The rotational correlation time of the lipid probe 1-palmitoyl-2-{6-[(7-nitro-2-1,3-benzoxadiazol-4-yl)amino]hexanoyl}-sn-glycero-3-phosphocholine (NBD-PC) is measured using fluorescence anisotropy for two lipid species. We measure the rotational diffusion in a monolayer of 1,2-Didecanoyl-sn-glycero-3-phosphocholine (DPPC) which displays a phase transition at room temperature from the liquid-expanded to the liquid-condensed phase. The constant rotational diffusion of the probe throughout the phase transition reflects the measurement of dynamics in only the liquid-expanded phase. We contrast the dynamic changes during this phase coexistence to the continuous density increase observed in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) at room temperature. We observe a non-exponential decay of the probe diffusion consistent with heterogeneity of the orientational dynamics.

Lipid structures exhibit complex and highly dynamic lateral structure; and changes in lipid density and fluidity are believed to play an essential role in membrane targeting and function. The dynamic structure of liquids on the molecular scale can exhibit complex transient density fluctuations. Here the lateral heterogeneity of lipid dynamics is explored in free standing lipid monolayers. As the temperature is lowered the probes exhibit increasingly broad and heterogeneous rotational correlation. This increase in heterogeneity appears to exhibit a critical onset, similar to those observed for glass forming fluids. We explore heterogeneous relaxation in in a single constituent lipid monolayer of 1, 2-dimyristoyl-sn-glycero-3-phosphocholine  by measuring the rotational diffusion of a fluorescent probe (1-palmitoyl-2-[1]-sn-glycero-3-phosphocholine), which is embedded in the lipid monolayer at low labeling density. Dynamic distributions are measured using wide-field time-resolved fluorescence anisotropy. The observed relaxation exhibits a narrow, liquid-like distribution at high temperatures (τ ∼ 2.4 ns), consistent with previous experimental measures (Dadashvand et al 2014 Struct. Dyn. 1 054701, Loura and Ramalho 2007 Biochim. Biophys. Acta 1768 467-478). However, as the temperature is quenched, the distribution broadens, and we observe the appearance of a long relaxation population (τ ∼ 16.5 ns). This supports the heterogeneity observed for lipids at high packing densities, and demonstrates that the nanoscale diffusion and reorganization in lipid structures can be significantly complex, even in the simplest amorphous architectures. Dynamical heterogeneity of this form can have a significant impact on the organization, permeability and energetics of lipid membrane structures.

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We have characterized the temperature and pressure dependent scaling of dynamic heterogeneity in a homogenous liquid phase of a lipid monolayer using timeresolved fluorescence anisotropy (TRFA) microscopy. The rotational correlation time of the lipid probe NBD-PC is measured using fluorescence anisotropy for two lipid species. We measure the rotational diffusion in a monolayer of DPPC which displays a phase transition at room temperature from the liquid-expanded to the liquidcondensed phase. The constant rotational diffusion of the probe throughout the phase transition reflects the measurement of dynamics in only the liquid-expanded phase.We contrast the dynamic changes during this phase coexistence to the continuous density increase observed in DMPC at room temperature. We observe a nonexponential decay of the probe diffusion consistent with heterogeneity of the orientational dynamics; as the free-volume is reduced the diffusion becomes increasingly heterogeneous. Rotational diffusion is highly influenced by the interaction of nearest neighbors. DMPC at lower temperatures is in the coexistence of LE-LC phase and for higher temperature it is in the pure liquid phase. Highly correlated motion results in regions of high-density, low mobility lipids, and lowdensity, high mobility lipids; and are observed as the bimodal distribution of rotational correlation times.iii Acknowledgment I would like to express my deepest appreciation to my advisor, Professor Christina M. Othon, for her full support, expert guidance, understanding and encouragement through my PhD study. I would like to thank the faculty of Physics department at Wesleyan University for their support over the past five years especially my committee members Prof. Fred M. Ellis and Prof. Francis W. Starr. Their thoughtful questions and comments were valued greatly. I would especially like to thank to Wesleyan machine shop members, David Strickland, Bruce Strickland and Thomas Castelli, for many discussion and hard works. I would like to thank my lab colleagues, Nimesh Shukla, Lee Chen, LaNell Williams and Inha Cho for being great colleagues. My gratitude also goes to my friends Mahboube Chitsazi, Ali Basiri, Masoud Nickaein, Swastik De, Jyoti Gupta and Phyllis LeMaire and all my other fellow Physics graduate students for their supports and understanding. I am indebted to my family. I would like to thank my dad and mom for sacrificing their life for me. I also want to thank my sisters Leila and Mina.

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