Biomedical subjects

Tigran V Chalikian

Publications and source records attributed to Tigran V Chalikian.

At least 19 recordsLinked to original sources

PubMed2006

Stability of DNA duplexes containing GG, CC, AA, and TT mismatches.

We employed salt-dependent differential scanning calorimetric measurements to characterize the stability of six oligomeric DNA duplexes (5'-GCCGGAXTGCCGG-3'/5'-CCGGCAYTCCGGC-3') that contain in the central XY position the GC, AT, GG, CC, AA, or TT base pair. The heat-induced helix-to-coil transitions of all the duplexes are associated with positive changes in heat capacity, DeltaC(p), ranging from 0.43 to 0.53 kcal/mol. Positive values of DeltaC(p) result in strong temperature dependences of changes in enthalpy, DeltaH degrees, and entropy, DeltaS degrees , accompanying duplex melting and cause melting free energies, DeltaG degrees, to exhibit characteristically curved shapes. These observations suggest that DeltaC(p) needs to be carefully taken into account when the parameters of duplex stability are extrapolated to temperatures distant from the transition temperature, T(M). Comparison of the calorimetric and van't Hoff enthalpies revealed that none of the duplexes studied in this work exhibits two-state melting. Within the context of the central AXT/TYA triplet, the thermal and thermodynamic stabilities of the duplexes in question change in the following order: GC > AT > GG > AA approximately TT > CC. Our estimates revealed that the thermodynamic impact of the GG, AA, and TT mismatches is confined within the central triplet. In contrast, the thermodynamic impact of the CC mismatch propagates into the adjacent helix domains and may involve 7-9 bp. We discuss implications of our results for understanding the origins of initial recognition of mismatched DNA sites by enzymes of the DNA repair machinery.

Base Composition↗

PubMed2006

Hydrophobic hydration in cyclodextrin complexation.

We report temperature-dependent acoustic and densimetric data on changes in volume, expansibility, and adiabatic compressibility accompanying the binding of 1-adamantanecarboxylic acid (AD) to beta-cyclodextrin (beta-CD). We interpret our volumetric results in terms of hydration. Based on our compressibility and expansibility data, we estimate that, at 25 degrees C, the binding of AD to beta-CD is accompanied by displacement of 20 to 25 water molecules from the hydration shells of the two interacting species. Comparison of the temperature-dependent compressibility changes accompanying the binding of AD to beta-CD with the compressibility contribution of aliphatic groups suggests that displaced water molecules predominantly come from the hydrophobic loci of AD and beta-CD. Thus, we conclude that hydrophobic interactions play a major role in stabilizing the AD-beta-CD complex. Our estimate of the number of water molecules released to the bulk is consistent with structural considerations. There is also good agreement between our volumetric data and osmotic stress results reported by Harries et al. (Harries, D.; Rau, D. C.; Parsegian, V. A. J. Am. Chem. Soc. 2005, 127, 2184). This observation is consistent with the picture in which the two techniques probe the same population of water molecules solvating AD and beta-CD.

Cyclodextrins↗

PubMed2006

Probing the transition state ensemble of a protein folding reaction by pressure-dependent NMR relaxation dispersion.

The F61A/A90G mutant of a redesigned form of apocytochrome b562 folds by an apparent two-state mechanism. We have used the pressure dependence of 15N NMR relaxation dispersion rate profiles to study the changes in volumetric parameters that accompany the folding reaction of this protein at 45 degrees C. The experiments were performed under conditions where the folding/unfolding equilibrium could be studied at each pressure without addition of denaturants. The exquisite sensitivity of the methodology to small changes in folding/unfolding rates facilitated the use of relatively low-pressure values (between 1 and 270 bar) so that pressure-induced changes to the unfolded state ensemble could be minimized. A volume change for unfolding of -81 mL/mol is measured (at 1 bar), a factor of 1.4 larger (in absolute value) than the volume difference between the transition state ensemble (TSE) and the unfolded state. Notably, the changes in the free energy difference between folded and unfolded states and in the activation free energy for folding were not linear with pressure. Thus, the difference in the isothermal compressibility upon unfolding (-0.11 mL mol(-1) bar(-1)) and, for the first time, the compressibility of the TSE relative to the unfolded state (0.15 mL mol(-1) bar(-1)) could be calculated. The results argue for a TSE that is collapsed but loosely packed relative to the folded state and significantly hydrated, suggesting that the release of water occurs after the rate-limiting step in protein folding. The notion of a collapsed and hydrated TSE is consistent with expectations based on earlier temperature-dependent folding studies, showing that the barrier to folding at 45 degrees C is entropic (Choy, W. Y.; Zhou, Z.; Bai, Y.; Kay, L. E. J. Am. Chem. Soc. 2005, 127, 5066-5072).

