Antipyretic Analgesics Remedial Agents Lower Temperature Body Pyrexia Biology Essay
Antipyretic anodynes are remedial agents that lower the temperature of the organic structure in fever.
Pyrexia is a status where the temperature of the organic structure rises to degrees above the normal organic structure temperature. They exert their action on the heat modulating Centre of the hypothalamus. Salicylates, aminophenol parallels, aminobenzines, pyrazolones and quinoline derived functions are the common group of compounds used as antipyretic anodynes.Antipyretics are agents which basically help to cut down fever to normal organic structure temperature.
The drug substances in this class possess the ability to relieve the esthesis of hurting threshold ranging from mild to severe position. Many substances in this class have been removed from the market because of assorted side effects seen such as tegument jobs, cardiac abnormalities, icterus and methemoglobinemia.Analgesics may be defined as ‘agents that relieve hurting by promoting the hurting threshold without upseting consciousness or changing sensory-modalites ‘ . ( Medicative Chemistry, K. Ashutosh, 2005 ) .
The mechanism by which these anodynes act can chiefly be attributed to the presence of opiate receptors located in selected parts of the CNS associated with hurting ordinance. These opiate receptors are located specifically in the median thalamus, beds I and II in the spinal cord and the brain-stem ‘s vagus karyon.The drug is wholly absorbed from the upper portion of the GI piece of land following unwritten soaking up.
Within one or two hours maximal plasma degree concentration is reached. Merely a fraction opf the drug os bound to plasma protein, the remainder ( about 99.8 % ) is metabolised and converted to acetyl p-aminophenol. The first measure in the metamorphosis of acetophenetidin is O- deethylation by cytochrome P450 1A2. Some of this metabolised drug is conjugated with glucorinioc acid or sulfate ions. Aboy 0.1 % is changed to p-phenetidin by deacetylation. 3.
5 % of the entire ingested dosage is found in the piss within 24 hours in the signifier of free aminophenol and and approximately 74 % in the signifier of conjugated N-acetyl- p- aminophenol.1.1.8. Dose: Typical doses of 300-500mg a twenty-four hours lead to analgetic effects.
Phenacetin is already withdrawn from the market but still general contraindication may be pointed out as:Should non be taken by patients holding high blood pressure.Patients holding nephritic inadequacy should avoid the drug as it has inauspicious effects on the kidney and may decline the status.
1.10 Adverse and toxicological effects:
Phenacetin was withdrawn from the market because of its terrible maltreatment and carcinogenic effects. Some inauspicious effects can be pointed out as:Nephrotoxicity and nephritic failure after prolonged usage is seen.
It has terrible carcinogenic effects and may take to malignant neoplastic disease.Another toxic consequence of acetophenetidin is that it converts hemoglobin to methemoglobin and sulfhemoglobin within the blood cells ladling to decrease in red blood cell endurance clip. This finally may take to less haemoglobin available for O transit and lead to chronic anoxemia.It may do intravascular haemolysis.High concentrations of acetophenetidin may hold depressant effects on the cardiovascular system circulation and nervous system which may take to vascular prostration and daze.
1.2.1. Iupac Name: A 2-hydroxy benzamide1.
2.2. Structural Formula: C7H7 NO21.2.3. Molecular mass: 137.
1g/mol1.2.4. Salicylamide has been used historically as an antipyretic anangesic but is non used well today ( Hart 1946 ) . It is described as an amide of salicylic acid.
It is prepared by responding methyl salicylate with ammonium hydroxide. In worlds it has antipyretic anodynes and even anti-inflammatory effects similar to salicylic acid. ( Insel, 1990 ) .
Figure: 2.1 Chemical construction of Salicylamide
Salicylamide appears as a white or pink crystalline pulverization or as needle shaped crystals holding faintly acrimonious gustatory sensation and is odorless. It has a runing point of 140 0C.It is meagerly soluble in H2O but soluble in ethyl alcohol, trichloromethane and quintessence.
It is soluble in a solution of base. It is stable, light sensitive. It is incompatible with strong bases and strong oxidising agents. It darkens on exposure to air ( Chemicalbook ) .
6. Mechanism of Action:
1.2.7. Curative usage:
Salicylamide is used as an antipyretic anodyne. It besides has a mild ataractic action. It has negligible anti-inflammatory consequence.
