Topography of nucleic acid helices in solutions. 3. Interactions of spermine and spermidine derivatives with polyadenylic-polyuridylic and polyinosinic-polycytidylic acid helices

The synthesis of spermine derivatives (II), \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm R}_1 {\rm R}_{\rm 2} {\rm R}_{\rm 3} \mathop {\rm N}\limits^ + \left( {{\rm CH}_2 } \right)_3 \mathop {\rm N}\limits^ + {\rm R}_{\rm 1} {\rm R}_{\rm 2} \left( {{\rm CH}_2 } \right)_2 ]_2 \cdot 4{\rm X}^ - $\end{document}, and spermidine derivatives (III), \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm R}_1 {\rm R}_{\rm 2} {\rm R}_{\rm 3} \mathop {\rm N}\limits^ + \left( {{\rm CH}_2 } \right)_4 \mathop {\rm N}\limits^ + {\rm R}_{\rm 1} {\rm R}_{\rm 2} \left( {{\rm CH}_2 } \right)_3 \mathop {\rm N}\limits^ + {\rm R}_{\rm 1} {\rm R}_{\rm 2} {\rm R}_3 \cdot 3{\rm X}^ - $\end{document}, are reported. The effects of these salts on the helix–coil transition of rA–rU and rI–rC helices were examined. Increasing the size of the hydrophobic substituents, R¹, R², and R³ lowers the degree of stabilization of the helical structure. The disproportionation reaction, 2rA–rU→rA–rU₂ + rA occurs readily with salts II and III, especially when the substituents, R₁, R₂, and R₃ are small, i.e., H or Me. Spermine is found to stabilize the rA–rU² and rI–rC helices to approximately the same extent; however, large differences between the degree of stabilization of rA–rU₂ and rI-rC helices are observed when the substituents R₁, R₂, and R₃ are large hydrophobic groups. Similar results are also obtained for the spermidine series. Finally, differences in the interactions of the salts II and III with rA–rU₂ and rI–rC helices suggest that the latter helix is denser.     

Optical anisotropy of polypeptide chains

Optical anisotropies γ² of N-t-butylacetamide (tBA), N-Methylacetamide (MA), and N, N-dimethylacetamide (DMA) have been determined from the Rayleigh ratios for depolarzed scattering by dilute solutions of the amides in p-dioxane. Traceless optical polarizability tensors \documentclass{article}\pagestyle{empty}\begin{document}$ \widehat{\rm \alpha } $\end{document} for the amides are derived from these results in conjunction with the Kerr constant for tBA determined by LeGèvre and co-workers. It is shown that the tensor \documentclass{article}\pagestyle{empty}\begin{document}$ \widehat{\rm \alpha } $\end{document}_i for the glycyle unit in a polypeptide chain may be identified with \documentclass{article}\pagestyle{empty}\begin{document}$ \widehat{\rm \alpha } $\end{document}MA . Methods for deriving corresponding tensors for other peptide units are indicated and the traceless polarizability tensor \documentclass{article}\pagestyle{empty}\begin{document}$ \widehat{\rm \alpha } $\end{document} for a polypeptide chain in any specified configuration is formulated.     

Sequence and age of eruption of deciduous dentition in the baboon (Papio sp)

A detailed eruption sequence and associated age of eruption for deciduous dentition in baboons (Papio sp) are presented in this paper. The sequence was determined by evaluation and comparison of the number and kinds of teeth present in nine age cohorts comprising the study sample of 88 males and 87 females who ranged in age from birth to 763 days. Eruption was assessed visually as present or absent. Several statistical methods used to derive the ages associated with the eruption sequence are described. The basic eruption sequence in the sample population is: i¹ i₁, i₂, i², \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm c}\limits_{\rm -} {\rm,}\mathop {\rm c}\limits^{\rm -} $\end{document} m¹ (m², m₂), M₁, M¹. Both sexes show the same pattern, with the exception of the second deciduous molar, where males show a sequence of m₂, m₂, while females show the opposite. Posterior dentition shows the greatest gender-specific variation in average age of eruption.     

