Through comparison of FTIR and NMR spectra, respectively, one can qual- itatively and quantitatively observe the effects of methylation and ﬂuorina- tion on DNA structure and dynamics. Preliminary results demonstrate that these different covalent modiﬁcations of the cytosine base affect the struc- ture and dynamics in different ways. Both modiﬁcations alter the BI/BII ra- tio but ﬂuorination does not appear to affect the dynamics as signiﬁcantly as methylation. We attribute the enhanced quenching of backbone dynamics by methylation to the formation of a strong dipolar interaction between the neg- atively charged backbone oxygen with the hydrogens of the methyl group whose proximity to the backbone is closer in BII than BI. Comparisons of ﬂuorination and methylation effects on sequences containing both the EcoRI and Cre binding sites, as well as their modulation due to sequence context, will be presented. 1778-Pos Board B622 Enthalpy-Entropy Contribution to Carcinogen-induced DNA Conforma- tional Heterogeneity Bongsup Cho, Fengting Liang. Univ of Rhode Island, Kingston, RI, USA. Aromatic amines are among the most notorious chemicals in the environ- ment. The formation of DNA adducts is thought to be a hallmark for the initiation of chemical carcinogenesis. Aromatic amine-DNA adducts are known to exist in a sequence-dependent equilibrium of the major groove B-type (B) and base-displaced stacked (S) conformations. We have con- ducted extensive calorimetry/NMR studies on the model lesions FAF and FABP in order to understand how thermodynamics inﬂuence the nature of S/B-conformational heterogeneity and subsequent molecular interactions with polymerases and repair proteins. Results indicated large differences in enthalpy-entropy compensations for FABP and FAF. The small and ﬂex- ible FABP exclusively adopts the less perturbed B-conformer, thus resulting in small enthalpy/entropy change. This is in contrast to FAF, which stacks better and exists as a mixture of B- and S-conformers, thus contributing to large enthalpy/entropy compensation. The results indicate that it is not just the stacking argument, but also the favorable entropy of the S-conformer over B-conformer that determines the S/B-conformational heterogeneity at an ambient temperature. 1779-Pos Board B623 Scaling Behavior of Single Stranded DNA Measured by Small Angle X-ray Scattering Adelene Y.L. Sim, Jan Lipfert, Daniel Herschlag, Sebastian Doniach. Stanford University, Stanford, CA, USA. Polyelectrolytes, or charged polymers, are prevalent in biological systems, yet their physical properties are far less well understood than those of neutral poly- mers. We report on measurements using small angle x-ray scattering to study bulk ensemble-averaged properties of small (up to 100 bases) poly-deoxythy- mine (poly-dT) and poly-deoxyadenine (poly-dA) molecules. By studying homomeric single stranded DNA (ssDNA), we can observe their polymeric properties without interference from secondary structure formation. This gives us insight to the conformational space explored by single stranded nucleic acids in folding processes, and the nucleotide dependence of loop ﬂexibility of DNA and RNA junctions. We observe, as is consistent with base-stacking of purines, that poly-dA is stiffer than poly-dT. For poly-dT, the radius of gyration (R g ) scales with the number of monomers with a Flory exponent (n) which decreases slowly with increasing salt, but drops sharply below that expected for a self- avoiding random walk (SAW) polymer (n ~ 0.588) with more than 500 mM of added sodium acetate. This is perhaps due to the condensation of charges around the DNA and/or the change in solvent quality with added salt. The ratio (r) of the square of the maximum pair-wise distance (D max ) to R g ﬂuctuates around 10, suggesting that ssDNA compacts locally. Assuming that D max is a fair estimate of the end-to-end distance of the polymer, a value of 12 is expected for r in the case of a rod, and about 6.3 for a SAW polymer. This lo- calized clustering is consistent with the electrostatic blob model of de Gennes et al. The persistence length of poly-dT - determined by ﬁtting the data to a worm-like chain model - increases in a sublinear fashion with increasing Debye screening length, unlike the behaviors predicted by polyelectrolyte theories. 