Effect of denaturants on the stability of light-harvesting complex

 The effect of denaturants such as urea and normal alcohols on the formation of light-harvesting (LH) polypeptides/bacteriochlorophyll a (BChla) complex (LH1 complex) in n-octyl-β-D-glucopyranoside (OG) micelle was examined to provide an insight into stability of the complex. The stabilities of the LH1 complex in OG micelle and of the complex in the chromatophore of photosynthetic bacteria were compared by addition of denaturants. The extent of stability of these complexes was monitored by the change in absorbance of Qy band of BChla in these complexes, resulting generally in the blue-shifting of the Qy band from near 870 nm to about 777 nm upon addition of these denaturants. Urea and guanidium hydrochloride (Gnd) showed a relatively weak denaturing effect. Normal alcohols showed stronger denaturing effect, depending on the hydrophobicity of the alcohols. These results imply that the stability of LH1 complex in OG micelle can be largely attributed to the hydrophobic interactions in the complex as well as that of the complex in the chromatophore of photosynthetic bacteria. Received: 23 May 1997 Accepted: 13 September 1997     

Probing the effect of the binding site on the electrostatic behavior of a series of carotenoids reconstituted into the light-harvesting 1 complex from purple photosynthetic bacterium Rhodospirillum rubrum detected by stark spectroscopy

Reconstitutions of the LH1 complexes from the purple photosynthetic bacterium Rhodospirillum rubrum S1 were performed with a range of carotenoid molecules having different numbers of C=C conjugated double bonds. Since, as we showed previously, some of the added carotenoids tended to aggregate and then to remain with the reconstituted LH1 complexes (Nakagawa, K.; Suzuki, S.; Fujii, R.; Gardiner, A.T.; Cogdell, R.J.; Nango, M.; Hashimoto, H. Photosynth. Res. 2008, 95, 339-344), a further purification step using a sucrose density gradient centrifugation was introduced to improve purity of the final reconstituted sample. The measured absorption, fluorescence-excitation, and Stark spectra of the LH1 complex reconstituted with spirilloxanthin were identical with those obtained with the native, spirilloxanthin-containing, LH1 complex of Rs. rubrum S1. This shows that the electrostatic environments surrounding the carotenoid and bacteriochlorophyll a (BChl a) molecules in both of these LH1 complexes were essentially the same. In the LH1 complexes reconstituted with either rhodopin or spheroidene, however, the wavelength maximum at the BChl a Qy absorption band was slightly different to that of the native LH1 complexes. These differences in the transition energy of the BChl a Qy absorption band can be explained using the values of the nonlinear optical parameters of this absorption band, i.e., the polarizability change Tr(Deltaalpha) and the static dipole-moment change |Deltamu| upon photoexcitation, as determined using Stark spectroscopy. The local electric field around the BChl a in the native LH1 complex (ES) was determined to be approximately 3.0x10(6) V/cm. Furthermore, on the basis of the values of the nonlinear optical parameters of the carotenoids in the reconstituted LH1 complexes, it is possible to suggest that the conformations of carotenoids, anhydrorhodovibrin and spheroidene, in the LH1 complex were similar to that of rhodopin glucoside in crystal structure of the LH2 complex from Rhodopseudomonas acidophila 10050.     

Probing the electronic structure and conformational flexibility of individual light-harvesting 3 complexes by optical single-molecule spectroscopy

We present fluorescence-excitation spectra of individual light-harvesting 3 (LH3 or B800-820) complexes of Rhodopseudomonas acidophila at 1.2 K. The optical single-molecule studies were employed to investigate the electronic structure as well as the conformational flexibility of the individual pigment-protein complexes. The optical spectra resemble those of individual light-harvesting 2 (LH2) complexes, in agreement with the structural similarity of both types of complexes. Although variations among the LH3 spectra are large, there is a distinct difference in the spectral features of the 800 and 820 nm region that appears in all the complexes studied. In the B800 region 4-6 narrow bands are present whereas in the B820 region a limited number of relatively broad bands are observed. These observations can generally be interpreted in terms of localized excitations in the 800 nm region and delocalized excitations in the 820 nm region. The observed heterogeneous spectral behavior, especially in the B820 band, indicates that the B820 pigments of LH3 are sensitive to light-induced local conformational changes. It is suggested that a rotation of the C(3)-acetyl chain of a BChl a pigment bound to the beta-subunit of the light-harvesting complex is the origin of the conformational flexibility and affects the optical properties of the whole pigment-protein complex.     