Patterns of tooth size variability in the dentition of primates

Published data on tooth size in 48 species of non-human primates have been analyzed to determine patterns of variability in the primate dentition. Average coefficients of variation calculated for all species, with males and females combined, are greatest for teeth in the canine region. Incisors tend to be somewhat less variable, and cheek teeth are the least variable. Removing the effect of sexual dimorphism, by pooling coefficients of variation calculated for males and females separately, reduces canine variability but does not alter the basic pattern. Ontogenetic development and position in functional fields have been advanced to explain patterns of variability in the dentition, but neither of these appears to correlate well with patterns documented here. We tentatively suggest another explanation. Variability is inversely proportional to occlusal complexity of the teeth. This suggests that occlusal complexity places an important constraint on relative variability within the dentition. Even when the intensity of natural selection is equal at all tooth positions, teeth with complex occlusal patterns must still be less variable than those with simple occlusion in order to function equally well. Hence variability itself cannot be used to estimate the relative intensity of selection. Low variability of the central cheek teeth ( \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm M}\frac{1}{1} $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm M}\frac{2}{2} $\end{document}) makes them uniquely important for estimating body size in small samples, and for distinguishing closely related species in the fossil record.     

Polarized fluorescence in an electric field. A model calculation with application to the geometry of the 2-hydroxy-4,4′-diamidinostilbene–DNA complex

Theoretical expressions are derived for the change in the polarized components of the fluorescence, resulting from the orientation of a rigid molecule bearing a chromophore with arbitrary angles for the absorption and transition moments \documentclass{article}\pagestyle{empty}\begin{document}$ \vec \mu _a $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ \vec \mu _e $\end{document} with respect to the molecular axis. The break in the symmetry relation H_V = V_H is related to the tilt angle between \documentclass{article}\pagestyle{empty}\begin{document}$ \vec \mu _a $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ \vec \mu _e $\end{document}. The theory is applied to a sonicated DNA–2-hydroxy-4,4′-diamidinostilbene complex, in the blue and red emission bands of this peculiar dye. Simultaneous measurements of linear dichroism and fluorescence lead to the determination of an angle of 47° between a fluorescent bound dye and the DNA axis, with no difference for the blue- and red-emitting species, but confirm the presence of nonfluorescent bound dye in a more perpendicular arrangement.     

Topography of nucleic acid helices in solutions. II. Structure of the double-stranded rA-rU, rI-rC, acid rA, and the triple-stranded rA-rU2 and rA-rI2 helices

Information concerning the structures of rA–rU, rA–rU₂ rI–rC, rA–rI₂, and acid rA helices in solutions is reported. Through the use of diquaternary ammonium salts of the general structure, \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm R}_1 {\rm R}_2 {\rm R}_3 \mathop {\rm N}\limits^ + ({\rm CH}_2 )n\mathop {\rm N}\limits^ + {\rm R}_1 {\rm R}_2 {\rm R}_3 \cdot 2{\rm Br}^ - $\end{document} (I), it is shown that (1) the distances between adjacent negatively charged oxygen atoms on the helix increases in the following order rA–rI₂ < rI–rC < rA–rU ? rA–rU₂; (2) the density of the helices increases in the order. rA–rI₂ < rA–rU < rA–rU₂ < rI–rC; (3) there is a large hydrophobia site in rA–rI₂ and possibly also in rA–rU, rA–rU₂, and rI–rC helices; (4) the results of the interactions between the salts of type I and the helices may be formulated in semi-quantitative terms by the use of two parameters, α, and β which are shown to be related to the charge separation and the density of the helices, respectively; (5) the studies in solutions compare favorably with the x-ray studies on the fibers; and (6) the acid rA helix differs significantly from the other helices by the fact that the electrostatic interstrand interactions between the negatively charged oxygen atom of a phosphate group and the positively charged 10-amino group of adenine contribute significantly to the stabilization of the helix, and thus it is found that the presence of the salts, I, leads to a significant destabilization of the acid rA helix.     

