to be 22 ± 3 kJ mol(-1), which is a factor of two higher compared to that obtained in alcohols. The significantly higher activation energy obtained in bis(trifluoromethylsulfonyl)imides compared to alcohols is probably due to the highly ordered nature of the ionic liquids, which hinders the twisting process. Kramers theory has been applied to understand the reduced isomerization rate constants in terms of solvent friction. As in case of alcohols, the isomerization data could not be explained by the Kramers model. However, a power law relation, which is a phenomenological functional form, could mimic the observed trend.

Solute–solvent friction kernels and solution properties of methyl oxazoline–phenyl oxazoline (MeOx–PhOx) copolymers in binary ethanol–water mixtures

Solvent mixtures often alter the solubility of polymeric substances. Statistical copolymers made from 2-methyl-2-oxazoline (MeOx) and 2-phenyl-2-oxazoline (PhOx) are known for their varying solubilities in pure ethanol, pure water and in binary mixtures of ethanol-water. Constrained Molecular Dynamics (MD) simulations have been carried out with an aim to explain the varying solubilities of the statistical MeOx-PhOx copolymers. The solute-solvent dynamic friction kernels calculated through constrained MD simulations corroborate the solubility pattern in these copolymers. The solvation characteristics have been analyzed in terms of the solute-solvent radial distribution functions (RDFs). The ethanol-soluble MeOx-PhOx copolymers exhibit characteristic solute-composition dependence in the dynamic solute-solvent friction kernels, indicating the strength of the solute-solvent correlations. The aggressive solvation by the ethanol molecules in the binary solvent mixtures has been brought out by the O(solute)-H(ethanol) RDFs which exhibit a characteristic dependence on the ethanol content in the solvent composition. The corresponding O(solute)-H(water) RDFs are devoid of any such composition dependence. For all the MeOx-PhOx copolymers, the O-site solvation is strongly dominated by the water molecules and the N-sites are solvated equally by both ethanol and water molecules.

Watching solvent friction impede ultrafast barrier crossings: A direct test of Kramers theory

A systematic investigation of the solvent's dynamic influence on activated barrier crossings on an electronic ground state is performed using ultrafast two-dimensional infrared chemical exchange spectroscopy. These measurements facilitate a direct comparison with the widely adopted Kramers theory of condensed phase reaction kinetics, and for the first time avoid the significant complication of electronic excitation to probe directly in the time domain a ground electronic state reaction with a well-defined transition state. The picosecond timescale interconversion between two stable isomers of the metal carbonyl complex Co(2)(CO)(8) in a series of linear alkane solvents shows negligible energetic variation with solvent carbon chain length, providing an exclusive probe of the effects of solvent friction. Relative to the linear alkane series, cyclohexane does alter the potential energy surface by preferentially stabilizing one of the isomers. Despite this pronounced modification of the reaction barrier energetics, combination of experiment and computation enables the removal of the nondynamical barrier contribution to the rate constant, isolating the dynamical influence of solvent friction. The experimental data, supported with quantum and classical computations, show agreement with a simple Markovian Kramers theory for the isomerization rate constant's dependence on solvent viscosity.

Solvent-Controlled Electron Transfer in Crystal Violet Lactone

Steady-state and picosecond time-resolved emission experiments are used to examine the excited-state charge transfer reaction of crystal violet lactone (CVL) in aprotic solvents. Solvatochromic analysis using a dielectric continuum model suggests dipole moments of 9-12 D for the initially excited (LE) state and ∼24 D for the charge-transfer (CT) state. Intensities of steady-state emission as well as kinetic data provide free energies for the LE → CT reaction that range from +12 kJ/mol in nonpolar solvents to -10 kJ/mol in highly polar solvents at 25 °C. Reaction rates constants, which lie in the range of 10-100 ns(-1) in most solvents, depend on both solvent polarity and solvent friction. In highly polar solvents, rates are correlated to solvation times in a manner that indicates that the reaction is a solvent-controlled electron transfer on an adiabatic potential surface having a modest barrier.

Solvent Viscosity and Friction in Protein Folding Dynamics

The famous Kramers rate theory for diffusion-controlled reactions has been extended in numerous ways and successfully applied to many types of reactions. Its application to protein folding reactions has been of particular interest in recent years, as many researchers have performed experiments and simulations to test whether folding reactions are diffusion-controlled, whether the solvent is the source of the reaction friction, and whether the friction-dependence of folding rates generally can provide insight into folding dynamics. These experiments involve many practical difficulties, however. They have also produced some unexpected results. Here we briefly review the Kramers theory for reactions in the presence of strong friction and summarize some of the subtle problems that arise in the application of the theory to protein folding. We discuss how the results of these experiments ultimately point to a significant role for internal friction in protein folding dynamics. Studies of friction in protein folding, far from revealing any weakness in Kramers theory, may actually lead to new approaches for probing diffusional dynamics and energy landscapes in protein folding.