It is used in multidrug combinations for intervention of a assortment of mild hurting conditions including musculoskeletal, soft tissue and joint upsets. It is besides used in arthiritic conditions.
1.2.8: Pharmacokineticss of salicylamide:
Salicylamide is readily absorbed when given orally.
It undergoes important first base on balls metamorphosis. It is seen widely distributed in the organic structure as it is lesser edge to plasma proteins. It has a half life of about 1 hr as seen from urinary elimination informations. It gets quickly excreted in the urine chiefly as sulfate and glucuronide conjugates. Merely hint sums are excreted unchanged.
9. Adult dosage:
Salicyclamide is given in doses of approximately 1 to 2.5 gms daily, normally with other anodynes. It is besides applied locally in about concentrations of approximately 5 % .
1.2.11 Adverse consequence:
3. Nuclear magnetic resonance spectrometry:1.3.1 Definition: Nuclear magnetic resonance spectrometry ( NMR spectrometry ) is a powerful technique used for clarifying the constructions of organic molecules. It involves the soaking up of electromagnetic radiation in the wireless frequence scope of 4-900 MHz. The basic rule underlying Nuclear magnetic resonance spectrometry is that atomic karyons have quantised spin provinces which may be differentiated in the presence of strong magnetic field.
A assortment of information about a compound can be obtained from an NMR spectrum.1.3.2. Principles: In NMR, radiation in radiofrequency part is used to excite atoms, normally protons or carbon-13 atoms. The spins of the aroused atoms switches the alliance from with an applied magnetic field to against an applied magnetic field.
The soaking up of energy consequences from the application of energy in the signifier of wireless frequence, extraneous to the applied magnetic field which consequences in passages between the allowed provinces ensuing in whole the phenomenon taking topographic point. The scope of frequences required for excitement and the complex splitting forms produced are due to the chemical construction of the molecule. The corresponding frequence of the captive energy depends on the karyon and besides its electronic environment. ( Traficante 1996 ) .1.3.3.
Theory of NMR: It was suggested by Pauli that certain magnetic karyon could hold spin every bit good as magnetic minute from rotary motion around their axes. He suggested that the karyon could aline themselves with or opposed to the magnetic field. The angular minute of the whirling charge is expressed by the spin quantum figure, I. The spin quantum figure can be a half whole number or an whole number and is expressed in units of h/2Iˆ , where H is Planck ‘s changeless. Any karyon which has I & gt ; 0 when placed in a magnetic field will presume a maximal figure of orientations equal to 2I+1. Now, I= 1/2 for a proton, hence two orientations exist: aligned with field ( low energy ) and aligned opposed to the field ( high energy ) .The separation of the energy degrees or whirl provinces is a map of the external magnetic field ( H0 ) , the spin quantum figure ( I ) and atomic magnetic minute ( Aµ ) .
E= Aµ H0/I
3.4. Basic footings and constructs in Nuclear magnetic resonance:
4.1: Absorption and relaxation: NMR spectrometry can be distinguished from other signifiers of spectrometry in that it has equal populations in the higher and lower energy provinces. The population of the two provinces is governed by the Boltzmann distribution in the absence of an applied magnetic field. For soaking up to occour, there must be an extra population in the lower province. This extra in NMR is little, the phenomenon of impregnation occours readily as the the lower provinces is depleted.The population in the upper province besides depends on the phenomenon known as relaxation, which refers to any procedure that removes any karyon from an aroused province.
Two basic relaxation mechanisms are operative in NMR: spin-spin relaxation and spin-lattice relaxation.1.3.4.
2: Chemical Shift: The term chemical displacement is used to depict the alone magnetic field strength required to accomplish resonance for any given proton. The needed magnetic field is influenced by the electromagnetic environment of the proton. Shielding is said to occour when these Fieldss counteract the applied magnetic field. Shielding can be reduced by the presence of negatron retreating substituents such as O or halogens.
The protons attached to negatively charged substituients are said to be deshielded and necessitate less magnetic field strength to accomplish resonance.1.3.5: Types of Nuclear magnetic resonance: There are fundamentally two types of NMR techniques viz. 1H or Proton NMR and 13C NMR.
It can be used for fingerprinting mixturesIt can be used to find the residuary constructions of unfolded proteins.It can be used for direct sensing of H bonding interactions.