Viscosity of heparin as a function of dielectric constant and of desulfation

The effect of dielectric constant (D) of the solvent on the viscosity of heparin was examined using the relation \documentclass{article}\pagestyle{empty}\begin{document}$ \eta _{{\rm sp}} /c = [\eta ]_\infty (1 + k/\sqrt c) $\end{document}, where [η]_∞ is the shielded intrinsic viscosity obtained by extrapolating \documentclass{article}\pagestyle{empty}\begin{document}$ \eta _{{\rm sp}} /c\,{\rm vs}{\rm . }\,1/\sqrt c ) $\end{document} to infinite concentration, and k is an interaction parameter independent of the dielectric constant of the solvent. This equation was previously reported by the authors⁹ for describing the reduced viscosities of strong polyelectrolytes in salt-free polar solvents. It was found that the [η]_∞ of heparin increases linearly with increasing dielectric constant of the solvent whereas the k values were, within experimental error, independent of D in the range 54.7 < D < 93.2 examined. Graded hydrolysis of heparin from its acid form (heparinic acid) at 57°C resulted in samples of varying degree of desulfation with corresponding decrease in biological activity. It was found that both [η]_∞ and k decrease with increasing desulfation.     

Peptaibol antibiotics: Conformational preferences of synthetic emerimicin fragments

The ir absorption and CD conformational analyses of solutions of the protected 2–9 fragment of the peptaibol antibiotics emerimicins III and IV \documentclass{article}\pagestyle{empty}\begin{document}$ \rlap{--} (Aib_3 \rlap{--} )L - Val - Gly - L - Leu\rlap{--} (Aib_2 \rlap{--} ) $\end{document} and related short sequences are consistent with the presence of a right-handed α-helix for the octapeptide, while the tri-, tetra-, and pentapeptides adopt a 3₁₀-helix, either right- or left-handed, depending on the amino acid sequences. The structural preferences of solid-state \documentclass{article}\pagestyle{empty}\begin{document}$ Z\rlap{--} (Aib_3 \rlap{--} )L - Val - OMe $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ Z\rlap{--} (Aib_3 \rlap{--} )L - Val - Gly - OMe $\end{document} have been determined by x-ray diffraction. In accord with the solution data, incipient 3₁₀-helices, formed by two and three β-turns, have been found for the tetra- and pentapeptides, respectively. The tetrapeptide helix has the left-handed screw sense, while that of the pentapetide is right-handed, thus confirming the conclusions of the CD analysis of the solution.     

Modellierung der enzymatischen Cellobiosehydrolyse – Vergleich linearer und nichtlinearer Parameterbestimmung

Experimental kinetic data (initial rate and high conversion) on the hydrolysis of cellobiose by 1,4-β-glucosidace (Gliocladium sp.) have been analysed and a competitive inhibition by glucose has been proposed. The determination of kinetic parameters from integral data is based upon algorithms for non-linear optimization and numerical integration. The values of kinetic constants \documentclass{article}\pagestyle{empty}\begin{document}$(v_{\max } = 1.02\frac{{\mu {\rm M}_{{\rm glucose}} }}{{{\rm mg}_{{\rm protein}} \cdot \min }},K_M = 2.6{\rm mM/l, and }K_P = 1.2{\rm mM/l)}$\end{document} agree well with the initialrate results. An important distinction is the confidence limit of parameters. Linear regression analysis shows a virtual accuracy and can lead to wrong conclusions.     

Effect of inhibitor complex formation on the hydration properties of alpha-chymotrypsin. Changes induced in protein hydration by toxylation of the native enzyme

The effect of enzyme-inhibitor complex formation on the hydration properties of the macromolecular moiety was investigated on the model system of α-chymotrypsin and its Ser-195 tosyl derivative. The primary (A-shell) hydration of the native and modified enzyme was compared by sorption measurements. The secondary (B-shell) hydration water was investigated by differential scanning calorimetry. Tosylation is known to induce pronounced conformational changes in the chymotrypsin molecule. These structural modifications have the following effects on the hydration of the native enzyme. The water binding capacity of the protein surface is significantly increased, as shown by both the calorimetric and the sorption results. The amount of unfreezable water of primary hydration is increased by 50 mol H₂O/mol chymotrypsin. The heats (ΔH ) and entropies (ΔS ) of the interaction of water with chymotrypsin are strongly reduced in the modified enzyme. This effect is interpretable by a reduction of the H bonding potential of the protein surface. Parallel to this decrease in δH , the heats of fusion of the secondary hydration water (Q_fus) are significantly increased by tosylation (Q_fus = 256.2 ± 7.8 and 294.2 ± 4.8 J g^?1 H₂O for the native and the tosylated enzyme, respectively). This increase in Q_fus reflects an increase in the extent of H bonding in the B-shell hydration sphere. These changes in the hydration of the native enzyme, associated with the reaction: native chymotrypsin → tosylchymotrypsin, are interpreted by cooperative phase transitions of water molecules in the primary and secondary hydration water. One of these transitions was found to exhibit a significant, linear enthalpy–entropy compensation effect. The compensation temperature \documentclass{article}\pagestyle{empty}\begin{document}$ \hat{\beta} $\end{document} is 290.7 ± 2.8°K. This \documentclass{article}\pagestyle{empty}\begin{document}$ \hat{\beta} $\end{document} value agrees well with compensation temperatures reported in the literature for a series of biochemical reactions in aqueous solution (250–320° K). This agreement in \documentclass{article}\pagestyle{empty}\begin{document}$ \hat{\beta} $\end{document} may point to a common source of both compensation phenomena.     

