A Method of Mapping the Molecular Structures - Assignment Example

Introduction Nuclear Magnetic Resonance , otherwise abbreviated as NMR, is a method of mapping the molecular structures and studying how they function as well as how they relate, using a technique which is non-destructive. This technique has its basis in the fact that many atoms have nuclei which behave as little magnets. Have magnetic qualities. When the tiny magnets are inside a larger one, the South Pole ends of the tiny magnets get in line with the bigger magnet’s North Pole. In so doing, the tiny magnets take in some energy and start acting in a “spin”, “wobble” or even “flip” manner. This action, also referred to as resonance, has the ability of being physically mapped to indicate the atoms present within the molecule and their location in relation to the others, hence the name Nuclear Magnetic Resonance. Nuclear Magnetic Resonance had been formerly expressed and evaluated by Isidor Rabi in terms of molecular beams(1938) 1. He was later awarded a Nobel Prize in physics for this. 2. Edward Mills Purcell and Felix Bloch extended this to be applied in liquids as well as solids, resulting in their sharing of the Nobel Prize in physics (1952).3 Purcell put more efforts on this development as well as the radar application, in the Radiation Laboratory of Massachusetts Institute of Technology during the period of the Second World War. In his project of identifying and generating the radio frequency energy as well as its up take by matter, he discovered Nuclear Magnetic Resonance. Edward Mills Purcell and Felix Bloch realized that the magnetic nuclei, such as 1H as well as 31P, were able to take in the radio frequency energy when located in any magnetic field with strength in line with the nuclei’s identity. NMR, though formerly applied in physics, has been used greatly as the analytical technique for revealing the chemical structures as well as the material properties. It parallels the electromagnetic technology development and the introduction of electromagnetic technology for use by civilians. Therefore, in an attempt to fully exploit the information regarding Nuclear Magnetic Resonance, this work expounds on various aspects including applications, types, and properties (Akitt, 2000). Types of nuclear magnetic resonance Nuclear magnetic resonance has two main types of instruments; the continuous wave instruments and the Fourier transform instruments. The continuous wave instruments were used formerly in experiments; however, in 1970 there was availability of the Fourier transform instrument which has currently taken the market. The continuous wave instruments These spectrometers are same as the optical spectrometers in principle. This entails a sample being held strongly in a magnetic field which is very strong, while the source frequency gets slowly scanned. However, some instruments have this frequency constantly held while field scanning takes place. Fourier transform instruments The amount of energy changes experienced in the NMR spectroscopy are quite small, hence sensitivity becomes a great limitations. In order to increase sensitivity, several spectra may be recorded and added together; and since there is random noise, this causes addition as square root of recorded spectra number. While using the instrument of continuous wave, there will be much time required for spectra collection since one scan uses approximately 2-8 minutes. However, Fourier transform instruments (FT-NMR) irradiates all spectrum frequencies simultaneously through the pulse of a radio frequency. The nuclei go back to the thermal equilibrium by tracking the pulse. The instrument records the emission signal with time domain as relaxation of the nuclei takes place. NMR applications Medicine The most well known application of NMR to the public is the magnetic resonance imaging (MRI) often applied in medical diagnosis as well as the MR microscopy applied in the research settings. NMR, however, is also applied in the studies involving chemistry, mostly in the NMR spectroscopy like the proton NMR, the phosphorus-31 NMR, the carbon-13 NMR, and the deuterium NMR. The information regarding biochemistry may also be collected from the living tissue (for instance the brain tumors in human beings) by using a technique referred to as in vivo magnetic resonance spectroscopy or even the chemical shift NMR Microscopy. These studies are able to be carried out because the nuclei are always encircled by the orbiting electrons. These are always charged particles which produce tiny, local fields of