Directed formation of micro- and nanoscale patterns of functional light-harvesting LH2 complexes

The precision placement of the desired protein components on a suitable substrate is an essential prelude to any hybrid "biochip" device, but a second and equally important condition must also be met: the retention of full biological activity. Here we demonstrate the selective binding of an optically active membrane protein, the light-harvesting LH2 complex from Rhodobacter sphaeroides, to patterned self-assembled monolayers at the micron scale and the fabrication of nanometer-scale patterns of these molecules using near-field photolithographic methods. In contrast to plasma proteins, which are reversibly adsorbed on many surfaces, the LH2 complex is readily patterned simply by spatial control of surface polarity. Near-field photolithography has yielded rows of light-harvesting complexes only 98 nm wide. Retention of the native optical properties of patterned LH2 molecules was demonstrated using in situ fluorescence emission spectroscopy.     

Time-dependent complex formation of dendritic poly(L-lysine) with plasmid DNA and correlation with in vitro transfection efficiencies

Dendritic poly(L-lysine) of the 6th generation shows high transfection efficiency into several cultivated cells with low cytotoxicity. In order to understand the mechanism of complex formation with plasmid DNA, the complex was observed using atomic force microscopy. After mixing for 15 min, 1-2 microns assemblies of complexes composed of several small particles (50-200 nm) were observed. At the same time, individual small complexes of 50 to 500 nm were observed on a mica surface. After incubation for 2 h, only the large complexes were found on the mica surface. As a result of further dynamic light scattering analysis and measurement of the transfection efficiency at different time points, the transfection efficiency of KG6 was found to increase with increasing size of the DNA-complexes. This result indicates that large complexes of more than 1 micron are major species that contribute to transfection in vitro.     

Characterization and deposition of various light-harvesting antenna complexes by electrospray atomization

Photosynthetic organisms have light-harvesting complexes that absorb and transfer energy efficiently to reaction centers. Light-harvesting complexes (LHCs) have received increased attention in order to understand the natural photosynthetic process and also to utilize their unique properties in fabricating efficient artificial and bio-hybrid devices to capture solar energy. In this work, LHCs with different architectures, sizes, and absorption spectra, such as chlorosomes, Fenna–Matthews–Olson (FMO) protein, LH2 complex, and phycobilisome have been characterized by an electrospray-scanning mobility particle-sizer system (ES-SMPS). The size measured by ES-SMPS for FMO, chlorosomes, LH2, and phycobilisome were 6.4, 23.3, 9.5, and 33.4?nm, respectively. These size measurements were compared with values measured by dynamic light scattering and those reported in the literature. These complexes were deposited onto a transparent substrate by electrospray deposition. Absorption and fluorescence spectra of the deposited LHCs were measured. It was observed that the LHCs have light absorption and fluorescence spectra similar to that in solution, demonstrating the viability of the process.     

Explaining the visible and near-infrared circular dichroism spectra of light-harvesting 1 complexes from purple bacteria: a modeling study

In this work, we investigate the origin and characteristics of the circular dichroism (CD) spectrum of various light-harvesting 1 (LH1) complexes. The near-infrared (NIR) CD signal of these core antennae is strongly nonconservative, and the nature of this nonconservativity is under examination in this paper. So far, on the basis of the high-resolution structures of LH2, we have been able to model the absorption and CD spectra in the bacteriochlorophyll (BChl) Q(Y) and Q(X) regions of LH2 (Georgakopoulou et al., Biophys. J. 2002, 82, 2184-2197), as well as in the carotenoid region (Georgakopoulou et al., Biophys. J. 2004, 87, 3010-3022). We proceed by applying the same modeling method in order to reproduce the LH1 spectra. We assume a ring of dimers in a perfect circular arrangement with 16-fold symmetry, and account for all excitonic interactions within the ring. Because LH1 complexes exhibit Q(Y) and Q(X) CD signals of very low intensity, higher transitions can easily affect these regions. Therefore, we expand the model and take into account also the Soret and carotenoid transitions. We can now understand the shape of the absorption and CD spectra and contemplate the structure of the LH1 complex. The latter is similar to LH2 in that it is a very symmetric ring dominated by excitonic interactions. The larger number of symmetry and the bigger diameter of LH1, combined with small rotations of the BChl transition dipole moments, are responsible for the display of CD signals that are very low in intensity. The interaction of the Q(Y) with the carotenoid transitions results in complete loss of the conservativity. Interaction energies between all the pigments in the ring are calculated, and their values are in good accordance with what is reported in the literature.     