It can be used for research in polymer chemical science and natural philosophies. ( Michigan province university ) .www2.chemistry.msu.edu/facilities/nmr/900Mhz/MCSB_NMR_applications.htmlUsed for the chemical analysis of molecular constructions.
It is used for the word picture of physical belongingss of affair in stuffs scientific discipline. ( Bernhard Blumich, 2005 )NMR is successfully used for the word picture of the exact construction of natural stuffs and finished merchandises.NMR is used for metabolic surveies.1.3.7.
Strength: NMR provides a batch more information about molecular construction of a compound than any other technique. ( Jens Duus 2000 )Can be used detect really all right structural constituents, plants for organic and inorganic compounds, is qualitative and quantitative, versatile.
It is a comparatively insensitive technique compared to the other spectroscopic techniques as it requires more than 5mg of sample for proton atomic magnetic resonance ( NMR ) and more than 20mg for carbon-13 NMR.The instrumentality for NMR is really expensive and hard to run.
A specializer operator is required to run this instrument, although mechanization is progressively available for everyday methods. ( Jens Duus 2000 )Overall, it is a really expensive, clip devouring procedure and the spectra take long clip to construe.Figure 1.
3. NMR sample tubing placed in a spinster, ( Norell, Inc. 2008 ) modified.1.4. Infrared spectrometry:
IR is a technique based on the quivers of the atoms of a molecule.
An IR spectrum is obtained by go throughing Ir radiation through a sample and finding what fraction of the incident radiation is absorbed at a peculiar energy.The energy at which any extremum in an soaking up spectrum appears corresponds to the frequence of a quiver of a portion of the sample molecule.( Referee: Biological applications of IR Spectroscopy by Barbara Stuart, David J. Ando )1.4.2. Principles: Electromagnetic radiations runing between 400cm-1 and 4000cm-1 ( 2500 and 20000 nanometer ) are passed through a sample. The bonds present in the molecule sample absorb the radiation which causes them to stretch or flex.
The wavelength of the radiation absorbed is characteristic of the bond absorbing it. ( Koichi Nishikida 1995 )
It is used to look into the individuality of a compound in man-made chemical science.It is used to look into the presence or absence of a carbonyl group, which is hard to look into by any other method.
Can be used to characterize samples in the solid and semi-solid provinces such as picks and tablets and designation of natural stuffs and merchandise quality, wet and solvent content in drying or solvent remotion procedures, residuary drug carryover in fabrication installations, blending quality rating, and imagination of tablets and boxing systems. .Can be used to observe polymorphs of drugs ( polymorphs are different crystal signifiers of a molecule that have different physical belongingss such as solubility and runing point, which may be of import in the fabrication procedure and bio-availability ) . ( Stuart 2004 )IR is used in reading of the spectra of drugs with the aid of FTIR for designation of drugs, excipients and the natural stuffs used in industry. ( Ref: Biological applications of IR Spectroscopy by B.
Stuart )A qualitative fingerprint cheque for the individuality of natural stuff used in fabrication and every bit good as for placing drugs.It has been used for analysis of all right chemicals, polymers gasoline and pharmaceuticals, both with diffusing and Fourier-Transform ( FT-NIR ) based instrument.FTIR is used for structural elucidation of penicillins. ( Ref: Biological applications of IR Spectroscopy by B.Stuart )
For every compound being studied, it provides a fingerprint which is alone to the compound.The matching of the compound to its standard fingerprint can be carried out as the instruments are computing machine controlled. ( James Robertson 1999 )IR procedure is rather inexpensive, various and is an easy method to place functional groups.
There is a trouble in sample readying and reading.
It can merely observe gross drosss in samples.Sample readying requires a grade of accomplishment, peculiarly when K bromide disc demands to be prepared. ( James Robertson 1999 )It is a clip devouring method and sample recovery is non possible.1.5. Mass spectroscopy:1.
5.1. Mass spectroscopy is an analytical technique that measures the mass-to-charge ratio ( m/z ) of gas-phase ions formed from molecules. The mass spectrometer measures the mass to bear down ratio of the gas stage ions and gives the copiousness of ionic species. Mass spectrometry is a destructive method of analysis wher the sample can non be recovered. Mass spectrometry is a extremely sensitive technique and requires really less sample compared to other spectroscopic techniques.1.5.