Photophysical Characterization of Benzylidene Malononitriles as Probes of Solvent Friction

Steady-state absorption and emission and femtosecond time-resolved emission spectroscopy of two benzylidene malononitriles, 2-[4-(dimethylamino)benzylidene])malononitrile (DMN) and julolidinemalononitrile (JDMN), are reported in a variety of room-temperature solvents. Solvatochromic shifts of these molecules are consistent with dielectric continuum descriptions and an S(1)-S(0) dipole moment change of 8.5 D. Time-resolved spectra show modest dynamic Stokes shifts of approximately 1000 cm(-1) occurring independently of fluorescence decay, which takes place in 0.5-5 ps in most room-temperature solvents. Absorption transition moments and fluorescence decay times are used to determine radiative rate constants: k(rad) = 0.32 +/- 0.02 ns(-1) in DMN and 0.28 +/- 0.02 ns(-1) in JDMN, assumed to be independent of solvent. Quantum yield data together with these radiative rates provide the reaction rate constants k(rxn) associated with the internal conversion process of these molecules in 33 representative solvents at 298 K and in several solvents as functions of temperature. Reaction rates of JDMN are systematically lower than those of DMN by a factor of 2.0. Values of k(rxn) in series of homologous solvents or in a single solvent at different temperatures are correlated to solvent viscosity eta and temperature T in the manner k(rxn)/T proportional to eta(-p) with exponents 0.2 < or = p < or = 0.8. Solvent polarity appears to influence these reactions such that for a given viscosity reaction in high polarity solvents is significantly slower than in nonpolar solvents. However, this conclusion is predicated on the assumption that reactive friction is identical in solvents of the same viscosity, which is unlikely to be quantitatively correct. The observed reaction rates and their solvent dependence are discussed in terms of isomerization about the C=C bond occurring on a shelf-like potential.

Solvent Viscosity and Friction in Protein Folding Dynamics

Solvent Viscosity and Friction in Protein Folding Dynamics

An Electrohydrodynamic Model and Extensive MD Simulations Agree on the Positional and Intra-Residual Relaxations Up to Sub-Microsecond Dynamics

Our recent simulations have indicated that the number of water molecules within a cutoff distance of each residue scales linearly with protein depth. At physiological temperatures, while the translational memory of water molecules around a residue is proportional to its depth, the orientational memory is independent of the residues position[1]. These corroborate the recently reported result that water density fluctuations around hydrophobic surfaces are considerably larger than those near hydrophilic surfaces[2].We develop an efficient, simple model that characterizes protein dynamics both at picosecond and sub-microsecond timescales[3], which are coupled through conformational motions and catalysis[4]. Our approach is based on our earlier two-degree-of-freedom model, coupling the protein's fluctuations to the vicinal layer[5]. It proves to be efficient in estimating dynamic transitions in different solvents. The model emanates from geometric Brownian motion, similar in spirit to those including hydrodynamic interactions[6]. In our formulation, however, the traditional intra-molecular interactions are coupled to an electrostatic field that increases the flexibility of the regions in contact with hydrophobic residues, and almost exhibits a decoupled dynamics from hydrophilic regions where the solvent friction term dominates. We derive analytically the decay of the positional fluctuations and the relaxation of the distances between the residues which are not in direct contact.We have performed 200ns MD simulations both above and below the dynamic transitions of lysozyme and myoglobin to validate our analytical results. We obtain a remarkable agreement between simulations and our model for the decay time of both positional fluctuations and intra-residual distances.1.J.Servantie, C.Atilgan, A.R.Atilgan, ArXiv:0905-4893.2.A.J.Patel and D.Chandler, ArXiv:0909-4097.3.O.B.Okan, A.R.Atilgan, C.Atilgan, BiophysJ, Oct2009.4.W. Min, X.S. Xie, B. Bagchi, JPhysChemB, 112,454(2008).5.C. Atilgan, A.O. Aykut, A.R. Atilgan, BiophysJ, 94,79(2008).6.E. Caballero-Manrique etal, BiophysJ, 93,4128(2007).

Non-Native Interactions Are Critical for Mechanical Strength in PKD Domains

SummaryExperimental observation has led to the commonly held view that native state protein topology is the principle determinant of mechanical strength. However, the PKD domains of polycystin-1 challenge this assumption: they are stronger than predicted from their native structure. Molecular dynamics simulations suggest that force induces rearrangement to an intermediate structure, with nonnative hydrogen bonds, that resists unfolding. Here we test this hypothesis directly by introducing mutations designed to prevent formation of these nonnative interactions. We find that these mutations, which only moderately destabilize the native state, reduce the mechanical stability dramatically. The results demonstrate that nonnative interactions impart significant mechanical stability, necessary for the mechanosensor function of polycystin-1. Remarkably, such nonnative interactions result from force-induced conformational change: the PKD domain is strengthened by the application of force.

