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    <import namespace="http://fasb.org/us-types/2020-01-31" schemaLocation="http://xbrl.fasb.org/us-gaap/2020/elts/us-types-2020-01-31.xsd" />
    <import namespace="http://www.xbrl.org/dtr/type/non-numeric" schemaLocation="http://www.xbrl.org/dtr/type/nonNumeric-2009-12-16.xsd" />
    <import namespace="http://www.xbrl.org/dtr/type/numeric" schemaLocation="http://www.xbrl.org/dtr/type/numeric-2009-12-16.xsd" />
    <import namespace="http://xbrl.sec.gov/country/2020-01-31" schemaLocation="https://xbrl.sec.gov/country/2020/country-2020-01-31.xsd" />
    <import namespace="http://fasb.org/srt/2020-01-31" schemaLocation="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd" />
    <import namespace="http://fasb.org/srt-types/2020-01-31" schemaLocation="http://xbrl.fasb.org/srt/2020/elts/srt-types-2020-01-31.xsd" />
    <element id="PTIX_ConvertibleLongTermNotesPayableRelatedParty" name="ConvertibleLongTermNotesPayableRelatedParty" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_DebtDiscountFromWarrantsIssuedToPlacementAgents" name="DebtDiscountFromWarrantsIssuedToPlacementAgents" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_DebtDiscountFromBeneficialConversionFeature" name="DebtDiscountFromBeneficialConversionFeature" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_ClassOfWarrantOrRightIssued" name="ClassOfWarrantOrRightIssued" nillable="true" xbrli:periodType="duration" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_DerivativeLiabilityMeasurementInputContractualTerm" name="DerivativeLiabilityMeasurementInputContractualTerm" nillable="true" xbrli:periodType="duration" type="xbrli:durationItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_EstimateCommitmentAmount" name="EstimateCommitmentAmount" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_SettlementOfLiabilityChargedToStockCompensationExpense" name="SettlementOfLiabilityChargedToStockCompensationExpense" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_AccruedAccountingFeesCurrent" name="AccruedAccountingFeesCurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_AccruedResearchAndDevelopmentCurrent" name="AccruedResearchAndDevelopmentCurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_AccruedLegalFeesCurrent" name="AccruedLegalFeesCurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_CommissionPercentageTerms" name="CommissionPercentageTerms" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_BeneficialConversionFeatureAndDebtDiscount" name="BeneficialConversionFeatureAndDebtDiscount" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_AdditionalNotesRelatedFee" name="AdditionalNotesRelatedFee" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_IssuanceOfOptionsForSettlementOfAccountsPayable" name="IssuanceOfOptionsForSettlementOfAccountsPayable" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_UnamortizedStockOptionExpense" name="UnamortizedStockOptionExpense" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_NumberOfOptionsSharesIssuedForSettlement" name="NumberOfOptionsSharesIssuedForSettlement" nillable="true" xbrli:periodType="duration" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_NumberOfOptionsSharesIssuedForSettlementValue" name="NumberOfOptionsSharesIssuedForSettlementValue" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_LossOnSettlementOfAccountsPayable" name="LossOnSettlementOfAccountsPayable" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriodDescription" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriodDescription" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRightsValue" name="ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRightsValue" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionsOutstandingWeightedAverageExercisePrice" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionsOutstandingWeightedAverageExercisePrice" nillable="true" xbrli:periodType="instant" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsNonOptionsAggregateIntrinsicValueOutstanding" name="SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsNonOptionsAggregateIntrinsicValueOutstanding" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_ClassOfWarrantOrRightExpirationPeriodDescription" name="ClassOfWarrantOrRightExpirationPeriodDescription" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageExercisePrice" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageExercisePrice" nillable="true" xbrli:periodType="instant" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGrantsInPeriodWeightedAverageExercisePrice" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGrantsInPeriodWeightedAverageExercisePrice" nillable="true" xbrli:periodType="duration" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageRemainingContractualTermBeginning" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageRemainingContractualTermBeginning" nillable="true" xbrli:periodType="duration" type="xbrli:durationItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_NumberOfWarrantsOutstandingWeightedAverageRemainingLifeGranted" name="NumberOfWarrantsOutstandingWeightedAverageRemainingLifeGranted" nillable="true" xbrli:periodType="duration" type="xbrli:durationItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageRemainingContractualTermEnding" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageRemainingContractualTermEnding" nillable="true" xbrli:periodType="duration" type="xbrli:durationItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_OptionsExpirationDateDescription" name="OptionsExpirationDateDescription" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_RoyaltyPaymentPercentage" name="RoyaltyPaymentPercentage" nillable="true" xbrli:periodType="instant" type="num:percentItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_UpFrontSubLicenseFeesPercentage" name="UpFrontSubLicenseFeesPercentage" nillable="true" xbrli:periodType="instant" type="num:percentItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_RoyaltyPaymentOnBehalfOfSublicenseePercentage" name="RoyaltyPaymentOnBehalfOfSublicenseePercentage" nillable="true" xbrli:periodType="instant" type="num:percentItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_InterestOnAmountsOwedUnderLicenseAgreementRate" name="InterestOnAmountsOwedUnderLicenseAgreementRate" nillable="true" xbrli:periodType="instant" type="num:percentItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_PercentageOfCorporationsTaxableIncome" name="PercentageOfCorporationsTaxableIncome" nillable="