Lateral organization of a membrane protein in a supported binary lipid domain: direct observation of the organization of bacterial light-harvesting complex 2 by total internal reflection fluorescence microscopy

A unique method is described for directly observing the lateral organization of a membrane protein (bacterial light-harvesting complex LH2) in a supported lipid bilayer using total internal reflection fluorescence (TIRF) microscopy. The supported lipid bilayer consisted of anionic 1,2-dioleoyl-sn-glycero-3-[phospho-rac-(1'-glycerol)] (DOPG) and 1,2-distearoly-sn-3-[phospho-rac-(1'-glycerol)] (DSPG) and was formed through the rupture of a giant vesicle on a positively charged coverslip. TIRF microscopy revealed that the bilayer was composed of phase-separated domains. When a suspension of cationic phospholipid (1,2-dioleoyl-sn-glycero-3-ethylphosphocholine: EDOPC) vesicles (approximately 400 nm in diameter), containing LH2 complexes (EDOPC/LH2 = 1000/1), was put into contact with the supported lipid bilayer, the cationic vesicles immediately began to fuse and did so specifically with the fluid phase (DOPG-rich domain) of the supported bilayer. Fluorescence from the incorporated LH2 complexes gradually (over approximately 20 min) spread from the domain boundary into the gel domain (DSPG-rich domain). Similar diffusion into the domain-structured supported lipid membrane was observed when the fluorescent lipid (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-lissamine-rhodamine B sulfonyl: N-Rh-DOPE) was incorporated into the vesicles instead of LH2. These results indicate that vesicles containing LH2 and lipids preferentially fuse with the fluid domain, after which they laterally diffuse into the gel domain. This report describes for first time the lateral organization of a membrane protein, LH2, via vesicle fusion and subsequent lateral diffusion of the LH2 from the fluid to the gel domains in the supported lipid bilayer. The biological implications and applications of the present study are briefly discussed.     

Function of membrane protein in silica nanopores: incorporation of photosynthetic light-harvesting protein LH2 into FSM

A high amount of functional membrane protein complex was introduced into a folded-sheet silica mesoporous material (FSM) that has nanometer-size pores of honeycomb-like hexagonal cylindrical structure inside. The photosynthetic light-harvesting complex LH2, which is a typical membrane protein, has a cylindrical structure of 7.3 nm diameter and contains 27 bacteriochlorophyll a and nine carotenoid molecules. The complex captures light energy in the anoxygenic thermophilic purple photosynthetic bacterium Thermochromatium tepidum. The amount of LH2 adsorbed to FSM was determined optically and by the adsorption isotherms of N2. The FSM compounds with internal pore diameters of 7.9 and 2.7 nm adsorbed LH2 at 1.11 and 0.24 mg/mg FSM, respectively, suggesting the high specific affinity of LH2 to the interior of the hydrophobic nanopores with a diameter of 7.9 nm. The LH2 adsorbed to FSM showed almost intact absorption bands of bacteriochlorophylls, and was fully active in the capture and transfer of excitation energy. The LH2 complex inside the FSM showed increased heat stability of the exciton-type absorption band of bacteriochlorophylls (B850), suggesting higher circular symmetry. The environment inside the hydrophobic silica nanopores can be a new matrix for the membrane proteins to reveal their functions. The silica-membrane protein adduct will be useful for the construction of new probes and reaction systems.     