1780-Pos Board B624 Macrosolute Effects on Nucleic Acid Interactions Sarai Obando, Jennifer L. Small, Chris R. MacKay, Eric D. Nellis, Karina L. Vivar, Steven J. Metallo. Georgetown University, Washington, DC, USA. The intracellular environment contains a variety of solutes that cumulatively occupy a signiﬁcant volume of the cell (20-30%). The high volume occupancy generates a system which is macromolecularly crowded. This crowding, also known as the excluded volume effect, can lead to an increase in the chemical activity of solutes and inﬂuence thermodynamic and kinetic values as com- pared to a dilute system. Using synthetic, inert cosolutes to provide a simpliﬁed mimic of the crowding in the intracellular environment, DNA structures were studied. We demonstrate that crowding can lead to the differential stabilization of a complementary DNA duplex over duplexes containing a single mis- matched base pair, effectively increasing the speciﬁcity of the hybridization re- action. In systems with molecularities ranging from one to four we demonstrate that as the molecularity of a system increases the crowding effects also increase. An increase in Tm of up to 12  C was noted for a multi-branch DNA structure with four arms. Crowding mediated enhancement of the rate of hybridization was found to be independent of sequence but dependent upon structure. 1781-Pos Board B625 The Interaction of Monovalent Cations with a Model DNA Hairpin Earle Stellwagen, Joseph Muse, Nancy C. Stellwagen. University of Iowa, Iowa City, IA, USA. Capillary electrophoresis was used to study the interactions of monovalent cat- ions with DNA hairpins, using as a model the 16 residue oligonucleotide ATCCTATTTTTAGGAT, which is known to form a stable hairpin with a 6 base pair stem and a 4 base loop. The unstructured 14 base oligonucleotide ACCTGATCACGTTA served as a reference analyte. All measurements were performed in the absence of Mg 2þ at pH 7.3 using diethylmalonate as the buffering anion. Increasing the concentration of Na þ in the buffer increased the melting temperature of the hairpin, as predicted by the mFOLD algorithm. Isothermal measurements at 20 o indicate that Na þ forms a saturable complex with the hairpin, with a K D of about 100 mM, but does not form a complex with the unstructured reference oligonucleotide. These measurements suggest that the increase in the melting temperature of the hairpin with increasing Na þ is due to the preferential binding of Na þ ions to the hairpin conformation. The cations Li þ ,K þ , Tris þ and tetramethylammonium þ (TMA þ ) bind equally well to the model hairpin and affect its melting temperature similarly. The tet- raethylammonium þ (TEA þ ) ion also binds equally well to the hairpin, but only to the extent of ~50% saturation. The tetrapropylammonium þ (TPA þ ) and tet- rabultylammonium þ (TBA þ ) ions bind to the hairpin very weakly if at all. Sur- prisingly, the melting temperature of the hairpin is systematically diminished as TMA þ is replaced in turn by TEA þ , TPA þ or TBA þ , suggesting that the larger tetraalkylammonium ions may destabilize the hairpin conformation by a through-solvent mechanism. 1782-Pos Board B626 Observation of Oligonucleotide Dynamics by means of Fluorescent Nucleoside analog 6MI Andrew T. Moreno, Joseph Knee, Ishita Mukerji. Wesleyan Univeristy, Middletown, CT, USA. To improve current understanding of the structural recognition mechanism of architectural DNA binding proteins such as HU and IHF, we are investigating the structure and dynamics of different DNA substrates. We are able to make these observations on both global and local levels by incorporating the ﬂuores- cent guanosine nucleoside analog 6-methylisoxanthopterin (6-MI), with H- bonds with cytosine similar to guanosine. We have previously shown this probe does not signiﬁcantly perturb the global structures of duplex DNA molecules. 6-MI was systematically incorporated into a 34 base oligonucleotide. Initial characterization of local DNA environment included time resolved ﬂuores- cence and rotational correlation measurements of the duplex oligomers relative to 6-MI monomer and single stranded DNA. Analysis of time-resolved ﬂuores- cence decay yields 3 lifetime components of 0.4 ns, 4 ns and 6.5 ns. The largest Monday, March 2, 2009 347a