Structural biology workflow for the expression and characterization of functional human sodium glucose transporter type 1 in Pichia pastoris

Heterologous expression of human membrane proteins is a challenge in structural biology towards drug discovery. Here we report a complete expression and purification process of a functional human sodium/D-glucose co-transporter 1 (hSGLT1) in Pichia pastoris as representative example of a useful strategy for any human membrane protein. hSGLT1 gene was cloned in two different plasmids to develop parallel strategies: one which includes green fluorescent protein fusion for screening optimal conditions, and another for large scale protein production for structural biology and biophysics studies. Our strategy yields at least 1 mg of monodisperse purified recombinant hSGLT1 per liter of culture, which can be characterized by circular dichroism and infrared spectroscopy as an alpha-helical fold protein. This purified hSGLT1 transports co-substrates (Na⁺ and glucose) and it is inhibited by phlorizin in electrophysiological experiments performed in planar lipid membranes.

Forearm pronation efficiency in A.L. 288-1 (Australopithecus afarensis) and MH2 (Australopithecus sediba): Insights into their locomotor and manipulative habits

OBJECTIVES

The locomotor and manipulative abilities of australopithecines are highly debated in the paleoanthropological context. Australopithecus afarensis and Australopithecus sediba likely engaged in arboreal locomotion and, especially the latter, in certain activities implying manipulation. Nevertheless, their degree of arboreality and the relevance of their manipulative skills remain unclear. Here we calculate the pronation efficiency of the forearm (E_rot ) in these taxa to explore their arboreal and manipulative capabilities using a biomechanical approach.

MATERIALS AND METHODS

Three-dimensional humeral images and upper limb measurements of A.L. 288-1 (Au. afarensis) and MH2 (Au. sediba) were used to calculate E_rot using a previously described biomechanical model.

RESULTS

Maximal E_rot in elbow flexion occurs in a rather supinated position of the forearm in Au. afarensis, similarly to Pan troglodytes. In elbow extension, maximal E_rot in this fossil taxon occurs in the same forearm position as in Pongo spp. In Au. sediba the forearm positions where E_rot is maximal are largely coincident with those for Hylobatidae.

CONCLUSIONS

The pattern in Au. afarensis suggests relevant arboreal capabilities, which would include vertical climbing, although it is suggestive of poorer manipulative skills than in modern humans. The similarity between Au. sediba and Hylobatidae is difficult to interpret, but the differences between Au. sediba and Au. afarensis suggest that the capacity to rotate the forearm followed different evolutionary processes in these australopithecine species. Although functional inferences from the upper limb are complex, the observed differences between both taxa point to the existence of two distinct anatomical models.

Activity-related sexual dimorphism in Alaskan foragers from Point Hope: Evidences from the upper limb

Ipiutak (100BCE-500CE) and Tigara (1200 - 1700CE) are two populations from Point Hope, Alaska. As commonly observed in forager communities, it may be expected males and females to have been involved in markedly different daily activities. Nevertheless, activity-related sexual dimorphism in these populations has been scarcely studied. Using humeral diaphyseal cross-sectional properties and forearm rotational efficiency, which are activity-dependent characteristics, we aim to assess differences between sexes and discuss what activities could have triggered them. Our results suggest that in Ipiutak males and females did not differ meaningfully in their cross-sectional properties. Conversely, in Tigara males had a greater rigidity of the entire humeral diaphysis than females, which suggests the existence of greater relative activity levels and more physically demanding tasks, possibly related to hunting activities. Concerning the differences between sexes in the forearm rotational efficiency, in Tigara females rotational efficiency in elbow flexion is maximal in a more supinated position than in males, which leads to an improvement of efficiency in those stages related to manipulation, and so improves the manipulative capacities of the upper limb. These differences in efficiency are caused by a more proximally oriented humeral medial epicondyle in females, which is thus confirmed to be a good feature to assess differences in labor. Therefore females in Tigara probably performed in a daily basis household activities, such as hide processing and other manipulative labors. In Ipiutak, the analysis of forearm rotational efficiency did not reveal differences between sexes. Overall, the results suggest that division of labor in Ipiutak was not as marked as in Tigara, where upper limb skeletal structure supports the idea that both sexes were involved in different daily activities. Nevertheless, the generalized lack of results in Ipiutak could be due to the small sample size, and thus interpretations should be considered with caution.