Cytochromes b↗

PubMed2006

Thermodynamics of interactions of water-soluble porphyrins with RNA duplexes.

We characterized the interactions of meso-tetrakis(4N-(2-hydroxyethyl)pyridinium-4-yl) porphyrin (TEtOHPyP4), meso-tetrakis(4N-allylpyridinium-4-yl) porphyrin (TAlPyP4), and meso-tetrakis(4N-metallylpyridinium-4-yl) porphyrin (TMetAlPyP4) with the poly(rA)poly(rU) and poly(rI)poly(rC) RNA duplexes between 18 and 45 degrees C by employing circular dichroism, light absorption, and fluorescence intensity spectroscopic measurements. Our results suggest that TEtOHPyP4 and TAlPyP4 intercalate into the poly(rA)poly(rU) and poly(rI)poly(rC) host duplexes, while TMetAlPyP4 associates with these RNA duplexes by forming outside-bound, self-stacked aggregates. We used our temperature-dependent absorption titration data to determine the binding constants and stoichiometry for each porphyrin-RNA binding event studied in this work. From the temperature dependences of the binding constants, we calculated the binding free energies, DeltaG(b), enthalpies, DeltaH(b), and entropies, DeltaS(b). For each RNA duplex, the binding enthalpy, DeltaH(b), is the most favorable for TEtOHPyP4 (an intercalator) followed by TAlPyP4 (an intercalator) and TMetAlPyP4 (an outside binder). On the other hand, for each duplex, external self-stacking of TMetAlPyP4 produces the most favorable change in entropy, DeltaS(b), followed by the intercalators TAlPyP4 and TEtOHPyP4. Thus, our results suggest that the thermodynamic profile of porphyrin-RNA binding may correlate with the binding mode. This correlation reflects the differential nature of molecular forces that stabilize/destabilize the two modes of binding-intercalation versus external self-stacking along the host duplex.

Kinetics↗

PubMed2005

Association of the minor groove binding drug Hoechst 33258 with d(CGCGAATTCGCG)2: volumetric, calorimetric, and spectroscopic characterizations.

We employed ultrasonic velocimetry, high-precision densimetry, circular dichroism and fluorescence spectroscopy, and isothermal titration calorimetry to characterize the binding of Hoechst 33258 to the d(CGCGAATTCGCG)(2) oligomeric duplex at 25 degrees C. We used this experimental combination to determine the full thermodynamic profile for the binding of Hoechst 33258 to the DNA. Specifically, we report changes in binding free energy, enthalpy, entropy, volume, and adiabatic compressibility accompanying the binding. We interpret our volumetric data in terms of hydration and evaluate the number of waters of hydration that become released to or taken up from the bulk. Our calorimetric data reveal that the drug-DNA binding event studied in this work is entropy-driven and proceeds with an unfavorable change in enthalpy. The favorable binding entropy predominantly results from hydration changes. In contrast to a large and positive change in hydrational entropy, the binding-induced change in configurational entropy is insignificant. The latter observation is consistent with the "lock-and-key" mode of minor groove binding.

Binding Sites↗

PubMed2005

Compressibility changes accompanying conformational transitions of apomyoglobin.

We used high-precision density and ultrasonic velocity measurements to characterize the native (N), molten globule (MG), and unfolded (U) conformations of apomyoglobin. The molten globule states that were studied in this work include the MG(pH4)(NaCl) state observed at pH 4 and 20 mM NaCl, the MG(pH4)(NaTCA) state observed at pH 4 and 20 mM sodium trichloracetate (NaTCA), the MG(pH2)(NaCl) state observed at pH 2 and 200 mM NaCl, and the MG(pH2)(NaTCA) state observed at pH 2 and 20 mM NaTCA. We used our densimetric and acoustic data to evaluate changes in adiabatic compressibility associated with the acid- or salt-induced N-to-MG, MG-to-U, MG-to-MG, and U-to-MG transitions of the protein. The N-to-MG(pH4)(NaCl) and N-to-MG(pH4)(NaTCA) transitions are accompanied by decreases in compressibility of -(3.0 +/- 0.6) x 10(-6) and -(2.0 +/- 0.6) x 10(-6) cm3 g(-1)bar(-1), respectively. The N-to-MG(pH2)(NaCl) and N-to-MG(pH2)(NaTCA) transitions are associated with compressibility changes of -(4.9 +/- 1.1) x 10(-6) and (0.7 +/- 0.9) x 10(-6) cm3 g(-1) bar(-1), respectively. We interpret these data in terms of the degree of unfolding of the various molten globule forms of apomyoglobin. In general, our compressibility data reveal significant disparities between the various equilibrium molten globule states of apomyoglobin while also quantitatively characterizing each of these states. Volumetric insights provided by our data facilitate gaining a better understanding of the folding pathways, intermediates, and kinetics of apomyoglobin folding.

