E. Gallego

On the fundamental role of the Glu 2- ... Arg 10+ salt bridge in the folding of isolated ribonuclease A S-peptide.

The fundamental role of the Glu 2- ... Arg 10+ salt bridge in the folding of isolated S-peptide (1-19 N-terminal fragment of Ribonuclease A) is demonstrated from the comparison of the helix contents, at 0 degrees C, of S-peptide and related peptides. Helix contents have been determined from the analysis of proton chemical shift vs. temperature curves. The observed data can be accounted for by assuming that two side-chain interactions contribute to stabilize the 3-13 helix of S-peptide, the salt bridges Glu 2- ... Arg 10+ and Glu 9-... His 12+, the former being more effective. The salt bridge Glu 9- ... Arg 10+ turns to a weaker interaction, a hydrogen bond Glu 2 (C delta = 0) ... Arg 10+, on protonation or esterification of the Glu 2 carboxylate.

Self-association of imidazole and its methyl derivatives in aqueous solution. A study by ultraviolet spectroscopy

Abstract The self-association of imidazole and its monomethyl derivatives, 1-methyl-, 2-methyl-, and 4(5)-methylimidazole, was studied in aqueous solution, at different pH values, by ultraviolet spectroscopy. The variation in molar absorptivity with the concentration of these compounds was measured for the band near 205 nm. Hypochromic deviations from Beers law with increasing concentration were detected for all compounds. These observations were interpreted in terms of two processes of self-association: dimerization and polymerization, similarly to other heteroaromatic systems, such as pyridine and pyrimidine, in aqueous solution. From the experimental data, self-association constants for dimerization ( K 2 ) and polymerization ( K n ) were calculated. Hypochromic effects were also found in a band near 195 nm, observed in dilute solutions of these compounds at basic pH. The results obtained are discussed with respect to the structural domains of imidazole and the location of the methyl group on the ring. A model of association based on the stacking of the rings, with the possible reinforcement due to hydrogen bonding between rings, is suggested for these compounds in aqueous solution. Self-association of another compound with the ring partially reduced, 2-methyl-2-imidazoline, was also studied, with the result that the aromatic character of the nucleus is not critical for the occurrence of self-association.

A critical study of the application of ultraviolet spectroscopy to the self-association of adenine, adenosine and 5′-AMP in aqueous solution

Abstract UV spectra of adenine, adenosine and 5′-AMP in aqueous solution have been measured over the concentration range 1 × 10 −6 −5 × 10 −2 M. The apparent molar absorptivity of these compounds changes upon concentration, showing two hypochromic effects at c −3 M and c &>; 5 × 10 −3 M, respectively, which may be explained in terms of self-association. A method for calculating self-association constants from these experimental data is developed, based on an association model in which the first hypochromic effect is interpreted in terms of formation of dimers, with an equilibrium constant K 2 , and the second effect is interpreted in terms of formation of polymers, with an equilibrium constant K n . The value of K 2 is of the order of magnitude of 10 4 for the three compounds. The value of K n is dependent on the model chosen for the analysis of the second effect, having an order of magnitude of 10 2 . The features of the self-association model are discussed, as well as the method for calculating self-association parameters from experimental data.

Self-association of pyridine and some of its derivatives in aqueous solution: influence of alkyl substituents

Abstract The self-association of pyridine and some of its alkyl derivatives (2-, 3- and 4-picoline, 2,4- and 2,6-lutidine, 4-tert-butyl- and 2,6-di-tert-butylpyridine) was studied in aqueous solution, at different pH values, by UV spectroscopy. The variation in molar absorptivity with concentration was measured not only for the main maximum, but also for the different component bands of the adsorption band in the mid-UV region (above 200 nm) of these compounds. From the experimental curves of hypochromic effects, self-association constants for dimerization (K2) and polymerization (Kn) were calculated. The results obtained are discussed in detail with relation to the position and nature of alkyl substituents on the pyridine ring. The most relevant result is the influence of alkyl substituents on self-association, particularly the special role of the methyl substituents in ortho positions with respect to the nitrogen atom in 2-picoline, 2,4- and 2,6-lutidine. In 2-picoline and 2,6-lutidine, polymerization can be studied separately from dimerization by measuring the band at longest wavelength, which suggests a possible relationship between the mechanism of formation of polymers and the π* ← n transitions which give rise to these bands.