magnet which increase or decrease the magnetic field in the external region, hence causing a part of the nuclei to be shielded. The extent to which shielding takes place is affected by the specific local environment. Being among the two main spectroscopic techniques applied in metabolomics, nuclear magnetic resonance is also applied in the generation of the metabolic fingerprints right from the biological fluids in order to gain information concerning states of diseases or even information concerning the toxic insults (Tyszka, 2005). Chemistry Through the study of the NMR spectra peaks, chemists are in a position to identify structures of various compounds. This technique may be very selective, differentiating between various atoms present in a given molecule or even in molecule collections having similar characteristics while only differing in the chemical environment within the local regions. Through the study o f Spin-spin relaxation time (F2*) information, chemists are in a position to identify a compound’s identity through comparison of the precession frequencies noted with the known frequencies. More data on the structures may be elucidated through observation of the spin-spin coupling, a method through which nucleus’ precession frequency may be affected through magnetic shift from a nuclei located nearby. Since the timescale of NMR is relatively slow in comparison to other techniques of spectroscopy, altering the temperature in a T2* (Spin-spin relaxation time) experiment may provide information concerning fast reactions, like the rearrangement of the cope or information concerning structural dynamics like cyclohexane’s ring-flipping. In lower temperatures, there is always a distinction made between the axial hydrogens and the equatorial hydrogens found in the cyclohexane. Buckminsterfullerene (composition C60)) is an instance of the NMR being applied to identify a structure. The currently well-known carbon form contains sixty atoms of carbon which make a sphere. The atoms are in the identical environments hence see equal H field located internally. However, buckminsterfullerene lack hydrogen hence 13C NMR is used. Non-destructive testing Nuclear magnetic resonance has been considered as a powerful method of analyzing various samples in a non-destructive manner. The radio waves as well as the magnetic fields which are static simply infiltrate various matter types including materials which have no ferromagnetic characteristics in them. For instance, many costly biological samples, like the nucleic acids, involving RNA as well as DNA, or even proteins, may be examined for a longer period(e.g. months) through nuclear magnetic resonance before applying biochemical experiments that are destructive. This also enables NMR to be the best choice for use in analyzing samples that are risky. Acquiring dynamic information Apart from giving static information concerning molecules through determination of their three dimension structures while in solution, NMR has also other remarkable advantages compared to X-ray crystallography. Among the advantages is the fact that NMR is crucial in acquiring vital dynamic information, among which include collective motion with low frequency in DNA 12. Acquiring data regarding petroleum industry Another application of NMR is in acquiring data for petroleum exploration as well as natural gas exploration. This is done through drilling of a borehole in sedimentary strata and rocks in which NMR logging equipment gets lowered. The NMR analysis of the boreholes is carried out for measuring the porosity of the rocks, approximation of permeability and identification of pore fluids, including oil, gas and water. The instruments are basically low field Nuclear magnetic resonance spectrometers. NMR spectroscopy flow probes Of recent, there has been development of flow probes that have been specifically designed to allow NMR application in a media of liquid. The flow probes are also referred to as the assemblies of flow cells and they have the ability of substituting the tube probes which are standardised. This has allowed the methods which have the ability of incorporating application of continuous flows to be able to sample the introduction devices. Process control NMR has become quite important in process control as well as process optimization, in oil refineries, petrochemical plants and other industries. There is the application of two NMR analysis types for the provision of immediate feed and result analysis so as to manage and optimize the unit operations. Time-domain NMR (TD-NMR) spectrometers