Biohybrid photosynthetic antenna complexes for enhanced light-harvesting

Biohybrid antenna systems have been constructed that contain synthetic chromophores attached to 31mer analogues of the bacterial photosynthetic core light-harvesting (LH1) β-polypeptide. The peptides are engineered with a Cys site for bioconjugation with maleimide-terminated chromophores, which include synthetic bacteriochlorins (BC1, BC2) with strong near-infrared absorption and commercial dyes Oregon green (OGR) and rhodamine red (RR) with strong absorption in the blue-green to yellow-orange regions. The peptides place the Cys 14 (or 6) residues before a native His site that binds bacteriochlorophyll a (BChl-a) and, like the native LH proteins, have high helical content as probed by single-reflection IR spectroscopy. The His residue associates with BChl-a as in the native LH1 β-polypeptide to form dimeric ββ-subunit complexes [31mer(-14Cys)X/BChl](2), where X is one of the synthetic chromophores. The native-like BChl-a dimer has Q(y) absorption at 820 nm and serves as the acceptor for energy from light absorbed by the appended synthetic chromophore. The energy-transfer characteristics of biohybrid complexes have been characterized by steady-state and time-resolved fluorescence and absorption measurements. The quantum yields of energy transfer from a synthetic chromophore located 14 residues from the BChl-coordinating His site are as follows: OGR (0.30) < RR (0.60) < BC2 (0.90). Oligomeric assemblies of the subunit complexes [31mer(-14Cys)X/BChl](n) are accompanied by a bathochromic shift of the Q(y) absorption of the BChl-a oligomer as far as the 850-nm position found in cyclic native photosynthetic LH2 complexes. Room-temperature stabilized oligomeric biohybrids have energy-transfer quantum yields comparable to those of the dimeric subunit complexes as follows: OGR (0.20) < RR (0.80) < BC1 (0.90). Thus, the new biohybrid antennas retain the energy-transfer and self-assembly characteristics of the native antenna complexes, offer enhanced coverage of the solar spectrum, and illustrate a versatile paradigm for the construction of artificial LH systems.     

Long range excitonic transport in a biomimetic system inspired by the bacterial light-harvesting apparatus

Photosynthesis, the process by which energy from sunlight drives cellular metabolism, relies on a unique organization of light-harvesting and reaction center complexes. Recently, the organization of light-harvesting LH2 complexes and dimeric reaction center-light-harvesting I-PufX core complexes in membranes of purple non-sulfur bacteria was revealed by atomic force microscopy [S. Bahatyrova et al., Nature (London) 430, 1058 (2004)]. Here, we discuss optimal exciton transfer in a biomimetic system closely modeled on the structure of LH2 and its organization within the membrane using a Markovian quantum model with dissipation and trapping added phenomenologically. In a deliberate manner, we neglect the high level detail of the bacterial light-harvesting complex and its interaction with the phonon bath in order to elucidate a set of design principles that may be incorporated in artificial pigment-scaffold constructs in a supramolecular assembly. We show that our scheme reproduces many of the most salient features found in their natural counterpart and may be largely explained by simple electrostatic considerations. Most importantly, we show that quantum effects act primarily to enforce robustness with respect to spatial and spectral disorder between and within complexes. The implications of such an arrangement are discussed in the context of biomimetic photosynthetic analogs capable of transferring energy efficiently across tens to hundreds of nanometers.     

Resonance Raman and surface-enhanced resonance Raman spectra of LH2 antenna complex from Rhodobacter sphaeroides and Ectothiorhodospira sp. excited in the Qx and Qy transitions

Well-resolved vibrational spectra of LH2 complex isolated from two photosynthetic bacteria, Rhodobacter sphaeroides and Ectothiorhodospira sp., were obtained using surface-enhanced resonance Raman scattering (SERRS) exciting into the Qx and the Qy transitions of bacteriochlorophyll a. High-quality SERRS spectra in the Qy region were accessible because the strong fluorescence background was quenched near the roughened Ag surface. A comparison of the spectra obtained with 590 nm and 752 nm excitation in the mid- and low-frequency regions revealed spectral differences between the two LH2 complexes as well as between the LH2 complexes and isolated bacteriochlorophyll a. Because peripheral modes of pigments contribute mainly to the low-frequency spectral region, frequencies and intensities of many vibrational bands in this region are affected by interactions with the protein. The results demonstrate that the microenvironment surrounding the pigments within the two LH2 complexes is somewhat different, despite the fact that the complexes exhibit similar electronic absorption spectra. These differences are most probably due to specific pigment-pigment and pigment-protein interactions within the LH2 complexes, and the approach might be useful for addressing subtle static and dynamic structural variances between pigment-protein complexes from different sources or in complexes altered chemically or genetically.     

Molecular model of LH1 light-harvesting complex of the photosynthetic center of Thermochromatium tepidum bacteria

A three-dimensional all-atom structure of the LH1 light-harvesting complex of the photosynthetic center of the Thermochromatium tepidum bacteria was constructed with molecular modeling techniques.     