Animals↗

PubMed2004

Energetics of nucleic acid stability: the effect of DeltaCP.

We report high-resolution differential scanning calorimetric data on the poly(dAdT)poly(dAdT), poly(dA)poly(dT), poly(dIdC)poly(dIdC), poly(dGdC)poly(dGdC), poly(rA)poly(rU), and poly(rI)poly(rC) nucleic acid duplexes. We use these data to evaluate the melting temperatures, TM, enthalpy changes, DeltaHM, and heat capacity changes, DeltaCP, accompanying helix-to-coil transitions of each polymeric duplex studied in this work at different NaCl concentrations. In agreement with previous reports, we have found that DeltaCP exhibits a positive, nonzero value, which, on average, equals 268 +/- 33 J mol(-1) K(-1). With DeltaCP, we have calculated the transition free energies, DeltaG, enthalpies, DeltaH, and entropies, DeltaS, for the duplexes as a function of temperature. Since, DeltaG, DeltaH, and DeltaS all strongly depend on temperature, the thermodynamic comparison between DNA and/or RNA duplexes (that may differ from one another with respect to sequence, composition, conformation, etc.) is physically meaningful only if extrapolated to a common temperature. We have performed such comparative analyses to derive differential thermodynamic parameters of formation of GC versus AT, AU, and IC base pairs as well as B' versus A and B helix conformations. We have proposed some general microscopic interpretations for the observed sequence-specific and conformation-specific thermodynamic differences between the duplexes.

Base Pairing↗

PubMed2004

Probing hydration of monovalent cations condensed around polymeric nucleic acids.

We report the relative molar sound velocity increments, [U], partial molar volumes, V(o), and partial molar adiabatic compressibilities, K(S)(o), of the Li(+), Na(+), K(+), Rb(+), Cs(+), NH(4)(+), and N(CH(3))(4)(+) salts of poly(dAdT)poly(dAdT), poly(dGdC)poly(dGdC), poly(dIdC)poly(dIdC), poly(rA)poly(rU), poly(rG)poly(rC), poly(rI)poly(rC), and poly(rU) at 25 degrees C. When analyzing these data, we take into account the Donnan membrane equilibrium effect. Comparison between the values of [U], V(o), and K(S)(o) exhibited by the nucleic acid salts and respective chlorides (LiCl, NaCl, KCl, RbCl, CsCl, NH(4)Cl, and N(CH(3))(4)Cl) yields information about the state of counterion hydration in the vicinity of each nucleic acid structure studied here. Our analysis reveals that the poly(dGdC)poly(dGdC), poly(dIdC)poly(dIdC), and poly(rI)poly(rC) duplexes and single-stranded poly(rU) do not significantly influence the hydration properties of their condensed counterions. In the vicinity of these polymers, counterions retain their full hydration shells (within +/-15%). By contrast, counterions condensed around the poly(dAdT)poly(dAdT), poly(rA)poly(rU), and poly(rG)poly(rC) duplexes are significantly dehydrated and retain, respectively, only 65(+/-18)%, 34(+/-21)%, and 33(+/-9)% of their original hydration shells. Taken together, the volumetric data reported here provide important new information that ultimately may help us understand the central role that hydration and counterions play in modulating the conformational preferences of nucleic acids and the energetics of DNA recognition events.

Cations, Monovalent↗

PubMed2004

Salt-induced oligomerization of partially folded intermediates of equinatoxin II.