Biomechanics of forearm rotation: force and efficiency of pronator teres

Biomechanical models are useful to assess the effect of muscular forces on bone structure. Using skeletal remains, we analyze pronator teres rotational efficiency and its force components throughout the entire flexion-extension and pronation-supination ranges by means of a new biomechanical model and 3D imaging techniques, and we explore the relationship between these parameters and skeletal structure. The results show that maximal efficiency is the highest in full elbow flexion and is close to forearm neutral position for each elbow angle. The vertical component of pronator teres force is the highest among all components and is greater in pronation and elbow extension. The radial component becomes negative in pronation and reaches lower values as the elbow flexes. Both components could enhance radial curvature, especially in pronation. The model also enables to calculate efficiency and force components simulating changes in osteometric parameters. An increase of radial curvature improves efficiency and displaces the position where the radial component becomes negative towards the end of pronation. A more proximal location of pronator teres radial enthesis and a larger humeral medial epicondyle increase efficiency and displace the position where this component becomes negative towards forearm neutral position, which enhances radial curvature. Efficiency is also affected by medial epicondylar orientation and carrying angle. Moreover, reaching an object and bringing it close to the face in a close-to-neutral position improve efficiency and entail an equilibrium between the forces affecting the elbow joint stability. When the upper-limb skeleton is used in positions of low efficiency, implying unbalanced force components, it undergoes plastic changes, which improve these parameters. These findings are useful for studies on ergonomics and orthopaedics, and the model could also be applied to fossil primates in order to infer their locomotor form. Moreover, activity patterns in human ancient populations could be deduced from parameters reported here.

3D analysis of the forearm rotational efficiency variation in humans

Pronosupination is a component of the hominoid orthograde corporal plane that enables primates to execute efficient and sure locomotion in their habitat and is an essential movement for the development of manipulative capacities. We analyze human variability in the rotational efficiency of the pronator teres muscle by applying the biomechanical model created by Galtés et al. (Am J Phys Anthropol 2008; 135:293-300; Am J Phys Anthropol 2009a; 140:589-594) to skeletal remains of a human sample (N = 29) and three nonhuman hominoid specimens (chimpanzee, gorilla, and orangutan) by means of 3D technology. We aim to examine whether there is a distinctive human pattern of rotational efficiency and determine which structural features of the upper-limb bones have the greatest influence on the determination of rotational efficiency. Our results show that the human pattern differs from efficiencies observed in nonhuman hominoids, which may be interpreted in the light of morphofunctional adaptations. We identify medial epicondylar form as the key structure of the upper-limb bones for the determination of the rotational efficiency of the forearm. Results indicate that the more medially projected epicondyle of nonhuman hominoids relative to humans leads to higher values of maximum rotational efficiency. Moreover, the orientation of the medial epicondyle determines the pronounced differences in the position of the maximum efficiencies in the pronosupination range between humans and the studied nonhuman hominoids. Proximodistal orientation of the medial epicondyle is suggested to be a more appropriate feature for distinguishing between humans and nonhuman hominoids than anteroposterior orientation and, therefore, for inferring behavioral aspects from skeletal remains and fossils of primate upper-limb bones.

On-chip photoactivation of heterologously expressed rhodopsin allows kinetic analysis of G-protein signaling by surface plasmon resonance spectroscopy

Surface plasmon resonance spectroscopy allows the study of protein interaction dynamics in real-time. Application of this technique to G-protein coupled receptors, the largest family of receptors involved in signal transduction, has been complicated by their low level of expression and the critical dependence of their native conformation on the hydrophobic transmembrane lipid environment. Here, we investigate and compare three different strategies to immobilize rhodopsin, a prototypical G-protein coupled receptor on a sensor chip surface using antibodies and a lectin for receptor capturing. By further probing of different experimental conditions (pH, detergent type) we identified the optimal factors to maintain rhodopsin in a functional conformation and extended this approach to recombinant rhodopsin that was heterologously expressed in COS cells. Functional operation of rhodopsin on the sensor chip surface was proven by its activation and subsequent light-stimulated G-protein coupling. The influence of these experimental parameters on the association and dissociation kinetics of G-protein receptor coupling was determined. Thereby, we found that the kinetics of G(t) interaction were not changed by the strategy of immobilization or the type of detergent. Regeneration of opsin directly on a chip allowed recycling of the immobilized native and recombinant receptor. Thus, the approach provides an experimental framework for choosing the most suitable conditions for the solubilization, immobilization, and for functional tests of rhodopsin on a biosensor surface.