Self-association of pyridine monocarboxylic acids and their esters in aqueous solution. A study by ultraviolet spectroscopy

Abstract The ultraviolet spectra of pyridine monocarboxylic acids and their methyl or ethyl esters in aqueous solution, at different pH values, show an hypochromic effect over the range 10 −6 –10 −1 M, which is interpreted in terms of dimerization and polymerization. In general, the values calculated for the association constants show that the studied interaction is a function of the nature and position of the substituent groups in the pyridine ring, and of the cationic, zwitterionic, anionic or neutral species which predominate at each of the selected pH values.

Studies of the self-association of 5′-GMP in aqueous solutions by Fourier transform infrared spectroscopy

Abstract Fourier transform infrared spectra of 5′-GMP in D2O solutions have been obta i ned and studied in the concentration range from 5.10−3M to 5.10−1M, at room temperature. For high concentrations marked changes occur in the frequency, intensity and shape of some bands. The changes are most clearly observed in the 1700–1500 cm−1 region. More dilute solutions show similar effects in presence of KCl. These changes in the infrared spectrum when the concentration of 5′-GMP increases have been attributed to the self-association of this mononucleotide in solution, in agreement with previous NMR results.

Quantum-chemical calculations of a proposed PHEn-HISn+4 stabilizing interaction in peptide α-helices

Abstract The interaction between the sidechains of the Phe (n) and His (n+4) which has been proposed as a stabilizing effect for α-helix formation has been modelled by means of totuene: 4-Me-imidazole. Intermolecular potentials and their extrema are calculated at several degrees of approximation for neutral and protonated forms of the imidazole partner. A marked increase in stability for the charged complex relative to the neutral one is found. The interaction energies are used to reparameterize an analytical atom-atom potential for C, N and H contacts.

Quantitative interpretation of the helix coil transition in RNase a S-peptide

Abstract The per-residue helix fractions of S-peptide 1–19 and S-peptide 3–19 have been computed at several pHs and temperatures. The Zimm-Bragg model for helix coil transitions, extended to include specific interactions, has been used. The helicity directly obtained from the Scheragas “host-guest” empirical parameters has been taken as reference value. The thermodynamic parameters of assumed interactions have been evaluated by fitting the data obtained from the analysis of the NMR chemical shifts vs. temperature curves changes. Using these values the helix fraction of Glu 2-Me-ester S-peptide 1–19 and C-peptide 1–13 carboxylate have been also computed.

Self-association of nicotinamide—adenine dinucleotides in aqueous solution

Abstract The self-association of the nicotinamide—adenine dinucleotides NAD and NADH was studied in aqueous solution by means of ultraviolet spectroscopy. Measurements were made of the hypochromic effects of these compounds as a function of concentration over the range 10 −6 −10 −1 M. The effects were interpreted in terms of the processes of dimerization and polymerization. The corresponding self-association constants were calculated. Both processes are evidenced for NAD by the band at 259.0 nm, whilst for NADH the two processes are evidenced separately by two bands at 259.0 and 338.0 nm, which also show different behaviour with temperature. A complementary study of the self-association of nicotinamide as a function of pH was carried out in order to interpret the results of the analysis for NAD and NADH; our previous results on the self-association of adenine derivatives were also used. The differences observed between NAD and NADH are discussed in detail with relation to the interaction of their constituent units. The results of this work show that the self-association processes of NAD and NADH could be significant in determining the mode of interaction of these coenzymes in experiments of binding to corresponding enzymes.

13C NMR spectral assignment of ribonuclease S-peptide. Some new structural information about its low-temperature folding

Abstract The 13 C NMR spectrum of S-peptide (Ribonuclease 19 residue N-terminal fragment) (pH 5.4, 32°C, 12 mM in D 2 O has been assigned with a basis on characteristic values for 13 C signals of amino acids included in short peptides, SFOR multiplicities, 1 J CH reduced values and spectral pH variations. The shift vs. temperature changes have been followed in the range 0°C–50°C and the corresponding curves analyzed by using the ΔH° and ΔS° values for the helix-coil transition obtained from 1 H NMR spectra. Values for chemical shifts in the coil and in the helix have been obtained in this way. Transition shifts are largest for CO and C α resonances in the fragment 3–13, confirming that isolated S-peptide folds in a manner closely ressembling the native structure.