Conformational properties of methionine homo-oligopeptides in solution

A conformational analysis was carried out in solution on a series of L -methionine oligomers having the general formula \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm BOC\rlap{--} (L - Met\rlap{--})}_n {\rm OMe (}n = 2 - 7)$\end{document}. We examined these oligopeptides in TFE, HFIP, EG, and mixed organic–water media. The critical size for helix formation was found to be seven residues in TFE, whereas the β-associated structure appears at the pentamer in EG and TFE–water (20 : 80, v/v). In HFIP, however, the oligomers exist essentially in an unordered conformation.     

Optimization of a stagewise biochemical reactor system with product feedback

A consecutive, first-order, irreversible, biochemical reaction, \documentclass{article}\pagestyle{empty}\begin{document}$ A{\textstyle{{k(\theta)} \over {{\rm Enzyme }1}}} \to B{\textstyle{{k(\theta)} \over {{\rm Enzyme 2}}}} \to C $\end{document}, taking place in a series of N reactors with product recycle is considered. A discrete version of the maximum principle is used to derive general equations necessary for maximizing the production of (1) the final product, C, by choosing the temperature or the pH value in each reactor, and (2) the intermediate product, B, by choosing the reactor volume. A numerical computation for a series of three reactors with recycle is illustrated. The effects of varying the recycle rates on the optimal state and decision variables are also presented.     

Equi-replicated balanced block designs generated by a balanced matrix

In this paper it is shown that if N= \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \sum \limits_{i = 1}^{S_h} $\end{document} c_ihN_ih, where c_ih are some non-negative integer numbers and N_ih are such incidence matrices that A_h = \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \sum \limits_{i = 1}^{S_h} $\end{document} i N_ih is a balanced matrix defined by SHAH (1959), for h = 1, 2,…, p, then a block design with an incidence matrix Ñ = [N, N,…,N] is an equi-replicated balanced block design. Here the balance of a block design is defined in terms of the matrix M₀ introduced by CALI?SKI (1971).     

A Measure of Plankton Community Similarity Utilizing Variable Weighting of Total Density and Taxonomic Proportionality

Four commonly used formulae for measuring percentage similarity (PS) of biological communities were tested for their usefulness in relating to two plankton community properties, species proportional differences and total density differences. The formula best combining species proportionality and total density in the expression of PS is new: where min (xi,yi) is the lesser percentage (doubly standardized) of a species in two samples X and Y and where 2 q, 2xi and 2yi are the total quantities of all species in samples 8,X and Y, where \documentclass{article}\pagestyle{empty}\begin{document}$ \sum\limits_i {z_i } ,\,\sum\limits_i {x_i } \,and\sum\limits_i {y_i } $\end{document} are the total quantities of all species in samples Z, X and Y, respectively. Sample 2 contains the highest density of all species in the set; \documentclass{article}\pagestyle{empty}\begin{document}$ \sum\limits_i {z_i \, > \,(\sum\limits_i {x_i ,\,} \sum\limits_i {y_i } )} $\end{document}. The new expression of PS is simple to use and has the additional advantage of offering the analyst an unlimited choice of weighting factors or importance values for proportionality of species content and total density. The method has been applied to data from Gravenhurst Bay (Ontario) and effectively demonstrates the consequences of phosphorus loading reductions for phytoplankton communities.     