2. Principle: The basic rule of mass spectroscopy the coevals of ions by a suited method, so divide these ions by their mass-to-charge ratio and so observe them quantitatively and qualitatively. The first measure in mass spectroscopic analysis is the production of charged molecules or molecular fragments.
These are generated in a high-vacuum part, or instantly anterior to try debut, utilizing a assortment of methods for ion production. The molecules are bombarded with a high energy beam of negatrons. Due to this barrage the molecules are broken up into many fragments. Now, each sort of ion has a mass to bear down ratio which is measured by the mass spectrometer. ( Morrison and BoydThe mass spectrometric instrument consists of an ion beginning, a mass analyser and a sensor.
The block diagram in Fig. shows the different parts of a mass spectrometer.Diagram courtesy: American society of Mass spectrometry, 2001.There are a figure of methods that can be used for ionization of the sample as described in Table 1.
1.Table 1.1 Methods of ionization in mass spectrometry, ( Chemical Education Group SA Branch n.d. ) modified.MethodDescriptionChemical ionisation ( CI )By either a proton or hydride transportation, a reagent ion reacts with the analyte molecules, to organize ions.CH4+ + CH4 — & gt ; CH5+ + CH3 ; CH3+ + CH4 — & gt ; C2H5+ + H2Electron impact ( EI )An negatron beam, from a tungsten fibril, knocks an negatron off of analyte atoms or molecules to make ionsElectrospray ionisation ( ESI )The ESI beginning consists of a really all right acerate leaf. Sample solution is dispersed into a all right aerosol.
The droplets carry charge when the issue the capillary and, as the dissolver evaporates, extremely charged analyte ions are left behindLaser ionisation ( LIMS )A optical maser pulsation causes both vaporisation and ionisation of the sample.Matrix-assisted optical maser desorption ionisation ( MALDI )MALDI is an LIMS method of zaping and ionising big molecules such as proteins or Deoxyribonucleic acid fragments. The sample is dispersed in a solid matrix such as nicotinic acid and a UV optical maser pulsation targets the matrix which carries some of the big molecules into the gas stage in an ionised signifierPlasma-desorption ionisation ( PD )Decay of 252Cf green goodss two fission fragments of which one strikes the sample strike harding out 1-10 analyte ions and the other work stoppages a sensorResonance ionisation ( RIMS )One or more optical maser beams are tuned in resonance to passages of a gas-phase atom or molecule to make an ionThermal ionisation ( TIMS )A sample in its elemental signifier is deposited on a metal thread, such as Pt, and an electric current heats the metal to a high temperatureFast-atom barrage ( FAB )A high-energy beam of impersonal atoms, like Xe or Ar, strikes a solid sample doing desorption and ionisationPlasma and glow dischargePlasma is a hot, partially-ionized gas that excites and ionizes atoms. A glow discharge is a low-pressure plasma maintained between two electrodes
It is used for finding and besides corroborating the construction of drugs and natural stuffs used for industry.It can be used for characterizing drosss in drugs and preparation excipients by concurrence with either gas chromatography ( GC-MS ) or liquid chromatography ( LC-MS )Mass spectroscopy has become an of import tool in proteomics, which is presently a major tool in drug find. ( Yinon 1995 )It is an of import method for the word picture of proteins.
It is the best method for acquiring rapid designation of hint drosss, which is ideally carried out utilizing chromatographic in concurrence with high-resolution mass spectroscopy so that elemental composings can be determined.
With the coming of electro spray mass spectroscopy and the re-emergence of clip of flight mass spectroscopy the technique will be of major usage in the quality control of curative antibodies and peptides. ( Gross 2004 )
MS is non presently used in everyday quality control ( QC ) but is placed in a research and development ( R & A ; D ) environment, where it is used to work out specific jobs originating from everyday procedures or in procedure development.The instrumentality is expensive and requires support by extremely trained forces and regular care. However, restrictions are bit by bit being removed. ( Gross 2004 )The greater the MS instrument ‘s declaration, the greater its utility for analysisFinding the right parent extremum in the mass spectra may be hard.
2. EXPERIMENTAL Section
2.1 Purpose: The synthesis and word picture of Analgesic Antipyretic drugs:1. Acetophenetidin.2. Salicylamide.The experiment besides includes the word picture of Phenacetin and Salicylamide by Mass spectrometry, FT-Infrared spectrometry and NMR spectrometry.