Electrochemical reduction of tert-butyl chloride—Computational study

In this work kinetics of the electrochemical reduction of the tert-butyl chloride molecule are studied. The two-dimensional potential surface E( x, y) was obtained from the Hamiltonian model used earlier [1] to study quantum effects on the reaction rate of the same reaction. In series of molecular dynamics simulations two different models, isotropic ( γ x = γ y ) and anisotropic ( γ x > γ y ), of the friction parameters for the solvent ( γ x ) and for the C–Cl bond ( γ y ) were considered. An influence of several factors, such as an overpotential, a solvent friction coefficient and a temperature, on the rate constants k obtained with the two models are presented and compared to the results obtained for the non-adiabatic mechanism. It is shown, that the anisotropic model predicts generally much smaller rates when compared to those calculated with the isotropic model and also with the non-adiabatic model. On the curves k( γ x ) the turnover region is very well defined for relatively low frictions in the isotropic case, while for the anisotropic case it is shifted towards much higher γ x values. In all simulations the saddle point avoidance phenomenon was observed and its extent for different models and reaction conditions is discussed. Transfer coefficients α obtained for adiabatic and non-adiabatic reaction mechanisms as a function of the overpotential and temperature are also presented.

Solvent Friction Changes the Folding Pathway of the Tryptophan Zipper TZ2

Because the rate of a diffusional process such as protein folding is controlled by friction encountered along the reaction pathway, the speed of folding is readily tunable through adjustment of solvent viscosity. The precise relationship between solvent viscosity and the rate of diffusion is complex and even conformation-dependent, however, because both solvent friction and protein internal friction contribute to the total reaction friction. The heterogeneity of the reaction friction along the folding pathway may have subtle consequences. For proteins that fold on a multidimensional free-energy surface, an increase in solvent friction may drive a qualitative change in folding trajectory. Our time-resolved experiments on the rapidly and heterogeneously folding β-hairpin TZ2 show a shift in the folding pathway as viscosity increases, even though the energetics of folding is unaltered. We also observe a nonlinear or saturating behavior of the folding relaxation time with rising solvent viscosity, potentially an experimental signature of the shifting pathway for unfolding. Our results show that manipulations of solvent viscosity in folding experiments and simulations may have subtle and unexpected consequences on the folding dynamics being studied.

Multiple Mechanisms for Electron Transfer at Metal/Self‐Assembled Monolayer/Room‐Temperature Ionic Liquid Junctions: Dynamical Arrest versus Frictional Control and Non‐Adiabaticity

Electrochemical devices consisting of gold electrodes coated by electronically well-behaved self-assembled alkanethiol monolayers of variable thickness, a ferrocene/ferrocenium redox probe and a typical room-temperature ionic liquid (RTIL) [bmim][NTf(2)] (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) as a unique reaction medium with an exceptionally broad spectrum of relaxational modes (probed under variable temperature and pressure conditions), have been used to vary the intrinsic electron-transfer (ET) rate constant over eight orders of magnitude (from 0.1 to 3x10(7) s(-1)) by further tuning of the overvoltage. A remarkable interplay of ET mechanisms was observed, which was accompanied by the stepwise drop in the reorganisation free energy of the medium from 1.0 to 0.1 eV. The first mechanistic changeover to the dynamically arrested regime, with a locking ultra-slow relaxation time of approximately 50 micros, occurred at donor-acceptor separations below 20 A. Another mechanistic changeover to the full solvent friction regime, controlled by a medium relaxation process of approximately 100 ns, emerged for ET distances smaller than 8 A.

Photodynamics of a PV Trimer in High‐Viscosity Solvents and in PMMA Films: A New Insight into Energy Transfer versus Conformational Relaxation in Conjugated Polymers

Non-Stokes-Einstein relaxation: The rate constant of conformational relaxation of a phenylenevinylene trimer (see picture) in different solvents is proportional to eta(-alpha), with alpha values decreasing from close to unity (low viscosity) to zero at sufficiently high solvent viscosity. This behaviour is attributed to the flexible methylbutyl side chains of the trimer, which partially screen the solvent friction. The p-phenylenevinylene (PV) trimer (MBOPV3) was used to probe the effect of conformational relaxation on fluorescence decays of PV-based polymers in the high solvent viscosity regime, from n-hexadecane (3.45 cP at 293 K) to liquid paraffin (123 cP at 293 K), and in dilute poly(methyl methacrylate) (PMMA) solid films. The effect of intermolecular energy transfer and radiative transport on the fluorescence decays was also analysed by increasing the concentration of MBOPV3 in the PMMA films up to a pure MBOPV3 film. The rate constant of conformational relaxation was found to decrease with increasing solvent viscosity up to about 23 cP, but then becomes viscosity independent. The non-Stokes-Einstein behaviour of k(CR) versus eta over the whole viscosity range apparently results from the presence of the flexible methylbutyl side chains of MBOPV3, which partially screens the solvent friction. Conformational relaxation is not observed in very dilute PMMA solid films, where the fluorescence decay becomes single exponential. The decays become multi-exponential again with an increase of MBOPV3 concentration in the PMMA films, but in this case it is due to intermolecular (interchain) energy transfer from less planar to more planar conformations of MBOPV3. In the pure MBOPV3 film, interchain energy transfer (radiative and non-radiative) is the major process responsible for the observed (tetra-exponential) decays.