which work at the low field, about 2–20 MHz for 1H, give vital information on free induction decay (FID), which is essential in absolutely finding out values of hydrogen content. The same is also valuable in determining the rheological composition as well as the component composition. The spectrometers are essential in coal analysis, polymer production, food manufacturing, mining as well as cosmetics. The FT-NMR spectrometers with high resolutions and which operate within the range of 60 MHz while having protected permanent magnet structure produce 1H NMR spectra (with high resolution) of the refinery as well as the petrochemical streams (Haner, 2009). NMR of the earth’s field In the magnetic field of the earth, the frequencies of the NMR are always in the range of audio frequency. The NMR of the earth’s field (EFNMR) is basically stimulated through the application of a pulse from a magnetic field generated by a strong direct current to the particular sample. After this, the resultant magnetic field which is alternating and which has low frequency is analyzed. Some magnetometers, MRI imagers and EFNMR spectrometers exploit these effects. Since they are inexpensibly portable, the instruments are valuable in field works and teaching of NMR principles. Other applications In quantum computing, NMR applies the molecule state of spin as qubits. NMR is different from the other quantum computer implementations since it applies the ensemble of many systems (molecules). This is viewed as the state of thermal equilibrium. There are also many magnetometers employing NMR effects in measuring the magnetic fields. These magnetometers comprise of proton precession magnetometers and the Overhauser magnetometers. Nuclear Magnetic Resonance Spectroscopy experiments gHSQC Gradient heteronuclear single quantum coherence, otherwise abbreviated as gHSQC, is an experiment that is mostly applied in organic molecule NMR spectroscopy. This plays a specific role in the protein NMR field. It provides information in an experiment relating to the correlation present in the carbon-13 atoms and the protons in a gradient version. The information provided here is similar to that received from gradient heteronuclear multiple quantum coherence (gHMQC) even though the two methods have some little differences (Keeler, 2005). ATable illustrating the comparison of the advantages and disadvantage in the two techniques:   gHSQC gHMQC Experiment-setting ease ( in EZ NMR) button GHSQC button GHMQC correlation peak form narrow fairly broad spectrum phasing required (somewhat difficult) not required(not phase-sensitive) appropriateness for samples having little T2 relaxation times (the broad lines *) low better time for reaching similar digital resolution in the F1 (=C13 dimension) x minutes 2x minutes Differentiating between CH|CH3 and CH2 ( style of APT) yes no * incase the couplings broaden the lines, gHSQC is absolutely alright. This is not quite well when there is broadening of lines because of short relaxation times of T2. During such instances, failure may be experienced while gHMQC still operates. In HSQC, the spectrum that is produced is always a two dimensional spectrum, with 1H and heteronucleus (an atomic nucleus and not a proton) each receiving an axis. This spectrum has a peak for every unique proton which is connected to the considered heteronucleus.therefore in case a proton’s chemical shift is known, the coupled heteronucleus’ chemical shift is able to be identified. NOSY NOSY, or otherwise known as NOESY, refers to nuclear overhauser effect spectroscopy, which is a technique in NMR used for identifying macromolecular motif structures. It is a two-dimensional spectroscopy technique assisting in recognizing the spins which are going through cross-relaxation as well as in measuring the rates of cross-relaxation. NOESY is frequently applied as a technique of homonuclear 1H. Here, the dipolar couplings that are direct offer the main method of cross-relaxation hence the spins going through cross-relaxation are the ones nearer to each other in space. As such, a NOESY spectrum’s cross peak shows which 1H’ s are nearer to which 1H’ s in the space. NOESY has various versions including noesy, tnnoesy, gnoesywg, wgnoesy, and WET NOESY (Vuister, 2008). A NOESY sequence basically has three 90 degrees pulses, with the first creating a spin magnetization which is transverse. This then precesses at the evolution time t1. This evolution time is increased during the 2D experiment. The next pulse forms longitudinal magnetization same as the component of transverse