2.2. Synthesis of Acetophenetidin:2.
2.1. Chemicals used: p-aminophenol, acetic anhydride, Na ethoxide, rectified spirit, ethyl iodide, H2O.2.2.2. Instruments used: unit of ammunition underside flask equipped with a reflux capacitor, H2O bath, beakers, mensurating cylinders, vacuity filter, filter paper, electronic weighing machine, etc.
Suspend 11g ( 0.1 mol ) of p-aminophenol in 30 mol of H2O contained in a 250-ml beaker or conelike flask and add 12 milliliters ( 0.127 mol ) of acetic anhydride. Stir ( or agitate ) the mixture smartly and warm on a H2O bath.
The solid dissolves. After 10 proceedingss, cool, filter the solid ethanoyl group derived functions at the pump and wash with a small cold H2O. Recrystallise from hot H2O ( about 75ml ) and dry upon filet-paper in the air. The output of p-hydroxyacetanilide, m.p.
169 0C ( 1 ) , is 14g ( 93 % ) .Warm a flask on H2O bath until solution is complete. Cool the mixture and add 10g ( 0.066 mol ) of p-hydroxyacetanilide. Introduce 15g ( 8 milliliter, 0.1 mol ) of ethyl iodide easy through the capacitor and reflux the mixture for 45-60 proceedingss. Pour 100 milliliter of H2O through the capacitor at such a rate that the crystalline merchandise does non divide ; if crystals do separate, reflux the mixture until they dissolve.
Then cool the flask in an ice batch: roll up the petroleum acetophenetidin with suction and wash with a small cold H2O. Dissolve the petroleum merchandise in 80ml of rectified spirit ; if the solution is coloured, add 2g of bleaching C, furuncle and filter. Treat the clear solution with 125 milliliters of hot H2O and let to chill. Roll up the pure acetophenetidin at the pump and dry in the air. The output is 9.5g ( 80 % ) , m.p.
137 0C.Note: ( 1 ) if the m.p. is unsatisfactory, fade out the merchandise in dilute base in the cold and so reprecipitate it by the add-on of acid to the neutralization point. This process will extinguish hints of the diacetate of p-aminophenol which may be present ; the ethanoyl group group attached to nitrogen is non affected by cold dilute base ; but that attached to oxygen is readily hydrolysed by the reagent.2.3. Synthesis of Salicylamide:2.
3.1. Chemicals used: salicylic acid, dry methyl alcohol, conc. sulfuric acid, H2O, Na H carbonate, Mg sulfate, conc. ammonium hydroxide, hydrochloric acid.
2.3.2. Instruments used: unit of ammunition underside flask equipped with a reflux capacitor, H2O bath, beakers, mensurating cylinders, vacuity filter, fluted filter paper, electronic weighing machine, separatory funnel, still head with 360 0C thermometer, air capacitor.
Measure 1. Use 28g ( 0.2 mol ) of salicylic acid, 64g ( 81 milliliter, 2 mol ) of dry methyl alcohol and 8 milliliter of concentrated sulfuric acid. Reflux the mixture for at least 5 hours.
( 1 ) . Distil off the surplus of intoxicant on a H2O bath ( rotary evaporator ) and let to chill. Pour the residue into 250 milliliter of H2O contained in a separatory funnel and rinse the flask with a few milliliter of H2O which are besides poured into the speratory funnel. If, owing to the relatively little difference between the denseness of the ester and of H2O, trouble is experienced in obtaining a crisp separation of the lower ester bed and H2O, add 10-15 milliliter of C tetrachloride ( 2 ) and agitate the mixture in the funnel smartly ; upon standing the heavy solution of methyl benzoate in the C tetrachloride separates aggressively and quickly the underside of the separatory funnel.
. Run off the lower bed carefully, reject the upper aqueous bed, return the methyl benzoate to the funnel and agitate it with a strong solution of Na H carbonate until all free acid is removed and no farther development of C dioxide occurs. Wash one time with H2O and dry by pouring into a little dry conelike flask incorporating about 5g of Mg sulfate. Stopper the flask, agitate for approximately 5 proceedingss and let to stand for at least half an hr with occasional shaking. Filter the methyl salicylate solution through a little fluted filter paper straight into a unit of ammunition -bottomed flask fitted with a still-head transporting a 360 0C thermometer and an air capacitor. Add a few boiling french friess and distil from a air bath ; raise the temperature easy at first until all C tetrachloride has passed over and so heat more strongly.