Response to various shock loadings of the constant recycle-sludge-concentration activated-sludge process

The stability of the model of a completely mixed activated-sludge process holding the recycle sludge concentration, X_R, as a system constant subjected to pH, temperature, potassium cyanide, and phenol shock loading was investigated. Soft-drink bottling wastewater was used and maintained at 1000 mg/liter chemical oxygen demand (COD). The hydraulic ratio and recycle sludge concentration were maintained at 0.3 and 7000 mg/liter, respectively. An initial dilution rate of ¼ hr^?1 was maintained for pH and temperature shock loading, with ¼ and ? hr^?1 for KCN shock loading and ¼, ?, and \documentclass{article}\pagestyle{empty}\begin{document}$\frac{1}{16}$\end{document} hr^?1 for phenol shock loading. It was found that the present system could handle pH shock loading as low as 4.0 and as high as 10.4 without any serious disruption of biological solid concentration and filtrate COD. At pH 4.0 shock loading, filamentous organisms were predominant. Temperature shock loading could be handled from 23 to 36°C without any leakage of effluent filtrate COD. At 46°C temperature shock, a 14 hr period was required to recuperate to the new steady state and provided only 85% of COD removal efficiency. For KCN (50 mg/liger) and phenol (85 mg/liter) shock loading, the dilution rates should be lower than \documentclass{article}\pagestyle{empty}\begin{document}$\frac{1}{16}$\end{document} hr^?1 in order to shorten the transient period and improve the effluent quality. Biological kinetic constants included cell yield value, maximum growth rate, and the saturation constant, which was varied with the qualitative shock applied.     

Thermodynamic functions of biopolymer hydration. II. Enthalpy–entropy compensation in hydrophilic hydration processes

The thermodynamic functions of biopolymer hydration were investigated by multitemperature vapor pressure studies. Desorption measurements were performed that allowed determination of reversible isotherms in the hydration range of 0.1 to 0.3–0.5 g H₂O/g dry polymer. These isotherms are accessible to thermodynamic interpretation and are relevant to the interaction of water with biopolymers in their solution conformation. The results obtained on a series of different biopolymers (lysozyme, α-chymotrypsin, apo-lactoferrin, and desoxyribonucleic acid), show the following common features of interest: (1) The differential excess enthalpies (ΔH^e ) and entropies (ΔS^e ) are negative, and exhibit pronounced anomalies in a well-defined low-humidity range (approx. 0.1 g H₂O/g dry polymer). These initial extrema are interpretable by structural changes, induced in the native biopolymer structures by water removal below a critical degree of hydration. (2) The ΔH^e and ΔS^e terms exhibit statistically significant linear enthalpy–entropy compensation effects in all biopolymer–water systems investigated. The compensation temperatures \documentclass{article}\pagestyle{empty}\begin{document}$ \hat \beta = \overline {\Delta H} ^e /\overline {\Delta S} ^e $\end{document} are approximately identical for all biopolymers, ranging from 360 to 500 K. The compensation effects are attributable to phase transitions of water molecules between the bulk liquid and the inner-sphere hydration shell of native biopolymers. (3) The negative excess free energies (ΔG^e ) decrease monotonically with increasing water content and are close to zero at 0.3 to 0.5 g H₂O/g polymer. This result indicates that only transitions between the bulk liquid and the inner-sphere hydration shell are associated with significant net free energy effects. The outer-sphere hydration water is thermodynamically comparable to bulk water. The importance of the proportionality factor \documentclass{article}\pagestyle{empty}\begin{document}$ \hat \beta $\end{document} in the control of the free energy term is discussed.     

Unilateral tunicamycin sensitivity of gametogenesis in dioecious isogamous chlamydomonas species

Sex cell adhesion in isogamous chlamydomonads is caused by a complementarity between sex-specific mating type substances, glycoproteins anchored in the flagella membrane of (+) and (\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop - \limits^. $\end{document}) gametes. The systems of mating type substances are species-specific and condition, by their individuality, gametic incompatibility between species. The adhesion systems of several species share one common feature: the attainment of the agglutination capacity is sensitive to tunicamycin, but in one sex only. The effect is interpreted as due to the interference of tunicamycin with the synthesis of the mating type substances by blocking of their glycosylation in one but not in the other sex. It is postulated that the tunicamycin-sensitive gametic adhesiveness depends, within the mating-type-specific glycoprotein, on an N-glycosidically bound ligand of carbohydrate nature. A concept on the origin of sibling species by mutative modulations within the proper ligands of the glycoproteinaceous mating type substances is developed.     