Critical role of electrostatic interactions of amino acids at the cytoplasmic region of helices 3 and 6 in rhodopsin conformational properties and activation

The cytoplasmic sides of transmembrane helices 3 and 6 of G-protein-coupled receptors are connected by a network of ionic interactions that play an important role in maintaining its inactive conformation. To investigate the role of such a network in rhodopsin structure and function, we have constructed single mutants at position 134 in helix 3 and at positions 247 and 251 in helix 6, as well as combinations of these to obtain double mutants involving the two helices. These mutants have been expressed in COS-1 cells, immunopurified using the rho-1D4 antibody, and studied by UV-visible spectrophotometry. Most of the single mutations did not affect chromophore formation, but double mutants, especially those involving the T251K mutant, resulted in low yield of protein and impaired 11-cis-retinal binding. Single mutants E134Q, E247Q, and E247A showed the ability to activate transducin in the dark, and E134Q and E247A enhanced activation upon illumination, with regard to wild-type rhodopsin. Mutations E247A and T251A (in E134Q/E247A and E134Q/T251A double mutants) resulted in enhanced activation compared with the single E134Q mutant in the dark. A role for Thr(251) in this network is proposed for the first time in rhodopsin. As a result of these mutations, alterations in the hydrogen bond interactions between the amino acid side chains at the cytoplasmic region of transmembrane helices 3 and 6 have been observed using molecular dynamics simulations. Our combined experimental and modeling results provide new insights into the details of the structural determinants of the conformational change ensuing photoactivation of rhodopsin.

Ca2+/recoverin dependent regulation of phosphorylation of the rhodopsin mutant R135L associated with retinitis pigmentosa

No single molecular mechanism accounts for the effect of mutations in rhodopsin associated with retinitis pigmentosa. Here we report on the specific effect of a Ca2+/recoverin upon phosphorylation of the autosomal dominant retinitis pigmentosa R135L rhodopsin mutant. This mutant shows specific features like impaired G-protein signaling but enhanced phosphorylation in the shut-off process. We now report that R135L hyperphosphorylation by rhodopsin kinase is less efficiently inhibited by Ca2+/recoverin than wild-type rhodopsin. This suggests an involvement of Ca2+/recoverin into the molecular pathogenic effect of the mutation in retinitis pigmentosa which is the cause of rod photoreceptor cell degeneration. This new proposed role of Ca2+/recoverin may be one of the specific features of the proposed new Type III class or rhodopsin mutations associated with retinitis pigmentosa.

Effect of dodecyl maltoside detergent on rhodopsin stability and function

Detergent-solubilized bovine rhodopsin produces mixed detergent/lipid/protein micelles. The effect of dodecyl maltoside detergent on the thermal stability of dark-state rhodopsin, and upon formation of the different intermediates after rhodopsin photobleaching (metarhodopsin II and metarhodopsin III), and upon transducin activation has been studied. No significant effect is observed for the thermal stability of dark-state rhodopsin in the range of detergent concentrations studied, but a decrease in the stability of metarhodopsin II and an increase in metarhodopsin III formation is observed with decreasing detergent concentrations. The transducin activation process is also affected by the presence of detergent indicating that this process is dependent on the lipid micro-environment and membrane fluidity, and this stresses the importance of the native lipid environment in rhodopsin normal function.

Specific isomerization of rhodopsin-bound 11-cis-retinal to all-trans-retinal under thermal denaturation

The natural ligand of the retinal photoreceptor rhodopsin, 11-cis-retinal, is isomerized to its all-trans configuration as a consequence of light absorption in the first step of the visual phototransduction process. Here we show, by means of difference spectroscopy and high-performance liquid chromatography analysis, that thermal denaturation of rhodopsin induces the same type of isomerization. This effect is likely due to thermally induced conformational rearrangements of amino acid residues in the retinal-binding pocket--possibly implying helical movements--and highlights the tight coupling between 11-cis-retinal and opsin. This effect could have implications in the instability and functional changes seen for certain mutations in rhodopsin associated with retinal disease, and in the stability of the different conformers induced by mutations in other G protein-coupled receptors.