magnetization that is orthogonal to pulse direction. Therefore the main aim is to provide the initial condition for mixing time tm. 2D NOESY experiment always maintain a constant mixing time. The last pulse generates transverse magnetization (which is observable) from the longitudinal magnetization that has remained. A NOESY diagram Diagram indicating pulse sequences in gradient NOESY and gradient EXSY gHMBC gHMBC is a term used in NMR spectroscopy to mean gradient Heteronuclear Multiple Bond Coherence. This is a gradient version of Heteronuclear Multiple Bond Coherence (HMBC). HMBC is a technique of two-dimensional inverse correlation which permits identification of the link between two distinct nuclear species. It provides couplings of a longer range. The gHMBC experiment is often carried out with the aim of showing the links existing in the carbon atoms and their directly bonded protons. Such experiment also provides information concerning carbon types, for example the quaternary carbons don’t have attached protons hence don’t indicate any correlation. The methine carbons also indicate one peak. In an inverse detection, like HMQC, the proton sensitivity is always higher compared to carbon sensitivity. Also, Incase the cross peaks of HMBC are collected through a mode which is phase-sensitive, they show a character of mixed phase, meaning the cross peaks of HMBC can not be phased to be entirely absorptive. Application of pulsed gradients of field for HMBC coherence selection in the experiment (gHMBC) creates a 2-D data which is nonphase-sensitive.the technique is preferred since phasing of spectrum is unnecessary (Harris, 2006). TOCSY TOCSY has the full meaning of Total Correlation Spectroscopy. It is like COSY since it detects any H that is attached to another. Correlation is evident in every H in the system of spin, not only the ones coupled. An instance of this is found in 3-heptanone, as illustrated in the diagram. The protons labeled a to d are in a single spin system, a coupled proton network that has not been broken. E and f, the groups of ethyl, are in a second system of spin which is also separate since a and e coupling doesn’t exist across the given carbonyl. As opposed to COSY spectrum which may indicate only correlations between CH2 a and CH2 b, the TOCSY spectrum also indicates CH2s c and CH2s d correlations. As such, TOCSY doesn’t only form the correlations in germinal and vicinal protons like COSY, but it does this to all protons. The couplings are visible in protons that are distant provided each intervening proton has couplings. It is greatly important in recognizing the protons that are present in amino acids or even sugar rings. All the protons in the same sugar ring have correlation while they don’t have correlation with other protons from a different ring (Radhakrishnan, 2002). 2DgC0SY This has the full meaning of two dimensional gradient correlation spectroscopy. This is a gradient version of the two-dimensional correlation spectroscopy. Correlation spectroscopy is among the various types of 2D nuclear magnetic resonance (NMR) spectroscopy. The experiments in 2D - COSY closely resembles that of 2D – TOCSY in that the cross peaks of the coupled protons may be observed. They only differ in the details where TOCSY has additional information that is the correlations for every proton present in a spin system. The first 2D experiment had been considered in 1971 by Jean Jeener who was a Université Libre de Bruxelles professor. Richard R. Ernst, Walter P. Aue, and Enrico Bartholdi later put it into practice hence publishing the work in 1976. The 2D spectrum arising from a COSY experiment indicates the frequencies in each isotope (frequently hydrogen, 1H) in both the axes. Magnetization transfers in COSY happen through chemical bonds and not through space. Homonuclear correlation spectroscopy sequence is an instance of 2D NMR experiment. It comprises of steps involving pulse ((p1)), evolution time (t1), pulse (p2) and measurement time (t2) respectively. The spectra are compiled by a Computer as an evolution time (t1) function. Finally, Fourier transform transforms the given time-dependent signals to a 2D spectrum. This 2D spectrum indicates single isotope frequency (mostly hydrogen, 1H) along the axes. Generation of heteronuclear correlation spectra (correspondence of the two axes to dissimilar isotopes, like the 13C and the 1H ) have also been made possible through some techniques (Slichter, 2000). A diagram of COSY spectrum showing cross- peaks  1D 1H This type of proton experiment is considered as