Equinatoxin II (EqTxII) is a cytolytic, water-soluble protein which binds to and forms cation-selective pores in lipid membranes. To characterize the native and denatured states of EqTxII and to elucidate the biological role of its oligomers, we have studied salt-dependent heat-induced conformational transitions of EqTxII. To this end, we have employed a variety of experimental techniques, including differential scanning calorimetry, circular dichroism and light absorption spectroscopy, ultrasonic velocimetry, electron microscopy, PAGE, and a hemolytic activity assay. This experimental combination has enabled us to monitor and structurally and thermodynamically characterize temperature-induced conformational transitions and oligomerization of EqTxII at different concentrations of NaCl. At pH 3.0 and 25 degrees C, EqTxII retains its native conformation and remains hemolytically active over a broad range of NaCl concentrations. However, an increase in the salt concentration results in a diminution of the thermal stability of EqTxII. Specifically, the calorimetrically determined denaturation temperature, T(d), and enthalpy, DeltaH(cal), of the toxin decrease with an increase in the salt concentration. Our CD data suggest that the heat-induced denatured state of EqTxII lacks rigid tertiary structure while exhibiting well-defined secondary structure. The amount of the induced, non-native secondary structure of EqTxII depends on the solution ionic strength, temperature, time of incubation at an elevated temperature, and protein concentration. Our combined results suggest that, in the presence of salt, an increase in temperature results in formation of the partially unfolded state of the toxin that oligomerizes and forms biologically inactive, water-soluble aggregates.

Biopolymers↗

PubMed2004

Binding of bovine pancreatic trypsin inhibitor to trypsinogen: spectroscopic and volumetric studies.

We have investigated the binding of bovine pancreatic trypsin inhibitor (BPTI) to bovine trypsinogen by combining ultrasonic velocimetry, high precision densimetry, and fluorescence spectroscopy. We report the changes in volume, adiabatic compressibility, van't Hoff enthalpy, entropy, and free energy that accompany the association of the two proteins at 25 degrees C and pH 8.0. We have used the measured changes in volume and compressibility in conjunction with available structural data to characterize the binding-induced changes in the hydration properties and intrinsic packing of the two proteins. Our estimate reveals that 110 +/- 40 water molecules become released to the bulk from the hydration shells of BPTI and trypsinogen. Furthermore, we find that the intrinsic coefficient of adiabatic compressibility of the two proteins decreases by 14 +/- 2%, which is suggestive of the binding-induced rigidification of the proteins' interior. BPTI-trypsinogen association is an entropy-driven event which proceeds with an unfavorable change in enthalpy. The favorable change in entropy results from partial compensation between two predominant terms. Namely, a large favorable change in hydrational entropy slightly prevails over a close in magnitude but opposite in sign change in configurational entropy. The reduction in configurational entropy and, consequently, protein dynamics is consistent with the observed decrease in intrinsic compressibility. In general, results of this work emphasize the vital role that water plays in modulating protein recognition events.

Animals↗

PubMed2003

Volumetric and spectroscopic characterizations of glucose-hexokinase association.

The binding of D-glucose to hexokinase PII at 25 degrees C and pH 8.7 has been investigated by a combination of ultrasonic velocimetry, high precision densimetry, and fluorescence spectroscopy. The binding of glucose to the enzyme results in significant dehydration of the two interacting molecules, while the intrinsic coefficient of adiabatic compressibility of hexokinase slightly decreases. Glucose-hexokinase association is an entropy-driven process. The favorable change in entropy results from compensation between two large contributions. The binding-induced increase in hydrational entropy slightly prevails over the decrease in the configurational entropy of the enzyme. Taken together, our results emphasize the crucial role of water in modulating the energetics of protein recognition.

Dehydration↗

PubMed2003

Volume and compressibility changes accompanying thermally-induced native-to-unfolded and molten globule-to-unfolded transitions of cytochrome c: a high pressure study.