[Rhodopsin structure: some light into the shadows of retinal degenerations]

Retinitis pigmentosa is a group of retinal degenerative diseases, within the broad family of hereditary retinopathies, for which there is no cure at present. Mutations in different genes coding for proteins related to the metabolism of photoreceptor cells, and to the visual phototransduction cascade, are the cause of this disease. Rhodopsin, the photoreceptor protein responsible for light absorption--and key in the first stages of vision--is one of the most studied molecules of the retina. Mutations in the opsin gene account for about 25% of all cases of autosomal dominant retinitis pigmentosa. Recent crystallization of this receptor in its inactive dark state has revealed new structural details yielding further insights into the intra and intermolecular mechanismsin which the protein is involved as a result of its activation.Furthermore, the in vitro study of recombinant rhodopsins carrying mutations previously found in retinitis pigmentosa patients (by means of spectroscopic and functional techniques) has shed new light on the structural requirements for its correct function, as well as the molecular defects underlying the mechanism of photoreceptor cell death. In this study, the main findings of the recent investigations carried out in this field are presented. The relevant information obtained at the molecular level is bound to facilitate our understandingof the molecular processes that will allow suitable therapiesfor different retinal degenerative diseases, particularly retinitis pigmentosa, to be proposed.

Altered functionality in rhodopsin point mutants associated with retinitis pigmentosa

Point mutations found in rhodopsin associated with the retinal degenerative disease retinitis pigmentosa have been expressed in mammalian COS-1 cells, purified, and characterised. The mutations characterised-most of them for the first time-have been Met44Thr, Gly114Asp, Arg135Leu, Val137Met, and Pro171Leu in the transmembrane domain; Leu328Pro and Ala346Pro in the C-terminal tail of the cytoplasmic domain; and Gly106Trp in the intradiscal domain. Several of these mutations cause misfolding which results in impaired 11-cis-retinal binding. Two of them, Met44Thr and Val137Met, show spectral and structural features similar to those of wild type rhodopsin (Type I mutants) but significantly increased transducin initial activation rates. We propose that, in the case of these mutants, abnormal functioning resulting in faster activation kinetics could also play a role in retinitis pigmentosa by altering the stoichiometric balance of the different proteins involved in the phototransduction biochemical reactions.

The eye photoreceptor protein rhodopsin. Structural implications for retinal disease

Rhodopsin is the membrane receptor responsible for photoreception in the vertebrate retina. Its characteristic seven-transmembrane helical structural motif is today widely recognised as a paradigm in signal transduction. Rhodopsin and the phototransduction system are frequently used as structural and mechanistic models for the G-protein coupled receptor superfamily. Recent advances in the activation mechanism (as derived from the structural available data) and the implications for normal and pathological - in retinal disorders - visual function will be reviewed.

Mutations at position 125 in transmembrane helix III of rhodopsin affect the structure and signalling of the receptor

Mutation of L125R in trasmembrane helix III of rhodopsin, associated with the retinal degenerative disease retinitis pigmentosa, was previously shown to cause structural misfolding of the mutant protein. Also, conservative mutations at this position were found to cause partial misfolding of the mutant receptors. We report here on a series of mutations at position 125 to further investigate the role of Leu125 in the correct folding and function of rhodopsin. In particular, the effect of the size of the substituted amino-acid side chain in the functionality of the receptor, measured as the ability of the mutant rhodopsins to activate the G protein transducin, has been analysed. The following mutations have been studied: L125G, L125N, L125I, L125H, L125P, L125T, L125D, L125E, L125Y and L125W. Most of the mutant proteins, expressed in COS-1 cells, showed reduced 11-cis-retinal binding, red-shifts in the wavelength of the visible absorbance maximum, and increased reactivity towards hydroxylamine in the dark. Thermal stability in the dark was reduced, particularly for L125P, L125Y and L125W mutants. The ability of the mutant rhodopsins to activate the G protein transducin was significantly reduced in a size dependent manner, especially in the case of the bulkier L125Y and L125W substitutions, suggesting a steric effect of the substituted amino acid. On the basis of the present and previous results, Leu125 in transmembrane helix III of rhodopsin, in the vicinity of the beta-ionone ring of 11-cis-retinal, is proposed to be an important residue in maintaining the correct structure of the chromophore binding pocket. Thus, bulky substitutions at this position may affect the structure and signallling of the receptor by altering the optimal conformation of the retinal binding pocket, rather than by direct interaction with the chromophore, as seen from the recent crystallographic structure of rhodopsin.