Sizes and Conformation of the Molecules of DNA–Surfactant Complexes in Dilute Solutions and on the Atomic Smooth Substrates

The conformations of the molecules of DNA–surfactant complexes in dilute solutions and on the atomic smooth surfaces of mica and highly oriented pyrolytic graphite were comparatively studied by the methods of isothermal diffusion, electric birefringence, and atomic force microscopy. The DNA–surfactant complexes were deposited onto the substrates from a chloroform solution. The number of particles of the DNA–surfactant complex on the substrate was changed by varying the concentration of the initial solution within three orders of magnitude. The particles of a shape close to ellipsoidal, 25–70 nm in diameter and 2–4 nm high, were observed at the lowest concentration of DNA–surfactant solution on the mica substrate. The shape and size of these particles correspond to those of a single DNA–surfactant complex, calculated from its translational diffusion coefficient and the time of orientational relaxation in dilute solutions. An increase in the number of molecules deposited onto the substrate leads to an increase in the characteristic sizes of DNA–surfactant complex particles observed by the atomic force microscopy. This may be associated with the aggregation of DNA–surfactant complexes.     

Hexameric macroring of gable-porphyrins as a light-harvesting antenna mimic

Construction of a self-assembled supramolecular macroring that has distances and orientations of porphyrin dimer units in close analogy to those of the natural light-harvesting complexes was achieved. In natural light-harvesting complexes, bacteriochlorophyll-a's are arranged in macroring structures by coordination from imidazolyl side chains. A structural determination of a light-harvesting antenna complex (LH2) elucidated the arrangement of 18 bacteriochlorophyll-a's in a slipped-cofacial way with C9 symmetry in B850 in 1995. To obtain such an elegant macroring architecture as an artificial light-harvesting complex, we connected slipped-cofacial dimers of imidazolylporphyrins in a gable-porphyrin orientation. The introduction of zinc assembled by coordination porphyrins with originally a broad molecular weight distribution (MWD). When coordination bonds were cleaved and reorganized under high dilution conditions using chloroform/methanol solution, the MWD was perfectly converged. This crop gave particle images of a uniform height by atomic force microscopy measurements. Further purification was successfully achieved by gel permeation chromatography, and the first eluting component gave a diameter corresponding to the cyclic hexamer of gable-porphyrins from a small-angle X-ray scattering measurement with synchrotron radiation. In summary, porphyrin assemblies in a macroring arrangement were constructed using the gable-porphyrin motif, and their photophysical properties are highly interesting.     

Molecular assembly of artificial photosynthetic antenna core complex on an amino-terminated ITO electrode

Bacterial photosynthetic membrane proteins, light-harvesting antenna complex (LH1), reaction center (RC), and their combined ‘core’ complex (LH1–RC) are functional elements in the primary photosynthetic events, i.e., capturing and transferring light energy and subsequent charge separation. These photosynthetic units (PSUs) isolated from Rhodospirillum rubrum (Rs. rubrum) were assembled onto an ITO electrode modified with 3-aminopropyltriethoxysilane (APS–ITO). The near IR absorption spectra of PSUs on the assembled electrodes were identical to those of solutions, indicating that the LH1 and LH1–RC core complexes were native on the electrode. Photocurrent response of PSUs on the electrode was examined upon illumination of the LH1 complex at 880 nm. The LH1–RC and a mixed assembly of LH1 and RC exhibited photocurrent response, but not LH1 only, consistent with the function of these PSUs, capturing light energy and transferring electron. This result provides useful methodology for building an artificial fabrication of PSUs on the electrode.     

Stability of integral membrane proteins under high hydrostatic pressure: the LH2 and LH3 antenna pigment-protein complexes from photosynthetic bacteria

The bacteriochlorophyll a-containing LH2 and LH3 antenna complexes are the integral membrane proteins that catalyze the photosynthetic process in purple photosynthetic bacteria. The LH2 complex from Rhodobacter sphaeroides shows characteristic strong absorbance at 800 and 850 nm due to the pigment molecules confined in two separate areas of the protein. In the LH3 complex from Rhodopesudomonas acidophila the corresponding bands peak at 800 and 820 nm. Using the bacteriochlorophyll a cofactors as intrinsic probes to monitor local changes in the protein structure, we investigate spectral responses of the antenna complexes to very high hydrostatic pressures up to 2.5 GPa when embedded into natural membrane environment or extracted with detergent. We first demonstrate that high pressure does induce significant alterations to the tertiary structure of the proteins not only in proximity of the 800 nm-absorbing bacteriochlorophyll a molecules known previously (Gall, A.; et al. Biochemistry 2003, 42, 13019) but also of the 850 nm- and 820 nm-absorbing molecules, including breakage of the hydrogen bond they are involved in. The membrane-protected complexes appear more resilient to damaging effects of the compression compared with the complexes extracted into mixed detergent-buffer environment. Increased resistance of the isolated complexes is observed at high protein concentration resulting aggregation as well as when cosolvent (glycerol) is added into the solution. These stability variations correlate with ability of penetration of the surrounding polar solvent (water) into the hydrophobic protein interiors, being thus the principal reason of the pressure-induced denaturation of the proteins. Considerable variability of elastic properties of the isolated complexes was also observed, tentatively assigned to heterogeneous protein packing in detergent micelles. While a number of the isolated complexes release most of their bacteriochlorophyll a content under high pressure, quite some of them remain apparently intact. The pigmented photosynthetic antenna complexes thus constitute a suitable model system for studying in detail the stability of integral membrane proteins.     