Dynamics and acoustical radiation of Porichthys notatus' swim bladder system

The swimbladder system of the plainfin midshipman consists of a gas-filled bladder and two intrinsic sonic muscles which are attached to the bladder at opposite sides. An experimental and analytical study was conducted to define the physical characteristics of this dynamic system, and to relate these characteristics to radiated acoustical pressure pulses. Results indicate that the system has two degrees of freedom, being comprised of two inertial, stiffness and damping components; the first and second mode components of a 23.1-centimeter midshipman are 0.002 and 0.019 kg (inertial) 2130 and 106,000 newtons per meter (stiffness) and 0.25 and 0.10 (damping) respectively. This system is excited by the sonic muscle forcing function which equals \documentclass{article}\pagestyle{empty}\begin{document}$ 0.00236{\rm}\sin \frac{{2\pi {\rm t}}}{{0.0045{\rm}\sec}}{\rm newtons}. $\end{document} Two system frequency response peaks were observed; the first was 110 hertz, at the flat section next to the sonic muscle, and was very near the repetition frequency of the sonic muscle pulses; the second was 350 hertz, at the hemispherical section, which was the frequency of the acoustical pressure pulse. These phenomena describe a dynamical system closely “tuned” to its forcing function, and a system which is highly responsive to acoustical pressure pulses radiated by neighboring midshipmen. The acoustical pressure pulse coincides in wave form with the hemispherical bladder wall acceleration.     

Bone remodeling rates: A test of an algorithm for estimating missing osteons

Frost (1987a) proposed an algorithm for estimating the number of missing osteons that correspond to observed osteon population densities (OPD). Such an algorithm should allow more accurate estimates of bone remodeling rates for skeletal remains for which in vivo labeling is not possible. In order to validate the algorithm, it was tested on an autopsy sample of 44 ribs. Estimates of activation frequency. ($ \mathop {\rm \mu }\limits^ - $_RC) and bone remodeling rate (V_f,r,t) using the new algorithm are in reasonable agreement with age-matched tetracycline-based values. Although mean values for activation frequencies ($ \mathop {\rm \mu }\limits^ - $_RC) and bone formation rate (V_f,r,t) generated by the algorithm were generally lower, they fell below 1 standard error for only an age category that included all ages above the 5th decade. It is now appropriate to apply the algorithm to archaeological skeletal remains. © 1994 Wiley-Liss, Inc.     

Spectrophotometric,potentiometric, and density gradient ultracentrifugation studies of the binding of silver ion by DNA

The equilibrium and the stoichiometry for the reversible complexing of silver ion by DNA have been studied by potentiometric titrations, proton release pH-stat titrations, and by spectrophotometry. The complexing reactions involve primarily the purine and pyrimidine residues, not the phosphate groups. There are at least three types of binding (types I, II, and III), of which the first two have been intensively studied in this work. The sum of type I and type II binding saturates at one silver atom per two nucleotide residues. In the type I and type II reactions, zero and one proton, respectively, are displaced per silver ion bound. At pH 5.6, the reactions occur stepwise, type I being first, while at pH 8.0, they occur simultaneously. The silver ion binding curve is very sharp at pH 8, indicating a cooperative reaction. The strength of the binding increases with increasing GC content. Type I binding is more important for GC-rich DNA's than for GC-poor ones. Denatured DNA binds more strongly than does native DNA. The silver ion complexing reaction is chemically and biologically reversible. We propose that type II binding essentially involves the conversion of an \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm N} - {\rm H} \cdots {\rm N} $\end{document} hydrogen bond of a complementary base pair to an N—Ag—N bond. The nature of type I binding is less clear, but it may involve a π interaction with stacked bases. The buoyant density (ρ₀) of DNA in a Cs₂SO₄ density gradient increases when the DNA reacts with silver ion. The buoyant density change is about 0.15 g./ml. for 0.5 silver bound per nucleotide. The large buoyant density changes and the selective nature of the complexing reaction make it possible to perform good separations between native and denatured DNA or between GC-rich and GC-poor native DNA's by density gradient centrifugation.