We have measured the transition temperatures, T(M), and van't Hoff enthalpies, DeltaH(M), of the thermally induced native-to-unfolded (N-to-U) and molten globule-to-unfolded (MG-to-U) transitions of cytochrome c at pressures between 50 and 2200 bar. We have used the pressure dependence of T(M) to evaluate the changes in volume, Delta(v), accompanying each protein transition event as a function of temperature and pressure. From analysis of the temperature and pressure dependences of Delta(v), we have additionally calculated the changes in expansibility, Delta(e), and isothermal compressibility, Delta(k)(T), associated with the thermally induced conformational transitions of cytochrome c. Specifically, if extrapolated to 25 degrees C, the native-to-unfolded (N-to-U) transition is accompanied by changes in volume, Delta(v), expansibility, Delta(e), and isothermal compressibility, Delta(k)(T), of -(5 +/- 3) x 10(-3) cm(3) g(-1), (1.8 +/- 0.3) x 10(-4) cm(3) g(-1) K(-1), and approximately 0 cm(3) g(-1) bar(-1), respectively. The molten globule-to-unfolded (MG-to-U) transition is accompanied by changes in volume, Delta(v), and isothermal compressibility, Delta(k)(T), of -(2.9 +/- 0.3) x 10(-3) cm(3) g(-1) at 40 degrees C and -(1.9 +/- 0.3) x 10(-6) cm(3) g(-1) bar(-1) at 35 degrees C, respectively. By comparing the volumetric properties of the N-to-U and N-to-MG transitions of cytochrome c, we have estimated the properties of the native-to-molten globule (N-to-MG) transition. For the latter transition, the changes in volume, Delta(v), and isothermal compressibility, Delta(k)(T), are approximately 0 cm(3) g(-1) at 40 degrees C and 1.9 cm(3) g(-1) bar(-1) at 35 degrees C, respectively. Our estimate for the change in expansibility, Delta(e), upon the N-to-MG is negative and equal to -(5 +/- 3) x 10(-4) cm(3) g(-1) K(-1). This finding contrasts with the results of previous studies all of which report positive changes in expansibility associated with protein denaturation. In general, our volumetric data permit us to assess the combined effect of temperature and pressure on the stability of various conformational states of cytochrome c.

Animals↗

PubMed2003

Hydration changes accompanying nucleic acid intercalation reactions:volumetric characterizations.

We use high precision ultrasonic velocimetric and densimetric techniques to determine at 25 degrees C the changes in volume, deltaV, and adiabatic compressibility, deltaK(S), that accompany the binding of ethidium to the poly(rA)poly(rU), poly(dAdT)poly(dAdT), poly(dGdC)poly(dGdC), and poly(dIdC)poly(dIdC) duplexes, as well as to the poly(rU)poly(rA)poly(rU) triplex. The binding of ethidium to each of the duplexes and the triplex is accompanied by negative changes in volume, deltaV, and adiabatic compressibility, deltaK(S). We discuss the basis for relating macroscopic and microscopic properties, particularly, emphasizing how measured changes in volume and compressibility can be quantitatively interpreted in terms of the differential hydration properties of DNA and RNA structures in their ligand-free and ligand-bound states. We also estimate the entropic cost of intercalation-induced changes in hydration of each of the nucleic acid structures and the drug. In general, our results emphasize the vital role of hydration in modulating the energetics of drug-DNA binding, while also underscoring the fact that hydration must be carefully taken into account in analysis and prediction of the energetics of nucleic acid recognition.

Circular Dichroism↗

PubMed2003

How large are the volume changes accompanying protein transitions and binding?

We present a simple model to describe volume changes accompanying protein folding and binding events. The model enables one to resolve the changes in volume accompanying conformational transitions of proteins as well as association of proteins with other molecules in terms of the intrinsic, thermal and interaction (hydration) contributions. The thermal contribution to protein volume results from thermally activated mutual vibrational motions of contacting solute and solvent molecules. Our calculations suggest that near zero volume changes accompanying protein folding and binding events reflect compensation between significant changes in the intrinsic, thermal and interaction terms. We have quantitatively estimated these terms as a function of the protein's molecular weight and degree of its unfolding. Results described in this work lay foundation for more reliable and physically justified interpretations of volumetric data on protein folding and binding events. We also discuss potential ways of extending applications of our model to analyzing other macromolecular systems and events, including drug-DNA and protein-DNA interactions and helix-to-helix and helix-to-coil transitions of nucleic acids.

Animals↗

PubMed2003

Volumetric effects of ionization of amino and carboxyl termini of alpha,omega-aminocarboxylic acids.

We have determined the partial molar volumes and adiabatic compressibilities of a homologous series of six alpha,omega-aminocarboxylic acids over a broad pH range at 25 degrees C. We interpret the resulting data in terms of the changes in hydration associated with neutralization of amino and carboxyl termini. By combining our volumetric results with pH-dependent data on 1-anilinonaphthalene-8-sulfonic acid fluorescence we propose the following explanation to the long-standing observation that changes in volume and compressibility accompanying neutralization of a carboxyl group depend on the type of the solute in contrast to solute-independent changes in these parameters accompanying neutralization of an amino group. Unlike amino groups, neutralized carboxyl groups are capable of forming hydrogen-bonded structures stabilized by hydrogen bonds between the carbonyl oxygen of one solute molecule and the hydroxyl group of another molecule. Formation of such hydrogen-bonded structures causes an additional decrease in solute hydration with concomitant increases in volume and compressibility. Furthermore, solutes with large aliphatic moieties may form larger associates stabilized, in addition to intermolecular hydrogen bonds, by hydrophobic interactions which will result in further increases in volume and compressibility. In the aggregate, our results emphasize the need for further studies focused on developing an understanding of the role of electrostatic interactions in stabilizing/destabilizing proteins and protein complexes.