Reconstitution of bacterial photosynthetic unit in a lipid bilayer studied by single-molecule spectroscopy at 5 K

As a model of photosynthetic unit (PSU), self-assembled aggregates of pigment-protein complexes from photosynthetic bacteria were prepared in a lipid bilayer by reconstitution of the light-harvesting 2 (LH2) complex and light-harvesting 1-reaction center (LH1-RC) complex through detergent removal of their micelles in the presence of lipids. By performing polarization-controlled fluorescence and fluorescence-excitation spectroscopy on single aggregates at a temperature of 5 K, the composition of individual aggregates was determined and excitation energy transfer (EET) between constituent complexes was observed. LH2 and LH1-RC from a bacterium, Rhodobacter (Rb.) sphaeroides, were found to form a trimeric aggregate in which EET takes place from one LH2 to two LH1-RCs. In contrast, a heterodimer of LH2 and LH1-RC in which EET works was found to assemble from a combination of complexes of different bacterial species, that is, LH2 from Rb. sphaeroides and LH1-RC from Rhodopseudomonas (Rps.) palustris.     

Ab inito study on triplet excitation energy transfer in photosynthetic light-harvesting complexes

We have studied the triplet energy transfer (TET) for photosynthetic light-harvesting complexes, the bacterial light-harvesting complex II (LH2) of Rhodospirillum molischianum and Rhodopseudomonas acidophila, and the peridinin-chlorophyll a protein (PCP) from Amphidinium carterae. The electronic coupling factor was calculated with the recently developed fragment spin difference scheme (You and Hsu, J. Chem. Phys. 2010, 133, 074105), which is a general computational scheme that yields the overall coupling under the Hamiltonian employed. The TET rates were estimated based on the couplings obtained. For all light-harvesting complexes studied, there exist nanosecond triplet energy transfer from the chlorophylls to the carotenoids. This result supports a direct triplet quenching mechanism for the photoprotection function of carotenoids. The TET rates are similar for a broad range of carotenoid triplet state energy, which implies a general and robust TET quenching role for carotenoids in photosynthesis. This result is also consistent with the weak dependence of TET kinetics on the type or the number of π conjugation lengths in the carotenoids and their analogues reported in the literature. We have also explored the possibility of forming triplet excitons in these complexes. In B850 of LH2 or the peridinin cluster in PCP, it is unlikely to have triplet exciton since the energy differences of any two neighboring molecules are likely to be much larger than their TET couplings. Our results provide theoretical limits to the possible photophysics in the light-harvesting complexes.     

Two-dimensional densely packed DNA nanostructure derived from DNA complexation with a low-generation poly(amidoamine) dendrimer

One of the keys for using deoxyribonucleic acid (DNA) as a nanomaterial relies on how the individual DNA chain can be aligned and how a multitude of DNA chains can be packed into ordered nanostructures. Here we present a simple method for constructing a 2-D densely packed DNA nanostructure using the electrostatic complex of DNA with a poly(amidoamine) (PAMAM) dendrimer of generation two. Ordered DNA arrays are formed by drop-casting an aqueous solution containing positively overcharged complexes onto mica followed by a prolonged incubation. During the incubation, the complexes tend to adsorb onto the negatively charged mica surface through electrostatic attraction. The rodlike complexes organize to form ordered arrays to increase the surface density of the adsorbed complexes and hence the attractive free energy of adsorption. The densely packed nanostructure obtained here is distinguished from the previously reported spheroid or toroid structure derived from DNA complexations with the higher-generation dendrimers.