very common in NMR experiments. The proton, also referred to as 1Hydrogen nucleus, considered as very sensitive and rated as second only to tritium in sensitivity; always yield signals which are sharp. In as much as it has a narrow range of chemical shift, the proton NMR’s sharp signals render it useful. The proton quantity of every type present in a pure sample’s spectrum may be directly acquired from every multiplet’s integral. This basically takes place when there is proper separation of the multiplets, with no overlapping of the solvent or remaining water signals. Apart from these, the molecule should not also be going through a gradual conformational exchange. The usual NMR spectrum gives integrals having a +/-10% accuracy. High accuracies, for instance +/-1%, may be attained through raising that relaxation delay so as to reach five times that of the required signal’s longitudinal relaxation time (T1). In case of multiplet overlaps then there may be application of the spectra region’s total integral (Laidler, 1999). Properties of 1D 1H and their values The proton has the following properties: Spin is ½, Natural abundance is 99.9845%, Chemical shift range is 13 ppm, from -1 to 12 , Frequency ratio (Ξ) is 100.000000% , Reference compound is TMS < 1% in CDCl3 = 0 ppm , reference Linewidth is 0.08 Hz, T1 of reference is 14 s ,Receptivity relative to 1H at natural abundance is 1.000, Receptive relative to 1H when enriched is 1.000, Receptivity relative to 13C at natural abundance is 5870, Receptivity relative to 13C when enriched is 5871 (Atkins, 2001). A diagram of Codeine’s 1H spectrum 13C/APT (Attached Proton Test) 13C, also known as Carbon-13 is a carbon’s natural and stable isotope. It is also among the well-known environmental isotopes. Carbon-13 constitutes 1.1% of the earth’s natural carbons. It has the following properties: Spin=½, Natural abundance=1.108%, range of Chemical shift =200 ppm, from 0 to 200, Frequency ratio (Ξ) =25.145020%, Reference compound= TMS < 1% in CDCl3 = 0 ppm, reference’s Linewidth and T1 = 0.19 Hz and 9 s respectively, Receptivity relative to 1H at natural abundance=1.70×10-4, Receptivity relative to 1H on enrichment =0.0159, Receptivity relative to 13C at natural abundance=1.00 and Receptivity relative to 13C on enrichment which is 93.5. (Havlíek, 2001) An illustration of 13C spectra    18 mg sample with acquisition time of 1.3 hrs On the other hand the Attached Proton Test, otherwise abbreviated as APT, is used in assigning multiplicities. Therefore 13C Attached-Proton-Test Spectra involves a situation where the Attached Proton Test assigns the C-H multiplicities in the 13C NMR spectra. This gives information about every carbon in the given experiment. According to the hydrogen number bound to a given carbon atom, n, the CHn spin vectors develop dissimilarly after the first pulse. It provides 13C NMR spectra with the attached proton number (multiplicity) being encoded in the very phase of 13C NMR signals. The APT spectra always have quaternary carbons, negative-phased carbons and positive-phased carbons of methane as well as methyl. The selection of multiplicity in the APT experiments has its basis on the 13C magnetization dephasing at the period of delay equivalent to reciprocal of one-bond 13C - 1H coupling constant (Lide,2002). A diagram of 13C spectra indicating the 13C couplings Conclusion As a technique used in identification of properties of nuclear structures, nuclear magnetic resonance has been applied widely in science, especially by using the continuous wave instruments and the Fourier transform instruments. Its characteristics has enabled it to be widely applied in non-destructive testing, chemistry, medicine, and acquiring dynamic information it is also applied in acquiring data applicable to petroleum industries, NMR spectroscopy flow probes, quantum computing , determining NMR of the earth’s field and magnetometers. As such therefore, it applies the spectroscopy experiments to determine the characteristics of molecular structures. The spectroscopy experiments used include Gradient heteronuclear single quantum coherence, 13C/APT (Attached proton test), 1D 1H (Hydrogen nucleus) and two dimensional gradient correlation spectroscopy. Others are total correlation spectroscopy, gradient heteronuclear multiple bond coherence, nuclear overhauser effect spectroscopy and gradient heteronuclear single quantum coherence. In essence, nulear magnetic resonance remains to be a vital tool in both minor and major scientific fields. 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