true" xbrli:periodType="instant" type="num:percentItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_OperatingLossCarryforwardsExpirationDescription" name="OperatingLossCarryforwardsExpirationDescription" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_EffectiveIncomeTaxRateReconciliationPermanentDifferencePercent" name="EffectiveIncomeTaxRateReconciliationPermanentDifferencePercent" nillable="true" xbrli:periodType="duration" type="num:percentItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsSharebasedCompensationCostNet" name="DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsSharebasedCompensationCostNet" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_DeferredTaxAssetsIncomeTaxLoss" name="DeferredTaxAssetsIncomeTaxLoss" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsGrantedInPeriodWeightedAverageRemainingContractualTerm" name="SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsGrantedInPeriodWeightedAverageRemainingContractualTerm" nillable="true" xbrli:periodType="duration" type="xbrli:durationItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm" name="SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm" nillable="true" xbrli:periodType="duration" type="xbrli:durationItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_AdjustmentsToAdditionalPaidInCapitalIssuanceOfOptions" name="AdjustmentsToAdditionalPaidInCapitalIssuanceOfOptions" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_IssuanceOfOptionsForSettlementOfAccruedPayroll" name="IssuanceOfOptionsForSettlementOfAccruedPayroll" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_DebtSettlementMember" name="DebtSettlementMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_StrategicBioPartnersMember" name="StrategicBioPartnersMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_UniversityOfTorontoMember" name="UniversityOfTorontoMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_SeriesBConvertiblePreferredStockMember" name="SeriesBConvertiblePreferredStockMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_ConversionFeatureSharesMember" name="ConversionFeatureSharesMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_VestBiWeeklyOverTwoMonthsMember" name="VestBiWeeklyOverTwoMonthsMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_ConvertibleNotesMember" name="ConvertibleNotesMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_ConsultantMember" name="ConsultantMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_UnsecuredConvertibleNotesMember" name="UnsecuredConvertibleNotesMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_UnsecuredConvertibleNotesWithRelatedPartiesMember" name="UnsecuredConvertibleNotesWithRelatedPartiesMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_FederalMember" name="FederalMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_StateAndLocalMember" name="StateAndLocalMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_ToxicologyStudiesMember" name="ToxicologyStudiesMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_EmployeesMember" name="EmployeesMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_NonEmployeesMember" name="NonEmployeesMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_SixteenIndividualsMember" name="SixteenIndividualsMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_TwelveRelatedPartiesMember" name="TwelveRelatedPartiesMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_SettlementOfAccruedCompensationMember" name="SettlementOfAccruedCompensationMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_OptionsVestedImmediatelyMember" name="OptionsVestedImmediatelyMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_OptionsVestedOverTwelveMonthsMember" name="OptionsVestedOverTwelveMonthsMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_OptionsVestedOverTwentyFourMonthsMember" name="OptionsVestedOverTwentyFourMonthsMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_OptionsVestedOverThirtySixMonthsMember" name="OptionsVestedOverThirtySixMonthsMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_OptionsVestedOverFourtyEightMonthsMember" name="OptionsVestedOverFourtyEightMonthsMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_ProtagenicWarrantsMember" name="ProtagenicWarrantsMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_AdditionalIssuanceOfWarrantsMember" name="AdditionalIssuanceOfWarrantsMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_AdditionalIssuanceOfWarrantsDebtMember" name="AdditionalIssuanceOfWarrantsDebtMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_AdditionalIssuanceOfWarrantsForAccruedInterestMember" name="AdditionalIssuanceOfWarrantsForAccruedInterestMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_DrDavidLovejoyMember" name="DrDavidLovejoyMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_ResearchAgreementsMember" name="ResearchAgreementsMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_RelatedPartyMember" name="RelatedPartyMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_ConvertibleNotesPayableRelatedPartyMember" name="ConvertibleNotesPayableRelatedPartyMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_OptionsToRelatedPartyMember" name="OptionsToRelatedPartyMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_DrBuellMember" name="DrBuellMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_RecognitionUpcomingServiceMember" name="RecognitionUpcomingServiceMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_PlacementAgentKatalystSecuritiesLLCMember" name="PlacementAgentKatalystSecuritiesLLCMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_LossesIncurredAfter2017Member" name="LossesIncurredAfter2017Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_LossesIncurredPriorTo2018Member" name="LossesIncurredPriorTo2018Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_FederalGeneratedPriorTo2018Member" name="FederalGeneratedPriorTo2018Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_StockOptionToConsultantsMember" name="StockOptionToConsultantsMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_PlacementAgentWarrantsMember" name="PlacementAgentWarrantsMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_BlackScholesModelMember" name="BlackScholesModelMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_BlackScholesModelTwoMember" name="BlackScholesModelTwoMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="PTIX_TwoThousandSixteenPlanMember" name="TwoThousandSixteenPlanMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
</schema>
