Towards User Operations of SONICC at GM/CA@APS

Second order nonlinear optical imaging of chiral crystals (SONICC), based on femtosecond laser scanning microscopy, has been implemented at GM/CA@APS undulator beamline 23ID-B for rapid protein crystal localization and centering. The technique is based on infrared laser light impinging on non-centrosymmetric crystals of proteins, which selectively may yield a frequency-doubled, visible signal generated by the anharmonic response of the electron cloud of the protein in response to the laser field. One aim of this method is to locate small crystals grown in opaque crystallization media for centering in X-ray beams of only a few microns or less in cross-section [1]. The optical system implemented at the beamline includes `trans' and `epi' detection of Second Harmonic Generation (SHG) signals [2]. In addition, scanning visible laser light across the sample and detecting two-photon excited UV fluorescence (TPE-UVF) provides complementary contrast based on the native fluorescence of proteins. An update on progress towards offering a user-friendly system to users will be provided. Different factors that influence imaging signals and the practice of successfully locating and accurately positioning a crystal via SONICC will be discussed.

PPM Detection Limits in PXRD by Integrating Nonlinear Optics and Synchrotron XRD

SHG microscopy allows rapid and selective identification of trace chiral crystals within amorphous media, enabling targeted XRD using a 5-10 micrometer diameter "minibeam". The sensitivity of PXRD is increased substantially by reducing the background scattering contributions of amorphous material otherwise encountered with a larger beam. In addition, performing diffraction only at the locations most likely to produce diffraction greatly reduced the overall beam-time required to perform the PXRD analyses. Integration of the SHG microscope directly into a synchrotron X-ray beamline at Argonne National Laboratory recovered high spatial registry between the regions of interest identified by SHG for positioning within the X-ray beam. Using this approach, diffraction was performed on individual griseofulvin nanocrystals suspended within an amorphous polymer, corresponding to a total of ~20 fg of total crystalline material. Additional measurements for ritonavir in hydroxypropylmethylcellulose (HPMC) were also performed, in which a bulk API concentration of 100 ppm produced diffraction peaks with a signal to noise ratio of >3000. Among other applications, sensitive detection of trace crystallinity can inform the design of amorphous formulations, in which the bioavailability of active pharmaceutical ingredients (APIs) is enhanced by maintaining them in an amorphous state. However, the long-term stability of a final dosage form can be negatively impacted by spontaneous transitioning to the typically more stable crystalline forms of the APIs, such that extensive quantitative characterization of the crystallization behaviors of amorphous formulations is routinely performed.

Protein Crystal Staining for Second Harmonic Generation Imaging

Recently, second harmonic generation (SHG) microscopy has become a useful tool in the field of structural biology for the detection of protein crystals. SHG, or the frequency doubling of light, is a process specific to crystalline media lacking inversion centers. Through theoretical models and experimental data, it is estimated that ~84% of the known protein crystal structures give detectable SHG signal using current SHG microscopy instrumentation. Extending this coverage could be extremely useful to structural biologists who routinely screen entire 96 well plates, with hundreds of crystallization conditions, in search of diffraction-quality protein crystals. A series of SHG active dyes, including Malachite Green (MG) and trans-4-[4-(dimethylamino)styryl]-1-methylpyridinium iodide (DMI) were investigated to assess their ability to enhance the nonlinear optical (NLO) response across a broad range of protein crystals with varying degrees of inherent SHG activity. MG and DMI were shown to enhance the SHG activity of tetragonal (P43212) lysozyme crystals, a protein that typically generates little to no SHG signal. SHG enhancements for lysozyme of approximately 16000x and 20x were achieved by intercalation of MG and DMI, respectively. These results are consistent with predictions based on the differences in symmetry and structure for the two dyes. The kinetics of the dye intercalation and uptake were investigated with SHG time-lapse images taken of a lysozyme crystal after the addition of MG dye into the crystallization well. Kinetic results indicate that an increase in SHG activity becomes easily noticeable within minutes of exposure to the dyes. These results show a significant progress towards increasing the coverage of SHG microscopy for protein crystal detection.