Amino Acids, Diamino↗

PubMed2003

The thermodynamics of protein-protein recognition as characterized by a combination of volumetric and calorimetric techniques: the binding of turkey ovomucoid third domain to alpha-chymotrypsin.

We have used ultrasonic velocimetry, high-precision densimetry, and fluorescence spectroscopy, in conjunction with isothermal titration and differential scanning calorimetry, to characterize the binding of turkey ovomucoid third domain (OMTKY3) to alpha-chymotrypsin. We report the changes in volume and adiabatic compressibility that accompany the association of these proteins at 25 degrees C and pH 4.5. In addition, we report the changes in free energy, enthalpy, entropy, and heat capacity upon the binding of OMTKY3 to alpha-chymotrypsin over a temperature range of 20-40 degrees C. Our volume and compressibility data, in conjunction with X-ray crytsallographic data on the OMTKY3-alpha-chymotrypsin complex, suggest that 454(+/-22) water molecules are released to the bulk state upon the binding of OMTKY3 to alpha-chymotrypsin. Furthermore, these volumetric data suggest that the intrinsic compressibility of the two proteins decreases by 7%. At each temperature studied, OMTKY3 association with alpha-chymotrypsin is entropy driven with a large, unfavorable enthalpy contribution. The observed entropy of the binding reflects interplay between two very large favorable and unfavorable terms. The favorable term reflects an increase in the hydrational entropy resulting from release to the bulk of 454 water molecules. The unfavorable term is related to a decrease in the configurational entropy and, consequently, a decrease in the conformational dynamics of the two proteins. In general, we discuss the relationship between macroscopic and microscopic properties, in particular, identifying and quantifying the role of hydration in determining the thermodynamics of protein recognition as reflected in volumetric and calorimetric parameters.

Animals↗

PubMed2003

Volumetric properties of proteins.

Structural and thermodynamic characterizations of a variety of intra- and intermolecular interactions stabilizing/destabilizing protein systems represent a major part of multidisciplinary efforts aimed at solving the problems of protein folding and binding. To this end, volumetric techniques have been successfully used to gain insights into protein hydration and intraglobular packing. Despite the fact that the use of volumetric measurements in protein-related studies dates back to the 1950s, such measurements still represent a relatively untapped yet potentially informative means for tackling the problems of protein folding and binding. This notion has been further emphasized by recent advances in the development of highly sensitive volumetric instrumentation that has led to intensifying volumetric investigations of protein systems. This paper reviews the volumetric properties of proteins and their low-molecular-weight analogs, in particular, discussing the recent progress in the use of volumetric data for studying conformational transitions of proteins as well as protein-ligand, protein-protein, and protein-nucleic acid interactions.

Crystallography↗

PubMed2003

Volumetric characterization of homopolymeric amino acids.

We have determined the partial molar volumes, expansibilities, and adiabatic compressibilities for poly(L-alanine), poly(L-proline), and poly(L-threonine) within the temperature range of 18-55 degrees C. In addition, we have determined at 25 degrees C changes in volume, DeltaV, and adiabatic compressibility, DeltaK(S), associated with the coil-to-helix transitions of poly(L-lysine) and poly(L-glutamic acid) and the alpha-to-beta transition of poly(L-lysine). We have interpreted our volumetric data as suggesting that poly(L-alanine) and poly(L-proline) are not fully unfolded and, probably, retain some solvent-inaccessible core. Further, we propose that poly(L-threonine) is fully unfolded with the majority of its atomic groups being solvent-exposed. Near zero changes in volume and compressibility accompanying the coil-to-helix transitions of poly(L-lysine) and poly(L-glutamic acid) suggest that, in the absence of fortuitous compensations, the coil-to-helix transitions of the polypeptides do not result in any significant enhancement of solute hydration. By contrast, the alpha-to-beta transition of poly(L-lysine) causes slight but statistically significant increases in volume and compressibility, an observation that may suggest that the beta-sheet conformation of poly(L-lysine) is slightly less hydrated than its alpha-helical conformation. In general, our results provide a quantitative volumetric description of the hydration properties of the homopolymeric polypeptides investigated. Such characterizations should prove useful in developing an understanding of the role that solvent plays in the stabilization/destabilization of folded protein structures and protein complexes.

Circular Dichroism↗