Roll up the pure methyl salicylate ( a colourless oil of delicious aroma, ‘oil of pyrola ‘ ) at 221-224 0C ; the output is 25g ( 81 % ) . The ester may besides be distilled under decreased force per unit area ; the b.p is 115 0C/20mmHg and a 2 0C fraction should be collected.Measure 2. 2 milliliter ( 2.
3g ) of methyl slayicyalte and 20ml of concentrated ammonium hydroxide ( 28 % NH3 ) are placed in a 50 milliliter unit of ammunition underside flask and stoppered tightly and agitate exhaustively. The ester may be dispersed lower by the add-on of a wetting agent ( .01g ) e.g.
Charcoal etc, but that is non necessary. The mixture is set aside for a hebdomad. During this clip the ester stage disappears. The clear solution is transferred to a beaker and diluted with 50 ml H2O.
The diluted solution is neutralised with hydrochloric acid. The liberated amide precipitates as the solution cools. The precipitate is collected by suction, washed with 50 milliliters H2O ( used in little part ) and dried ; m.
p. is 136/137 0C output is 70 % .
2.4. FT Infra-Red Spectroscopy
FT infra-red spectra were recorded on a Perkin Elmer, Paragon 1000 FT-IR spectrometer ( Figure 2.
1 ) . The spectra were recorded at a declaration of 1 cm-1 in the 450-4000 cm-1 scope. A dry N gas purging was maintained in the sample compartment to ease a simpler background minus. The samples which were solid in nature were examined as pressed KBr phonograph record which were prepared by triturating the sample exhaustively with K bromide and spectra were recorded over the 4000-450 cm-1 part.
The instruments used to fix the K bromide phonograph record with the sample were exhaustively cleaned so as to avoid any transverse taint.
Figure 2.1. Image of Perkin Elmer, Paragon 1000 FT-IR spectrometer
All the samples needed to be examined by the mass spectroscopic method were dissolved in an appropriate dissolver and so were run on VG-QUATTRO, where the recess investigation was heated. Mass spectra were obtained utilizing electrospray ionisation ( ESI ) method of ionisation. The beginning consists of a really all right acerate leaf.
The sample solution is dispersed into a all right aerosol. When the droplets exit at the capillary terminal they carry a charge and as the solvent gets evaporated, extremely charged analyte ions are left behind.In positive ion manner, ESI normally produces multiply charged ions [ M + nH ] n+ .
The ions in solution province are transferred to gaseous province at atmospheric force per unit area by ESI. The sample flows through a capillary tubing to which a high electromotive force is applied. There is a strong electric field due to the counter electrode. The sample emerging from the tip is dispersed into an aerosol of extremely charged droplets. Positive ions will roll up at the surface of the liquid and will travel towards the negative aggregator electrode if a positive potency is applied to the capillary.
As they move towards the electrode they are either subjected to heat or heated N gas due to which the dissolver evaporates and the droplets cut down in size. The droplet interrupt down into smaller atoms when electrostatic repulsive force is greater than surface tenseness of the liquid and the charged sample ions free from dissolver are released from droplets ( Lee 2005 ) .
Figure 2.2. Image of Mass spectrometer
2.6. Nuclear Magnetic Resonance
Samples ( + ) -3, 6-bis- ( aminoxymethyle ) -2, 5-piperazinedione and 3, 6-dimethylene-2, 5-piperazinedione were analyzed utilizing proton NMR spectrometry and C13 spectrometry.
Samples were run in a Jeol EX, 270 MHz FT NMR Spectrometer, integrating a Tuneable H-5/270 investigation. Samples were dissolved in DMSO d6 ( ~20mg/ml ) and placed into a 5 millimeter o.d.
borosilicate glass NMR tubing. The NMR tubing was loaded into the instrument and spun at 15 Hz. Samples were locked and shimmed with regard to the deuteriated dissolver. 1H spectra were acquired utilizing a individual pulsation experiment, with a relaxation hold of 4 seconds, the figure of spectral accretions was 32 scans.
2.3. Image of NMR Spectrometer