Fourier transform infrared spectroscopy indicates a major conformational rearrangement in the activation of rhodopsin

The study of the structural differences between rhodopsin and its active form (metarhodopsin II) has been carried out by means of deconvolution analysis of infrared spectra. Deconvolution techniques allow the direct identification of the spectral changes that have occurred, which results in a significantly different view of the conformational changes occurring after activation of the receptor as compared with previous difference spectroscopy analysis. Thus, a number of changes in the bands assigned to solvent-exposed domains of the receptor are detected, indicating significant decreases in extended (beta) sequences and in reverse turns, and increases in irregular/aperiodic sequences and in helices with a non-alpha geometry, whereas there is no decrease in alpha-helices. In addition to secondary structure conversions, qualitative alterations within a given secondary structure type are detected. These are seen to occur in both reverse turns and helices. The nature of this spectral change is of great importance, since a clear alteration in the helices bundle core is detected. All these changes indicate that the rhodopsin --> metarhodopsin II transition involves not a minor but a major conformational rearrangement, reconciling the infrared data with the energetics of the activation process.

An A-form of poly[d(A-C)].poly[d(G-T)] induced by mercury (II) as studied by UV and FTIR spectroscopies

The conformational changes of poly[d(A-C)].poly[d(G-T)] induced by Hg(ClO4)2 in aqueous solution have been studied using UV absorption and fourth derivative spectrophotometries, and FTIR spectroscopy. The UV absorption and fourth derivative spectra reflect changes in the polynucleotide stacking interactions as a result of the metal-polynucleotide interaction. The fourth derivative spectra do not indicate a Z-form either at low or at high metal-to-polynucleotide ratios. Furthermore, the infrared spectrum at high metal-to-polynucleotide ratio (r = 1.2; r = [Hg(ClO4])2/[nucleotide] molar ratio) has the main features of an A-form, in contrast with previous CD studies which proposed that the polynucleotide adopts a Z-form under these conditions. The nature of a different conformation of the polynucleotide induced at low r-ratios (r < or = 0.2) is discussed.

Quantitative characterization of the structure of rhodopsin in disc membrane by means of Fourier transform infrared spectroscopy

Fourier transform infrared (FTIR) spectroscopy has been used for the detailed characterization and quantification of the secondary structure of bovine rhodopsin in native disc membranes. FTIR spectra were obtained in aqueous media, both in 1H2O and in 2H2O. Analysis of spectra by means of Fourier self-deconvolution, complemented with maximum likelihood restoration and Fourier derivative, has allowed the characterization of major amide I secondary structure-sensitive component bands of structural relevance which had not been detected before. In consequence, we show a richer secondary structure for rhodopsin than previously described. Our results indicate a total regular helix content around 51%, which would include not only the main alpha 1-type helix but also 3(10)-like helix. The presence of distorted helicoid sequences might furthermore increase to a certain extent the total helix amount. It is also indicated that a significant proportion of the amino acid residues are involved in extended/beta-structures and in reverse turns, as well as in "random" segments, which had not been directly demonstrated before. 61 +/- 4% of rhodopsin is determined to be solvent-accessible, which is a substantially higher value than previously reported. Helices account for most of the inaccessible moiety.

An A-form of poly(amino2dA-dT).poly(amino2dA-dT) induced by polyamines

The effect of the naturally occurring polyamines spermidine and spermine on poly(amino2-dA-dT).poly(amino2dA-dT) conformation has been studied by UV, CD, and IR spectroscopies. It is shown that a conformational transition is induced in poly(amino2dA-dT).poly(amino2dA-dT) by micromolar concentrations of the polyamines (30 microM) in low-salt aqueous solution. The analysis of our results, in view of previously published studies on conformational properties of the amino polynucleotide, indicates the resulting conformer to be an A-form. Interestingly, the polyamine concentration at the midpoint of the transition is the same in both cases. This provides further evidence that the coordination of positively charged counterions to DNA is determined largely from the DNA structure, probably with an important role for the sequence, and less from the nature of the ions.

Study of polynucleotide conformation by resolution-enhanced ultraviolet spectroscopy poly(rC) and poly(dC)

Self-deconvolution and the fourth derivative of ultraviolet absorption spectra have been used to study stacked single-stranded and double-helix structures of different cytosine-containing polynucleotides for the first time. These compounds were studied under different solution conditions (pH and organic solvents) and at low temperatures. The red shift of the lower band (B2u band plus possibly some n-->pi* transition) of the absorption spectra in the cytosine-containing polynucleotides and the appearance of new peaks in the deconvoluted and derivative spectra in the 280-310 nm region are attributed mainly to cytosine-cytosine stacking interactions. In particular, the fourth-derivative peaks at wavelengths higher than 290 nm can be associated to coupling of electronic transitions of cytosine bases. The nature of the electronic transitions producing the absorption bands which are resolved in the aforementioned fourth-derivative peaks is discussed. It is concluded that the resolution-enhancement techniques used in this work, i.e. self-deconvolution and fourth derivative, complement each other and are useful methods to study structural changes of single-stranded and double-stranded polynucleotides allowing, at the same time, more information to be obtained about specific stacking interactions than classical absorption spectrophotometry.

Study of the structure of arrestin (S-antigen) from bovine photoreceptors by FTIR spectroscopy

Fourier transform-infrared spectroscopy has been used for the study of the secondary structure of arrestin from bovine retina rod cells. Spectra have been obtained in H2O and in D2O media. Resolution enhancement of the amide I secondary structure-sensitive overlapped component bands has been achieved by means of Fourier self-deconvolution and Fourier derivation. In order to obtain a quantitative estimation of the proportion of amino acid residues involved in each type of secondary structure, bands at the resolved frequencies have been curve-fitted to the deconvolved amide I contour by means of a least-squares best-fitting iterative program. The analysis of the results suggests that the secondary structure of arrestin comprises 56-63% of extended strands, 12-19% of turns and bends, 15% of alpha-helices and 10% of undefined and irregular segments.

9-aminoacridine inhibits the B-Z transition of poly(dA-dT)

9-Aminoacridine is the parent compound of a family of pharmacologically active model substances that bind to DNA through intercalation between base pairs. In the present study we show that 9-aminoacridine inhibits the B-to-Z isomerization of poly(dA-dT) in conditions that otherwise cause it to occur (5 M NaCl and 123 mM Ni(ClO4)2). Higher concentrations of Ni(ClO4)2 (155 mM) are able to induce the Z-form due to the disruption of the drug-polynucleotide interaction by the metal ion. Additionally, the dye reverses the Z-form in certain conditions. Thus, the data from this study indicate that 9-aminoacridine binds preferentially to the B-form of poly(dA-dT).

FT-IR spectroscopic study of the poly(amino2dA-dT) duplex in Mg(2+)-containing solution and in films

The alternative structures of the synthetic poly(amino2dA-dT) duplex have been studied using infrared spectroscopy in films and in solution (D2O and H2O) in the presence and in the absence of magnesium salt. In solution without magnesium salt, the polynucleotide exists in a B genus conformation with some of the sugar puckers possibly in the C3'-endo/anti geometry. In magnesium-containing solution (66 mM MgCl2), however, we report infrared spectra of Mg(2+)-poly(amino2dA-dT) which have characteristic marker bands of the A form. Film samples in 70% relative humidity (RH) give similar infrared spectra to those of the polynucleotide obtained using Mg2+. Thus, when analyzed in comparison with previously reported infrared spectra of other oligo and polynucleotides, our data show that double helical poly(amino2dA-dT) goes into the same (or very closely related) conformation in dehydrated films as in solutions containing Mg2+.

Conformational isomerizations of the poly(dA-dT) and poly(amino2dA-dT) duplexes involving the unusual X-DNA double helix: a fourth derivative spectrophotometric study

Fourth derivative spectrophotometry has been applied to monitor conformational isomerizations of polynucleotides for the first time. The transitions studied have been the B-A and A-X isomerizations of poly(dA-dT) and the B-X one of poly(amino2dA-dT). Parameters obtained from the fourth derivative spectra have been used to follow these conformational changes. The A form of poly(dA-dT) has been characterized by a new fourth derivative peak at 293.0 nm which can be associated to interstrand adenine-adenine interactions. Furthermore, some of the fourth derivative peaks in the long wavelength region (270-310 nm) can be related to stacking interactions present in the polynucleotide double helices. The tentative assignment of these peaks, particularly that at 299.0 nm in the derivative spectra of poly(amino2dA-dT), to n----pi electronic transitions is discussed.